The stability of a ship depends on the attitude. Fundamentals of ship theory. Operational, seaworthy and maneuverable qualities. Factors affecting ship stability

Vessel performance

The most characteristic operational qualities of a small vessel are: passenger capacity,load capacity, displacement and speed.

Passenger capacity is an indicator equal to the number of equipped places to accommodate people on the ship. Passenger capacity depends on the carrying capacity:

P = G/100, people (with luggage), or P =G/75 people (without luggage)

In this case, the result is rounded to a smaller integer. On a small vessel, the availability of equipped seats must correspond to the passenger capacity established for the vessel.

Passenger capacity can be approximately calculated using the formula:

N=Lnb Bnb/K, people,

Where TO - empirical coefficient taken equal to: for motor and rowing boats - 1.60; for boats - 2.15.

Load capacity— the payload of the ship, including the mass of people and luggage according to passenger capacity. A distinction is made between deadweight and net tonnage.

Deadweight - this is the difference between the displacement when fully loaded and when unladen.

Net load capacity - This is the mass of only the payload that the ship can take.

For large vessels, the unit of change in carrying capacity is ton, for small vessels - kg. Load capacity C can be calculated using formulas, or can be determined experimentally. To do this, when the vessel is empty, but with supplies and a reserve of fuel, cargo is sequentially placed until the vessel reaches the waterline corresponding to the minimum freeboard height. The mass of the placed cargo corresponds to the carrying capacity of the vessel.

Displacement . There are two types of displacement - mass (weight) and volumetric.

Mass (weight) displacement - this is the mass of the ship afloat, equal to the mass of the water displaced by the ship. The unit of measurement is ton.

Volumetric displacement V - this is the volume of the underwater part of the vessel in m3. The calculation is made through the main measurements:

V = SL VT,

where S is the coefficient of complete displacement, equal to 0.35 - 0.6 for small vessels, and a lower value of the coefficient is typical for small vessels with sharp contours. For displacement boats S = 0.4 - 0.55, planing boats S = 0.45 - 0.6, motor boats 5 - 0.35 - 0.5, for sailing ships this coefficient ranges from 0.15 to 0.4.

Speed.

Speed ​​is the distance traveled by a ship per unit time. On seagoing vessels, speed is measured in knots (miles per hour), and on inland vessels - in kilometers per hour (km/h). The navigator of a small vessel is recommended to know three speeds: the highest (maximum) that the vessel develops at maximum engine power; the smallest (minimum) at which the ship obeys the rudder; medium - the most economical for relatively large transitions. The speed depends on the engine power, the size and shape of the hull, the loading of the vessel and various external factors: waves, wind, currents, etc.

Seaworthiness of the vessel

The ability of a vessel to stay afloat, interact with water, and not capsize or sink when flooded is characterized by its seaworthiness. These include: buoyancy, stability and unsinkability.

Buoyancy. Buoyancy is the ability of a ship to float on the surface of the water, having a given draft. The more weight you place on the boat, the deeper it will sink into the water, but will not lose buoyancy until water begins to flow into the hull.

In the event of a leak in the hull or a hole, as well as water entering the vessel during stormy weather, its weight increases. Therefore, the ship must have a reserve of buoyancy.

Buoyancy reserve - This is the water-tight volume of the ship's hull, located between the load waterline and the upper edge of the side. If there is no reserve of buoyancy, the ship will sink if even a small amount of water gets inside the hull.

The reserve of buoyancy necessary for safe navigation of a vessel is ensured by giving the vessel a sufficient freeboard height, as well as the presence of waterproof closures and bulkheads between compartments and buoyancy blocks - structural elements inside the hull of a small vessel in the form of a solid block of material (for example, polystyrene) having a density of less than one . In the absence of such bulkheads and buoyancy blocks, any hole in the underwater part of the hull leads to a complete loss of buoyancy reserve and the death of the vessel.

The reserve of buoyancy depends on the height of the freeboard - the higher the freeboard, the greater the reserve of buoyancy. This reserve is standardized by the minimum freeboard height, depending on the value of which the safe navigation area and permissible distance from the shore are established for a particular small vessel. However, the freeboard height cannot be abused, as this affects another equally important quality - stability

Stability. Stability is the ability of a ship to withstand the forces that cause it to tilt, and after the cessation of these forces (wind, wave, movement of passengers, etc.) to return to its original equilibrium position. The same vessel may have good stability if the cargo is located close to the bottom and may partially or completely lose stability if the cargo or people are placed slightly higher

There are two types of stability: transverse and longitudinal. Transverse stability manifests itself when the ship rolls, i.e. when tilting it on board. During navigation, two forces act on the ship: gravity and support. The resultant D (Fig. 1, a) of the vessel's gravity force, directed downwards, will be conditionally applied at point G, called the center of gravity (CG), and the resultant A of the support forces, directed upwards, will be conditionally applied at the center of gravity C of the part immersed in water vessel, called the center of magnitude (CV). When the ship has no trim and roll, the CG and CV will be located in the centerline plane of the ship (DP).

Fig. 1 Location of the resultant forces of gravity and support relative to each other at different positions of the vessel

The ho value characterizes the stability of the vessel at low inclinations. The position of point M under these conditions is almost independent of the roll angle f.

The force D and the equal supporting force A form a pair of forces with the shoulder /, which creates a restoring moment MB=Dl. This moment tends to return the ship to its original position. Note that the CG is below point M.

Now imagine that an additional load is placed on the deck of the same ship (Fig. 1, c). As a result, the CG will be located significantly higher, and during a roll, point M will be below it. The resulting pair of forces will no longer create a restoring moment, but an overturning moment Mopr. Consequently, the ship will be unstable and capsize.

On lateral stability The width of the vessel is greatly influenced by the width of the hull: the wider the hull, the more stable the vessel, and, conversely, the narrower and taller the hull, the worse the stability.

For small high-speed vessels (especially when moving on high speed during rough seas) the problem of maintaining longitudinal stability is not always solved.

For small keel vessels, the initial metacentric height is, as a rule, 0.3 - 0.6 m. The stability of the vessel depends on the loading of the vessel, the movement of cargo, passengers and other reasons. The greater the metacentric height, the greater the righting moment and the more stable the vessel, however, with high stability the vessel has a sharp roll. Stability is improved by the low position of the engine, fuel tank, seats and appropriate placement of cargo and people.

In heavy winds, a strong wave hitting the side, and in some other cases, the ship's roll increases quickly and a dynamic heeling moment occurs. In this case, the ship's roll will increase even after the heeling and righting moments are equal. This occurs due to the action of inertial force. Typically, such a roll is twice as large as the roll from the static action of the same heeling moment. Therefore, sailing in stormy weather, especially for small vessels, is very dangerous.

Longitudinal stability acts when the ship is tilted to the bow or stern, i.e. during pitching. The navigator should take this stability into account when moving at high speeds during waves, because Having buried its nose in the water, a boat or motorboat may not restore its original position and sink, and sometimes even capsize.

Factors affecting ship stability:

a) The stability of a vessel is most significantly affected by its width: the greater it is in relation to its length, side height and draft, the higher the stability.

b) The stability of a small vessel increases if the shape of the submerged part of the hull is changed at large angles of heel. This statement, for example, is the basis for the action of side boules and foam fenders, which, when immersed in water, create an additional righting moment.

c) Stability deteriorates if the ship has fuel tanks with a surface mirror from side to side, so these tanks must have internal partitions

d) Stability is most strongly influenced by the placement of passengers and cargo on the ship; they should be located as low as possible. On a small vessel, people should not be allowed to sit on board or move around arbitrarily while it is moving. Cargoes must be securely fastened to prevent their unexpected displacement from their stowage locations e) In strong winds and waves, the effect of heeling moment is very dangerous for the vessel, therefore, with deterioration weather conditions it is necessary to take the ship to shelter and wait out the bad weather. If this is impossible to do due to the considerable distance to the shore, then in stormy conditions you should try to keep the ship “head to the wind”, throwing out the sea anchor and running the engine at low speed.

Unsinkable. Unsinkability is the ability of a ship to remain buoyant after part of the ship has been flooded.

Unsinkability is ensured structurally - by dividing the hull into waterproof compartments, equipping the vessel with buoyancy blocks and drainage means.

The non-flooded volumes of the hull are most often made of foam blocks. Its required quantity and location are calculated to ensure an emergency reserve of buoyancy and maintain the emergency vessel in the “even keel” position.

Of course, in conditions of strong excitement, not everyone who received a hole powerboat and the boat will ensure that these requirements are met.

Maneuverability of a small vessel

The main maneuvering qualities of a vessel include: controllability, circulation, propulsion and inertia

Controllability. Controllability is the ability of a vessel to maintain a given direction of movement while moving with a constant rudder position (heading stability) and to change the direction of its movement while moving under the influence of the rudder (agility).

Course stability is the property of a vessel to maintain a straight direction of motion. If the ship, with the rudder in a straight position, deviates from the course, then this phenomenon is usually called the yaw of the ship.

If the ship, with the rudder in a straight position, deviates from the course, then this phenomenon is usually called the yaw of the ship.

The causes of yaw can be permanent or temporary. Constant reasons include those related to the design features of the vessel: blunt bow contours of the hull, discrepancy between the length of the vessel and its width, insufficient rudder blade area, the influence of propeller rotation

Temporary yaw can be caused by improper loading of the vessel, wind, shallow water, uneven currents, etc.

The concepts of “course stability” and “agility” are contradictory, but these qualities are inherent in almost all ships and characterize their controllability.

Controllability is influenced by many factors and reasons, the main ones being the action of the steering wheel, the operation of the propeller and their interaction.

Agility- the property of a ship to change the direction of movement under the influence of the rudder. This quality primarily depends on the correct ratio of the length and width of the hull, the shape of its contours, as well as the area of ​​the rudder blade.

Features of vessel controllability when moving from forward to reverse

When carrying out mooring operations or the need to urgently stop the vessel (risk of collision, preventing grounding, assisting a person overboard, etc.), it is necessary to switch from forward to reverse. In these cases, the navigator must take into account that in the first seconds, when changing the operation of the right-hand rotation propeller from forward to reverse, the stern will rapidly roll to the left, and with a left-hand rotation propeller - to the right.

Reasons affecting controllability

In addition to the rudder and the rotating propeller, the stability and agility of the vessel are influenced by other factors, as well as a number of design features of the vessel: the ratio of the main dimensions, the shape of the hull contours, the parameters of the rudder and propeller. Controllability also depends on sailing conditions: the nature of the vessel’s loading, hydrometeorological factors.

Circulation If you move the rudder to any side while the ship is moving, the ship will begin to turn and describe a curved line on the water. This curve, described by the vessel’s center of gravity during a turn, is called the circulation line (Fig. 2), and the distance between the centerline plane of the ship on the forward course and its centerline plane after turning on the return course (180) is the tactical circulation diameter. The less tactical diameter circulation, the better the maneuverability of the vessel is considered. This curve is close to a circle, and its diameter serves as a measure of the maneuverability of the ship

The circulation diameter is usually measured in meters. For small motor vessels, the size of the tactical circulation diameter in most cases is equal to 2-3 ship lengths. Every driver needs to know the circulation diameter of the vessel he has to control, since correct and safe maneuvering largely depends on this. The speed of the vessel during circulation is reduced to 30%. We should never forget that when moving along a curve, a centrifugal force acts on the ship (Fig. 3), directed from the center of curvature to the outer side and applied to the center of gravity of the ship.

Fig 2 Circulation /—circulation line, 2—tactical circulation diameter, 3—steady circulation diameter

The drift of the vessel arising from the centrifugal force is prevented by the force of water resistance - lateral resistance, the point of application of which is located below the center of gravity. As a result, a pair of forces arises that creates a roll on board, opposite to the direction of rotation. Roll increases as the vessel's center of gravity increases above the center of lateral resistance and as the metacentric height decreases.

An increase in turning speed and a decrease in the circulation diameter significantly increase the roll, which can lead to the vessel capsizing. Therefore, never make sharp turns when the boat is moving at high speed.

Unlike conventional displacement vessels, vessels with planing contours on the circulation turn to the inside (Fig. 4). This occurs from the additional lifting force that occurs on the hull during lateral displacement due to planing contours. At the same time, sliding occurs under the influence of centrifugal force to the outside, which is why planing ships have a slightly greater circulation compared to displacement ships.

In addition to the circulation diameter, you should also know its time, i.e. the time it takes the ship to make a 360° turn.

The named circulation elements depend on the displacement of the vessel and the nature of the placement of cargo along its length, as well as on the speed. At low speed the circulation diameter is smaller.

Mobility. Propulsion is the ability of a vessel to move at a certain speed with a given engine power, while overcoming the forces of resistance to movement.

The movement of the vessel is possible only if there is a certain force that can overcome the resistance of the water - the thrust. At a constant speed, the amount of stop is equal to the amount of water resistance. Vessel speed and thrust are related following dependency:

R. V=ho-N.Where: V - ship speed; K - water resistance; N - engine power; ho -Efficiency=0.5.

This equation shows that as speed increases, water resistance also increases. However, this dependence has a different physical meaning and character for displacement vessels and planing vessels.

For example, at a speed of a displacement vessel up to a value equal to V = 2 ÖL, km/h (L is the length of the vessel, m), water resistance K consists of the friction resistance of water on the hull skin and the shape resistance that is created by water turbulence. When the speed of this vessel exceeds the specified value, waves begin to form and a third resistance is added to the two resistances - wave resistance. Wave drag increases sharply with increasing speed.

For planing vessels, the nature of water resistance is the same as for displacement vessels and the speed value is V = 8 ÖL km/h. However, with a further increase in speed, the ship receives a significant trim to the stern and its bow rises. This mode of movement is called transitional (from displacement to planing). A characteristic sign of the beginning of planing is a spontaneous increase in the speed of the vessel. This phenomenon is caused by the fact that after the bow rises, the overall resistance of the water to the vessel decreases, it seems to “float up” and increase speed while maintaining constant power.

When planing, another type of water resistance arises - splash resistance, and the wave resistance and shape resistance are sharply reduced and their values ​​are practically reduced to zero.

Thus, four types of resistance affect the propulsion of the vessel:

friction resistance- depends on the area of ​​the wetted surface of the vessel, on the quality of its processing and the degree of fouling (algae, mollusks, etc.);

shape resistance- depends on the streamlining of the vessel’s hull, which in turn is better, the sharper the stern end and the greater the length of the vessel compared to the width;

characteristic impedance- depends on the shape of the bow and the length of the vessel, the longer the vessel, the less wave formation;

splash resistance- depends on the ratio of the width of the body to its length.

Conclusion: 1. Displacement vessels with a narrow hull, round bilge lines and pointed bow and stern ends experience the least water resistance.

2. For planing vessels, in the absence of waves, a wide flat-bottomed hull with a transom stern provides the least water resistance with the greatest hydrodynamic lift.

More seaworthy planing vessels with a keeled or semi-keeled hull. Increasing the speed of these vessels is achieved by longitudinal steps and bilge splash guards.

Inertia. A very important maneuvering quality of a vessel is its inertia. It is usually estimated by the lengths of the braking distance, coasting and acceleration paths, as well as their duration. The distance that a ship travels during the period of time from the moment the engine switches from full forward to reverse until the ship finally stops is called braking distance. This distance is usually expressed in meters, less often in ship lengths. The distance covered by the vessel during the period of time from the moment the engine is stopped running in forward motion until the vessel comes to a complete stop under the influence of water resistance is called coasting. The distance that the ship travels from the moment the engine is switched on to forward speed until full speed is acquired at a given engine operating mode is called the acceleration path. Accurate knowledge by the driver of the above qualities of his vessel greatly ensures the safety of maneuvering in narrow areas and roadsteads with cramped navigation conditions. Remember! Motorized boats do not have brakes, so they often require significantly more distance and time to absorb inertia than, say, a car.

By the relative position of the cargo on the ship, the navigator can always find the most favorable value of the metacentric height, at which the ship will be sufficiently stable and less subject to pitching.

The heeling moment is the product of the weight of the cargo moved across the vessel by a shoulder equal to the distance of movement. If a person weighs 75 kg, sitting on a bank will move across the ship by 0.5 m, then the heeling moment will be equal to 75 * 0.5 = 37.5 kg/m.

Figure 91. Static stability diagram

To change the moment that heels the ship by 10°, it is necessary to load the ship to full displacement completely symmetrically relative to the center plane.

The vessel's loading should be checked by drafts measured on both sides. The inclinometer is installed strictly perpendicular to the center plane so that it shows 0°.

After this, you need to move loads (for example, people) at pre-marked distances until the inclinometer shows 10°. The test experiment should be carried out as follows: tilt the ship on one side and then on the other side.

Knowing the fastening moments of a ship heeling at various (up to the greatest possible) angles, it is possible to construct a static stability diagram (Fig. 91), which will evaluate the stability of the ship.

Stability can be increased by increasing the width of the vessel, lowering the center of gravity, and installing stern bulges.

If the center of gravity of the vessel is located below the center of magnitude, then the vessel is considered very stable, since the supporting force during a roll does not change in magnitude and direction, but the point of its application shifts towards the tilt of the vessel (Fig. 92, a).

Therefore, when heeling, a pair of forces is formed with a positive restoring moment, tending to return the ship to its normal vertical position on a straight keel. It is easy to verify that h>0, with the metacentric height equal to 0. This is typical for yachts with a heavy keel and atypical for larger ones. large ships with a conventional housing design.

If the center of gravity is located above the center of magnitude, then three cases of stability are possible, which the navigator should be well aware of.

The first case of stability.

Metacentric height h>0. If the center of gravity is located above the center of magnitude, then when the vessel is in an inclined position, the line of action of the supporting force intersects the center plane above the center of gravity (Fig. 92, b).

Rice. 92. The case of a stable ship

In this case, a couple of forces with a positive restoring moment is also formed. This is typical for most conventionally shaped boats. Stability in this case depends on the hull and the position of the center of gravity in height.

When heeling, the heeling side enters the water and creates additional buoyancy, tending to level the ship. However, when a ship rolls with liquid and bulk cargo that can move towards the roll, the center of gravity will also shift towards the roll. If the center of gravity during a roll moves beyond the plumb line connecting the center of magnitude with the metacenter, then the ship will capsize.

The second case of an unstable vessel in indifferent equilibrium.

Metacentric height h = 0. If the center of gravity lies above the center of magnitude, then during a roll the line of action of the supporting force passes through the center of gravity MG = 0 (Fig. 93).

In this case, the center of magnitude is always located on the same vertical as the center of gravity, so there is no recovering pair of forces. Without the influence of external forces, the ship cannot return to an upright position.

In this case, it is especially dangerous and completely unacceptable to transport liquid and bulk cargo on a ship: with the slightest rocking, the ship will capsize. This is typical for boats with a round frame.

The third case of an unstable ship with unstable equilibrium.

Metacentric height h<0. Центр тяжести расположен выше центра величины, а в наклонном положении судна линия действия силы поддержания пересекает след диаметральной плоскости ниже центра тяжести (рис. 94).

Stability is one of the most important seaworthiness of a vessel, which is associated with extremely important issues regarding navigation safety. Loss of stability almost always means the death of the ship and, very often, the crew. Unlike changes in other seaworthiness, the decrease in stability is not visible, and the crew of the ship, as a rule, is unaware of the impending danger until the last seconds before capsizing. Therefore, the greatest attention must be paid to the study of this section of the theory of the ship.

In order for a ship to float in a given equilibrium position relative to the water surface, it must not only satisfy the conditions of equilibrium, but also be able to resist external forces tending to take it out of the equilibrium position, and after the cessation of the action of these forces, return to its original position. position. Therefore, the balance of the ship must be stable or, in other words, the ship must have positive stability.

Thus, stability is the ability of a vessel, brought out of a state of equilibrium by external forces, to return to its original equilibrium position again after the action of these forces ceases.

The stability of the vessel is associated with its balance, which serves as a characteristic of the latter. If the ship's equilibrium is stable, then the ship has positive stability; if its equilibrium is indifferent, then the ship has zero stability, and, finally, if the ship's equilibrium is unstable, then it has negative stability.

Tanker Captain Shiryaev
Source: fleetphoto.ru

This chapter will examine the lateral inclinations of the ship in the midship frame plane.

Stability during transverse inclinations, i.e. when a roll occurs, is called transverse. Depending on the angle of inclination of the vessel, lateral stability is divided into stability at small angles of inclination (up to 10-15 degrees), or the so-called initial stability, and stability at large angles of inclination.

The tilting of the ship occurs under the influence of a pair of forces; the moment of this pair of forces, causing the vessel to rotate around the longitudinal axis, will be called heeling Mkr.

If Mcr applied to the ship increases gradually from zero to the final value and does not cause angular accelerations, and therefore inertia forces, then stability with such an inclination is called static.

The heeling moment acting on the ship instantly leads to the emergence of angular acceleration and inertial forces. The stability that appears with such an inclination is called dynamic.

Static stability is characterized by the occurrence of a restoring moment, which tends to return the vessel to its original equilibrium position. Dynamic stability is characterized by the work of this moment from the beginning to the end of its action.

Let us consider the uniform transverse inclination of the vessel. We will assume that in the initial position the ship has a straight landing. In this case, the supporting force D' acts in the DP and is applied at point C - the center of the vessel’s size (Center of buoyancy-B).

Rice. 1

Let us assume that the vessel, under the influence of a heeling moment, has received a transverse inclination at a small angle θ. Then the center of the magnitude will move from point C to point C 1 and the supporting force, perpendicular to the new existing waterline B 1 L 1, will be directed at an angle θ to the center plane. The action lines of the original and new direction of the support force will intersect at point m. This point of intersection of the line of action of the supporting force at an infinitesimal equal-volume inclination of a floating vessel is called the transverse metacentre.

We can give another definition to the metacenter: the center of curvature of the curve of displacement of the center of magnitude in the transverse plane is called the transverse metacenter.

The radius of curvature of the curve of displacement of the center of a quantity in the transverse plane is called the transverse metacentric radius (or small metacentric radius). It is determined by the distance from the transverse metacenter m to the center of magnitude C and is denoted by the letter r.

The transverse metacentric radius can be calculated using the formula:

i.e., the transverse metacentric radius is equal to the moment of inertia Ix of the area of ​​the waterline relative to the longitudinal axis passing through the center of gravity of this area, divided by the volumetric displacement V corresponding to this waterline.

Stability conditions

Let us assume that the ship, which is in a direct equilibrium position and floating along the waterline of the overhead line, as a result of the action of the external heeling moment Mkr, has heeled so that the original waterline of the overhead line with the new existing waterline B 1 L 1 forms a small angle θ. Due to the change in the shape of the hull part submerged in water, the distribution of hydrostatic pressure forces acting on this part of the hull will also change. The center of the vessel's size will move towards the roll and move from point C to point C 1.

The supporting force D', remaining unchanged, will be directed vertically upward perpendicular to the new effective waterline, and its line of action will intersect the DP at the original transverse metacenter m.

The position of the ship's center of gravity remains unchanged, and the weight force P will be perpendicular to the new waterline B 1 L 1. Thus, the forces P and D', parallel to each other, do not lie on the same vertical and, therefore, form a pair of forces with the arm GK, where point K is the base of the perpendicular lowered from point G to the direction of action of the supporting force.

The pair of forces formed by the weight of the vessel and the supporting force, tending to return the vessel to its original equilibrium position, is called a restoring pair, and the moment of this pair is called the restoring moment Mθ.

The issue of stability of a heeled ship is decided by the direction of action of the righting moment. If the restoring moment tends to return the ship to its original equilibrium position, then the restoring moment is positive, the stability of the ship is also positive - the ship is stable. In Fig. Figure 2 shows the location of the forces acting on the ship, which corresponds to a positive restoring moment. It is easy to verify that such a moment occurs if the CG lies below the metacenter.

Rice. 2 Rice. 3

In Fig. Figure 3 shows the opposite case, when the restoring moment is negative (the center of gravity lies above the metacenter). It tends to further deflect the ship from its equilibrium position, since the direction of its action coincides with the direction of action of the external heeling moment Mkr. In this case, the ship is not stable.

Theoretically, it can be assumed that the restoring moment when the vessel tilts is equal to zero, i.e. the force of the weight of the vessel and the supporting force are located on the same vertical, as shown in Fig. 4.

Rice. 4

The absence of a righting moment leads to the fact that after the heeling moment ceases, the ship remains in an inclined position, i.e., the ship is in indifferent equilibrium.

Thus, according to the relative position of the transverse metacenter m and C.T. G can be judged on the sign of the righting moment or, in other words, on the stability of the vessel. So, if the transverse metacenter is above the center of gravity (Fig. 2), then the ship is stable.

If the transverse metacenter is located below the center of gravity or coincides with it (Fig. 3, 4), the ship is not stable.

This gives rise to the concept of metacentric height: transverse metacentric height is the elevation of the transverse metacenter above the center of gravity of the vessel in the initial equilibrium position.

The transverse metacentric height (Fig. 2) is determined by the distance from the center of gravity (i.e. G) to the transverse metacenter (i.e. m), i.e., the segment mG. This segment is a constant value, since and C.T. , and the transverse metacenter do not change their position at small inclinations. In this regard, it is convenient to accept it as a criterion for the initial stability of a vessel.

If the transverse metacenter is located above the center of gravity of the vessel, then the transverse metacentric height is considered positive. Then the condition for the stability of the vessel can be given in the following formulation: the vessel is stable if its transverse metacentric height is positive. This definition is convenient in that it allows one to judge the stability of the vessel without considering its inclination, i.e., at a roll angle of zero, when there is no righting moment at all. To establish what data is necessary to obtain the value of the transverse metacentric height, let us turn to Fig. 5, which shows the relative location of the center of magnitude C, the center of gravity G and the transverse metacenter m of a vessel having positive initial lateral stability.

Rice. 5

The figure shows that the transverse metacentric height h can be determined by one of the following formulas:

h = Z C ± r - Z G ;

The transverse metacentric height is often determined using the last equality. The applicate of the transverse metacenter Zm can be found from the metacentric diagram. The main difficulties in determining the transverse metacentric height of a vessel arise when determining the applicate of the center of gravity ZG, which is determined using a summary table of the vessel's mass load (the issue was discussed in the lecture -).

In foreign literature, the designation of the corresponding points and stability parameters may look as shown below in Fig. 6.

Rice. 6

• where K is the keel point;
• B - center of buoyancy;
• G—center of gravity;
• M - transverse metacentre;
• KV - applicate of the center of magnitude;
• KG - applicate of the center of gravity;
• KM - applicate of the transverse metacenter;
• VM - transverse metacentric radius (Radius of metacentre);
• BG - elevation of the center of gravity above the center of magnitude;
• GM - transverse metacentric height.

The static stability arm, denoted in our literature as GK, is denoted in foreign literature as GZ.

Suggested reading:

The theory of lateral stability considers the inclination of the ship occurring in the midship plane, and an external moment, called the heeling moment, also acts in the midship plane.

Without limiting ourselves to small inclinations of the vessel for now (they will be considered as a special case in the section “Initial Stability”), let us consider the general case of heeling of the vessel under the action of an external heeling moment constant in time. In practice, such a heeling moment can arise, for example, from the action of a constant wind force, the direction of which coincides with the transverse plane of the vessel - the midsection plane. When exposed to this heeling moment, the ship has a constant roll to the opposite side, the magnitude of which is determined by the wind force and the righting moment on the part of the ship.

In the literature on ship theory, it is customary to combine in the figure two positions of the ship at once - straight and with a list. The heeled position corresponds to a new position of the waterline relative to the ship, which corresponds to a constant submerged volume, however, the shape of the underwater part of the heeled ship no longer has symmetry: the starboard side is submerged more than the left (Fig. 1).

All waterlines corresponding to one value of the vessel’s displacement (at constant weight of the vessel) are usually called equal volume.

The accurate representation in the figure of all equal-volume waterlines is associated with great calculation difficulties. In ship theory, there are several techniques for graphically depicting equal-volume waterlines. At very small angles of heel (at infinitesimal equal-volume inclinations), one can use a corollary from L. Euler’s theorem, according to which two equal-volume waterlines, differing by an infinitely small angle of heel, intersect along a straight line passing through their common center of gravity of the area (for finite inclinations this the statement loses its validity, since each waterline has its own center of gravity of the area).

If we abstract from the real distribution of forces of the ship's weight and hydrostatic pressure, replacing their action with concentrated resultants, we arrive at the diagram (Fig. 1). At the center of gravity of the vessel, a weight force is applied, directed in all cases perpendicular to the waterline. In parallel to it, there is a buoyancy force applied in the center of the underwater volume of the vessel - in the so-called center of magnitude(dot WITH).

Due to the fact that the behavior (and origin) of these forces are independent of each other, they no longer act along one line, but form a pair of forces parallel and perpendicular to the acting waterline B 1 L 1. Regarding weight force R we can say that it remains vertical and perpendicular to the surface of the water, and the tilted ship deviates from the vertical, and only the convention of the drawing requires that the vector of the weight force be deviated from the center plane. The specifics of this approach are easy to understand if you imagine a situation with a video camera mounted on a ship, showing on the screen the surface of the sea inclined at an angle equal to the angle of roll of the ship.

The resulting pair of forces creates a moment, which is usually called restoring moment. This moment counteracts the external heeling moment and is the main object of attention in the theory of stability.

The magnitude of the restoring moment can be calculated using the formula (as for any pair of forces) as the product of one (either of two) forces and the distance between them, called static stability shoulder:

Formula (1) indicates that both the shoulder and the moment itself depend on the angle of roll of the vessel, i.e. represent variable (in the sense of roll) quantities.

However, not in all cases the direction of the restoring moment will correspond to the image in Fig. 1.

If the center of gravity (as a result of the peculiarities of the placement of cargo along the height of the vessel, for example, when there is excess cargo on the deck) turns out to be quite high, then a situation may arise when the weight force is to the right of the line of action of the supporting force. Then their moment will act in the opposite direction and will contribute to the ship's heeling. Together with the external heeling moment, they will capsize the ship, since there are no other counteracting moments.

It is clear that in this case this situation should be assessed as unacceptable, since the vessel does not have stability. Consequently, with a high center of gravity, the ship may lose this important seaworthiness quality - stability.

On sea-going displacement vessels, the ability to influence the stability of the vessel, to “control” it, is provided to the navigator only through the rational placement of cargo and reserves along the height of the vessel, which determine the position of the vessel’s center of gravity. Be that as it may, the influence of the crew members on the position of the center of magnitude is excluded, since it is associated with the shape of the underwater part of the hull, which (with a constant displacement and draft of the vessel) is unchanged, and in the presence of a roll of the vessel, it changes without human intervention and depends only on the draft. Human influence on the shape of the hull ends at the design stage of the vessel.

Thus, the vertical position of the center of gravity, which is very important for the safety of the ship, is in the “sphere of influence” of the crew and requires constant monitoring through special calculations.

To calculate the presence of “positive” stability of a vessel, the concept of metacenter and initial metacentric height is used.

Transverse metacenter- this is the point that is the center of curvature of the trajectory along which the center of the value moves when the ship heels.

Consequently, the metacenter (as well as the center of magnitude) is a specific point, the behavior of which is exclusively determined only by the geometry of the shape of the vessel in the underwater part and its draft.

The position of the metacenter corresponding to the landing of the vessel without a roll is usually called initial transverse metacenter.

The distance between the center of gravity of the vessel and the initial metacenter in a particular loading option, measured in the center plane (DP), is called initial transverse metacentric height.

The figure shows that the lower the center of gravity is located in relation to the constant (for a given draft) initial metacenter, the greater will be the metacentric height of the vessel, i.e. the greater is the leverage of the restoring moment and this moment itself.

Thus, the metacentric height is an important characteristic that serves to control the stability of the vessel. And the greater its value, the greater at the same roll angles will be the value of the righting moment, i.e. resistance of the ship to heeling.

For small heels of the vessel, the metacenter is approximately located at the site of the initial metacenter, since the trajectory of the center of magnitude (point WITH) is close to a circle and its radius is constant. From a triangle with a vertex at the metacenter, a useful formula follows that is valid at small roll angles ( θ <10 0 ÷12 0):

where is the roll angle θ should be used in radians.

From expressions (1) and (2) it is easy to obtain the expression:

which shows that the static stability arm and metacentric height do not depend on the weight of the vessel and its displacement, but represent universal stability characteristics with which the stability of ships of different types and sizes can be compared.

So for ships with a high center of gravity (timber carriers), the initial metacentric height takes the values h 0≈ 0 – 0.30 m, for dry cargo ships h 0≈ 0 – 1.20 m, for bulk carriers, icebreakers, tugs h 0> 1.5 ÷ 4.0 m.

However, the metacentric height should not take negative values. Formula (1) allows us to draw other important conclusions: since the order of magnitude of the righting moment is determined mainly by the magnitude of the vessel’s displacement R, then the static stability arm is a “control variable” that affects the range of torque changes M in at a given displacement. And from the slightest changes l(θ) Due to inaccuracies in its calculation or errors in the initial information (data taken from ship drawings, or measured parameters on the ship), the magnitude of the moment significantly depends M in, which determines the vessel’s ability to resist inclinations, i.e. determining its stability.

Thus, the initial metacentric height plays the role of a universal stability characteristic, allowing one to judge its presence and size regardless of the size of the vessel.

If we follow the stability mechanism at large roll angles, new features of the righting moment will appear.

For arbitrary transverse inclinations of the vessel, the curvature of the trajectory of the center of magnitude WITH changes. This trajectory is no longer a circle with a constant radius of curvature, but is a kind of flat curve that has different values ​​of curvature and radius of curvature at each point. As a rule, this radius increases with the roll of the vessel and the transverse metacenter (as the beginning of this radius) leaves the center plane and moves along its trajectory, tracking the movements of the center of magnitude in the underwater part of the vessel. In this case, of course, the very concept of metacentric height becomes inapplicable, and only the righting moment (and its shoulder l(θ)) remain the only characteristics of ship stability at high inclinations.

However, in this case, the initial metacentric height does not lose its role as a fundamental initial characteristic of the stability of the vessel as a whole, since the order of magnitude of the righting moment depends on its value, as on a certain “scale factor,” i.e. its indirect effect on the stability of the vessel at large angles of roll remains.

So, to control the stability of the vessel before loading, it is necessary at the first stage to estimate the value of the initial transverse metacentric height h 0, using the expression:

where z G and z M 0 are applicates of the center of gravity and the initial transverse metacenter, respectively, measured from the main plane in which the beginning of the OXYZ coordinate system associated with the vessel is located (Fig. 3).

Expression (4) simultaneously reflects the degree of participation of the navigator in ensuring stability. By choosing and controlling the position of the vessel's center of gravity in height, the crew ensures the stability of the vessel, and all geometric characteristics, in particular, Z M 0, must be provided by the designer in the form of graphs of settlement d, called curves of theoretical drawing elements.

Further control of the vessel's stability is carried out according to the methods of the Maritime Register of Shipping (RS) or according to the methods of the International Maritime Organization (IMO).

Righting moment arm l and the moment itself M in have a geometric interpretation in the form of a Static Stability Diagram (SSD) (Fig. 4). DSO is graphical dependence of the restoring moment arm l(θ) or the moment itselfM in (θ) from roll angle θ .

This graph, as a rule, is depicted for a ship’s roll only to the starboard side, since the whole picture when a ship rolls to the left side for a symmetrical ship differs only in the sign of the moment M in (θ).

The importance of DSO in the theory of stability is very great: it is not only a graphical dependence M in(θ); The DSO contains comprehensive information about the state of the vessel's loading from the point of view of stability. The ship's DSO allows you to solve many practical problems on a given voyage and is a reporting document for the ability to begin loading the ship and sending it on a voyage.

The following properties can be noted as DSO:

• The DSO of a particular vessel depends only on the relative position of the vessel’s center of gravity G and the initial transverse metacenter m(or metacentric height value h 0) and displacement R(or draft d avg) and takes into account the availability of liquid cargo and supplies using special adjustments,
• the hull shape of a particular vessel is evident in the DSO over the shoulder l (θ), rigidly connected to the shape of the body contours , which reflects the displacement of the center of the quantity WITH towards the side entering the water when the vessel is heeling.
• metacentric height h 0, calculated taking into account the influence of liquid cargo and reserves (see below), appears on the DSO as the tangent of the tangent to the DSO at the point θ = 0, i.e.:

To confirm the correctness of the construction of the DSO, a construction is made on it: the angle is set aside θ = 1 rad (57.3 0) and construct a triangle with a hypotenuse tangent to the DSO at θ = 0, and horizontal leg θ = 57.3 0. The vertical (opposite) leg should be equal to the metacentric height h 0 on axis scale l(m).

• no actions can change the type of DSO, except for changing the values ​​of the initial parameters h 0 And R, since the DSO reflects, in a sense, the unchanged shape of the ship’s hull through the value l (θ);
• metacentric height h 0 actually determines the type and extent of the DSO.

Roll angle θ = θ 3, at which the DSO graph intersects the x-axis is called the sunset angle of the DSO. Sunset angle θ 3 determines only the value of the roll angle at which the weight force and the buoyancy force will act along one straight line and l(θ 3) = 0. Judge the capsizing of the vessel during a roll

θ = θ 3 will not be correct, since the capsizing of the vessel begins much earlier - soon after overcoming the maximum DSO point. Maximum point of DSO ( l = l m (θ m)) indicates only the maximum distance between the weight force and the supporting force. However, the maximum leverage l m and maximum angle θm are important quantities in stability control and are subject to verification for compliance with relevant standards.

DSO allows you to solve many problems of ship statics, for example, determining the static angle of roll of a ship under the influence of a constant (independent of the ship’s roll) heeling moment M cr= const. This heel angle can be determined from the condition that the heeling and righting moments are equal M in (θ) = M cr. In practice, this problem is solved as the task of finding the abscissa of the intersection point of the graphs of both moments.

The static stability diagram reflects the ship's ability to generate a righting moment when the ship is tilted. Its appearance has a strictly specific character, corresponding to the loading parameters of the vessel only on a given voyage ( R = Ri , h 0 = h 0 i). The navigator, who is involved in planning the loading voyage and stability calculations on the ship, is obliged to build a specific DSO for two states of the ship on the upcoming voyage: with the original location of the cargo unchanged and at 100% and 10% of the ship's stores.

In order to be able to construct static stability diagrams for various combinations of displacement and metacentric height, he uses auxiliary graphic materials available in the ship's documentation for the design of this vessel, for example, pantokarens, or a universal static stability diagram.

Pantocares are supplied to the ship by the designer as part of information on stability and strength for the captain. are universal graphs for a given vessel, reflecting the shape of its hull in terms of stability.

Pantokarens (Fig. 6) are depicted in the form of a series of graphs (at different heel angles (θ = 10,20,30,….70˚)) depending on the weight of the vessel (or its draft) of some part of the static stability arm, called the stability arm forms – lf(R, θ ).

The shape arm is the distance by which the buoyancy force will move relative to the original center of magnitude C o when the ship rolls (Fig. 7). It is clear that this displacement of the center of magnitude is associated only with the shape of the body and does not depend on the position of the center of gravity in height. A set of shape arm values ​​at different heel angles (for a specific vessel weight P=Pi) are removed from the pantocaren graphs (Fig. 6).

To determine the stability arms l(θ) and construct a static stability diagram for the upcoming voyage, it is necessary to supplement the form arms with weight arms l in, which are easy to calculate:

Then the ordinates of the future DSO are obtained by the expression:

Having performed calculations for two load states ( R zap.= 100% and 10%), two DSOs are constructed on a blank form, characterizing the stability of the vessel on this voyage. It remains to check the stability parameters for their compliance with national or international standards for the stability of sea vessels.

There is a second way to construct a DSO, using the universal DSO of a given vessel (depending on the availability of specific auxiliary materials on the ship).

Universal DSO(Fig. 6a) combines the transformed pantocarenes to determine lf and weight shoulder charts lV(θ). To simplify the appearance of graphical dependencies lV(θ) (see formula (6)) it was necessary to change the variable q = sin θ , resulting in sinusoidal curves lV(θ) transformed into straight lines lV (q(θ)). But in order to do this, it was necessary to adopt an uneven (sinusoidal) scale along the abscissa axis θ .

On the universal DSO, presented by the ship designer, there are both types of graphical dependencies - l f (P,θ) And l in (z G ,θ). Due to the change in the x-axis, the graphs of the shoulder shape l f no longer resemble pantocarenes, although they contain the same amount of information about the shape of the body as pantocarenes.

To use the universal DSO, you need to use a meter to remove the vertical distance between the curve from the diagram l f (θ, P *) and curve l in (θ, z G *) for several values ​​of the ship's roll angle θ = 10, 20, 30, 40 ... 70 0, which will correspond to the application of formula (6a). And then, on a blank DSO form, line up these values ​​as the ordinates of the future DSO and connect the points with a smooth line (the axis of roll angles on the DSO is now taken with a uniform scale).

In both cases, both when using pantocaren and when using a universal DSO, the final DSO should be the same.

On the universal DSO there is sometimes an auxiliary axis of metacentric height (on the right), which facilitates the construction of a specific straight line with the value z G * : corresponding to a certain value of the metacentric height h 0 * , because the

Let us now turn to the method of determining the coordinates of the vessel’s center of gravity X G And Z G. In the information on the stability of the vessel you can always find the coordinates of the center of gravity of an empty vessel, the abscissa x G 0 and ordinate z G 0.

The product of the vessel's weight and the corresponding coordinates of the center of gravity is called the static moments of the vessel's displacement relative to the midsection plane ( M x) and the main plane ( Mz); for an empty ship we have:

For a loaded ship, these values ​​can be calculated by summing the corresponding static moments for all cargo, stores in tanks, ballast in ballast tanks and an empty ship:

For static moment MZ it is necessary to add a special positive amendment taking into account the dangerous influence of free surfaces of liquid cargo, stores and ballast, available in the tables of the ship’s tanks, ∆MZh:

This correction artificially increases the value of the static moment so that worse values ​​of the metacentric height are obtained, thereby the calculation is carried out with a margin in the safe direction.

Having now divided the static moments M X And M Z correct by the total weight of the vessel on a given voyage, we obtain the coordinates of the vessel’s center of gravity along the length ( X G) and corrected ( Z G correct), which is then used to calculate the corrected metacentric height h 0 correct:

and then - to build the DSO. The value Z mo (d) is taken from the curved elements of the theoretical drawing for a specific average settlement.

“...Be careful! - the one-eyed captain squeaked. But it was already too late. Too many amateurs have accumulated on the starboard side of the Vasyukin dreadnought. Having changed the center of gravity, the barge did not hesitate and, in full accordance with the laws of physics, capsized.”

This episode from classical literature can be used as an illustrative example loss of stability from moving the center of gravity due to the accumulation of passengers on one side. Unfortunately, the matter is not always limited to a fun swim: loss of stability often leads to the death of the ship, and often people, sometimes several hundred people at a time (let us remember the very recent tragedy - the death of the motor ship "Bulgaria" ... - editor's note .).

In the history of world shipbuilding, a number of cases have been recorded similar to what happened at the beginning of the century with the American multi-deck river steamer General Slocum. Its designers provided everything for the convenience of passengers, but did not check how the ship would behave if all 700 of its inhabitants climbed to the upper promenade deck at once and simultaneously approached the side to admire the view...

Loss of stability is one of the most common causes of small ship accidents. That is why each of the captains, regardless of what his vessel looks like - a kayak or, say, a displacement boat, each of those who relax on the water, must have an understanding of the “laws of physics,” ignorance of which cost the Vasyukinites dearly. In other words, about the seaworthy quality of the vessel, which shipbuilders call stability.

Stability- this is the ability of a ship to resist the heeling action of external forces and return to an upright position after the cessation of this action. This term appeared in our country in the 18th century, when Russia became a maritime power; in origin and meaning it is a variation of the common word “sustainability”.

We constantly encounter the stability of balance in everyday life. It’s no secret to us that it’s easier to knock over a chair than a sofa; and an empty closet is easier than a book-filled one. When turning a heavy box over an edge, we first apply the greatest effort, then it becomes easier for us, and finally, when a conventional line drawn vertically through the center of gravity of the box passes over the edge, the box turns over on its own, without our participation. Having made sure that a low, wide box is more difficult to turn over than a tall and narrow one, and a heavy one is more difficult than a light one, we can come to the conclusion that the stability of a body on a hard surface is determined by its weight and the horizontal distance from the center of gravity to the edge of the supporting plane - the shoulder lever The greater the weight and leverage, the more stable the body.

This simple law is also valid for a floating vessel, but here the matter is complicated by the fact that instead of a solid surface, water serves as a support for the “overturning” vessel. In principle, as in the case just described, the stability of a ship is determined by its weight and leverage - the relative position of the points of application of two forces.

One of them is weight, i.e. the force of gravity applied at the vessel’s center of gravity (CG) and always directed vertically downward.

The other is the buoyancy force or maintaining force. According to Archimedes' law, for a floating ship this force is equal in magnitude to the force of gravity, but is directed vertically upward. The point of application of the resultant supporting forces is the fulcrum of the vessel! This point is located in the center of the hull volume submerged in water and is called the center of buoyancy or center of magnitude(CV).

When a ship floats freely in an upright position, the center of gravity is always on the same vertical with the center of gravity, and the equal and opposite forces acting on the ship are balanced. But then heeling forces began to act on the ship. It's not necessarily about moving passengers; this could be a gust of wind or, if we are talking about a yacht, just its pressure on the sails, a steep wave, a jerk of the tow rope, centrifugal force in a steep circulation, lifting a bather out of the water over the side, etc., etc.

The action of the moment of this heeling force, i.e. heeling moment, tilts - the ship heels. In this case, the ship’s CG does not change its position, unless, of course, this is the same “Vasyukin” case and there are no loads on the ship that can move towards the tilt. Since the ship continues to float even when heeling, i.e., Archimedes’ law continues to operate, an increase in the immersed volume on the side entering the water corresponds to an equal decrease in the immersed volume on the opposite side leaving the water. Let's not forget: the weight of the vessel does not change due to the heeling moment; therefore, the total value of the immersed volume should remain unchanged!

Because of this redistribution of the underwater volume, the position of the central point changes - it moves away in the direction of the ship’s heeling; as a result, a moment of supporting forces arises, tending to restore the straight position of the vessel and is therefore called restoring moment.

While the ship maintains stability, the righting moment, increasing as the roll increases, becomes equal to the heeling moment and, since it is directed in the opposite direction, completely “paralyzes” its action. This means that if the magnitude of the heeling forces no longer changes, the ship will continue to float with a constant list; if the action of the heeling forces stops and there is no heeling moment, the righting moment will immediately straighten the ship.

Referring to diagram 2, we can assume that the magnitude of the righting moment arising during a roll will be greater, the larger the shoulder - the horizontal distance between the new position of the center of gravity and the unchanged position of the center of gravity; that's why it's called stability shoulder. As long as this shoulder is there, the righting moment is active - the ship retains , but as soon as the shoulder disappears with a further increase in roll - the center of gravity will be on the same vertical with the center of gravity, no further efforts will be required to capsize the ship, it will lose stability - it will capsize.

The further the center of magnitude can go towards the inclination - the larger the stability arm, the more difficult it is to turn the ship over, i.e., the more stable it is. That is why a wide ship will always be noticeably more stable than a narrow one. On a four-oar yawl with a width of 1.6 m, rowers can get up and walk without much risk, but on an academic eight-oar yawl with a width of 0.7 m, it is enough for one rower to press his foot harder or raise the oar a little higher for a threatening list to arise!

It is especially important to have sufficient beam on the smallest boats. Their stability is also significantly influenced by the completeness of the waterline, i.e., an indicator of what proportion of a rectangle, the sides of which are composed of maximum length and width, is occupied by the area of ​​the current waterline. All other things being equal, ships with a larger waterline are always more stable than those with sharp waterlines at the bow and stern.

Stability, especially at small angles of inclination, largely depends on the shape of the hull - on the distribution of volumes of the underwater part of the hull. After all, ultimately, stability is determined not simply by the width of the effective waterline, but by the position of the “fulcrum” - the center of the actually submerged volume.

From the point of view of stability, the least advantageous are semicircular sections, which, due to sailing conditions, are often used for displacement vessels; The hulls of rowing academic boats, as well as relatively narrow and long boats not designed for planing, have a close to semicircular cross-section. The rectangular section has higher initial stability characteristics; This kind of section is made on boats of minimal length - tugs and punt shuttles. If you move the underwater volumes to the sides by reducing the draft (and volume) in the middle part, stability will benefit even more: the hulls of the latest universal small boats, such as the Sportiac and the Dolphin, have a similar shape.

Following the same path, you can further increase stability by cutting the body lengthwise - along the DP - and arranging the narrow halves to some width. This is how we came to the idea of ​​a double-hulled vessel, which is embodied in the designs of both low-speed floating cottages or inflatable rafts, and racing motor or sailing catamarans designed for record speeds.

With increasing angles of inclination, the shape of the surface part of the hull in the area entering the water during a list also becomes increasingly important. A clear example is the lack of stability of a log with a circular cross-section: for any “roll” - rotation around an axis - no additional volume enters the water, the shape of the immersed part and the position of the central point do not change, and a righting moment does not arise.

For the same reason, the once fashionable obstruction of sides on motorboats is also harmful. This is understandable: with increasing heel, the width of the waterline not only does not increase, but sometimes, on the contrary, it decreases! Therefore, on sharp turns, the old Kazankas, which had the sides tilted inward in the already rather narrow aft part, often overturned.

And vice versa: measures that increase stability are the camber of the sides and the attachment of additional buoyancy elements along their upper edges. The explanation is simple: when heeling, volumes enter the water exactly where they are most needed for support - where they provide a large leverage. In principle, a ship with a camber of frames on the surface and a relatively narrow running waterline combines good speed characteristics with high stability. For example, ancient galleys had this hull shape, where, as is known, the power of the “engine” was limited, and the requirements for speed and seaworthiness were quite high. For the same purpose, bundles of dry reeds were tied above the water along the sides of light Cossack “gulls.”

In fact, our sailboat tourists use the same technique, attaching inflatable cylinders to the sides of their kayaks. An even more effective means of increasing the stability of kayaks when sailing are side floats mounted on crossbars. On an even keel they move above the water and do not slow down. When the wind pressure on the sail tilts the trimaran kayak, the leeward float enters the water and serves as an additional support, located very advantageously - far from the DP.

Various side fittings on planing motor vessels - bulges and sponsons - serve a similar purpose: they improve the stability of the boat or motorboat both when stationary and while moving. The same "Kazanka" becomes safer even when operating with the "Vikhrem" thanks to the installation of additional volumes of buoyancy - stern boules, which enter the water when the stern is clearly overloaded or when heeling at rest. When moving straight forward, the lower working surface of the boules is above the running waterline, and during sharp turns that are dangerous for the Kazanka, this surface begins to “work”: the hydrodynamic lift force generated on it during planing prevents an increase in roll during circulation.

Effective waterline length, although to a lesser extent than width, also significantly affects the stability of the smallest ships. Here's a case in point. Once a sectional tourist kayak was tested. In the single-seater, three-section version, the boat turned out to be too “sporty”: those who did not have experience rowing in “academic” boats invariably capsized near the shore. However, it was enough to add another middle section 0.8 m long, and the same boat became a “calm” tourist vessel.

Stability is very closely related to another seaworthy quality of a vessel - unsinkability. Let us emphasize: both of these qualities are largely determined by the actual freeboard. If the freeboard is low, then even at small angles of heel the deck will enter the water, the width of the effective waterline will begin to decrease, and from this moment the stability arm and the righting moment will begin to decrease. Open - deckless boats, after entering the water at the upper edge of the side, immediately flood and capsize (this is exactly how the Vasyukinites, who were not experienced in ship theory, suffered!). It is clear that the higher the freeboard, the greater the permissible heel angle, the critical value of which is called the flood angle.

The most obvious indicator of a dangerous increase in roll and approaching the flooding angle is a decrease in the freeboard height on the side of the boat's roll. Needless to say, the smaller the boat, the more dangerous any list, the more important every centimeter of actual freeboard is! It is absolutely unacceptable to exceed the boat's carrying capacity specified by the manufacturer (overload)! It is dangerous to arrange the loads in such a way that the boat has a list already at the moment of departure from the shore: after all, this immediately reduces the actual height of the side and the margin of stability of your boat!

It is no coincidence that we are talking about the actual freeboard height. The history of “large” shipbuilding knows many cases when intact and undamaged ships lost stability only because, during a list, some open holes in the side accidentally appeared near the surface of the water.

Academician A.P. Krylov tells an interesting story. Before the 84-gun ship "King George" set out on its maiden voyage (this happened in 1782 in Portsmouth), it was specially heeled to correct some kind of malfunction in the kingstons. The edges of the lower row of open gun ports turned out to be at a level only 5-8 cm above the surface of the water. The senior officer, not realizing the dangerous position of the ship, when these 5-8 cm, and not the usual 8 m, was the actual height of the side, ordered the team to be called to the guns to raise the flag. Obviously, the sailors were running along the tilted side and a slight increase in the roll was enough for the ship to fall on board and carry more than 800 people to the bottom...

So, the necessary conditions for the stability of a vessel are its sufficient width and height of the side. Let us now make a clarification. The fact is that stability is usually divided into initial (within the roll angle of up to 10-20°) and stability at greater inclinations. For small ships, what is important, first of all, is the width and characteristics of initial stability: stability at large angles of heel most often “does not come to pass”, since the angle of flooding usually lies within the limits of initial stability. For larger seaworthy and closed - decked vessels, the freeboard height is more important, ensuring stability at large inclinations.

Now let us note one more completely obvious and practically very important condition: the more stable the vessel is, the lower its center of gravity is located. Everyone knows to what the roly-polys and tumblers owe their high “stability”! From our own experience, everyone knows well how any small boat begins to sway when they stand up to their full height and try to walk from one bank to another: with an increase in the height of the CG (shoulder), the magnitude of the heeling moment increases significantly, although the weight of the person itself does not change ...

That is why on the same kayaks, the width of which, as a rule, is at a dangerous minimum limit, you have to sit almost directly on the bottom. Another example. When a mast is placed on the yawls, a force of wind pressure on the sails appears at a certain height; in order to compensate for the significant heeling moment that arises, it is necessary to increase stability in the same way - the entire team transfers from the cans to the bottom.

And the third example. The editors of the collection got acquainted with a rather narrow two-seater boat (see photo), designed for rowing with long oars. The performance of the boat turned out to be excellent, but there was one “but”: while the author of the project was driving the boat to the testing site, he had already capsized! The editors who tried the boat also found themselves in the water. However, it was enough to lower the height of the cans by 150 mm - the situation changed.

Despite the most stringent weight saving regime, those ships whose stability is subject to particularly stringent requirements have to take on “dead weight” - ballast - specifically to lower the central gravity. Typically, cruising yachts and rescue boats carry permanent solid ballast, secured as low as the vessel's design allows. (The lower you can place the ballast, the less of it will be needed to ensure a certain height of the CG of the entire ship!) On such ships they try to place the CG under the CG. Then the maximum value of the stability arm will be achieved at a very large roll - up to 90". For comparison, it is enough to say that most conventional sea boats capsize already at a roll of 60-75°.

Sometimes temporary liquid ballast is used. Thus, on seaworthy motorboats and boats with keeled bottoms, low initial stability when parked (rolling) often has to be compensated by receiving water into special ballast tanks in the bottom, which are emptied automatically during movement.

It is very important that the center of gravity of a heeled vessel remains in its place: it is no coincidence that on sailing boats all heavy objects are securely fastened to prevent them from moving. There are, however, loads that are considered dangerous because they can cause loss of stability. These are all kinds of bulk cargo - from grain and salt to fresh fish, randomly poured in the direction of the ship's tilt. (It was because of the displacement of bulk cargo - grain - that the huge four-masted barque Pamir, the last large cargo sailing ship with a deadweight of 4500 tons, capsized and perished in 1957 during a hurricane!) Liquid cargo is especially dangerous. We will not go into the depths of the theory of the ship, but we will emphasize that in this case it is not so much the weight of the iridescent liquid cargo that reduces stability, but rather its free surface area.

How then, the reader may ask, do tankers carrying this dangerous liquid cargo sail across the seas and oceans? Firstly, the tanker hull is divided by transverse and longitudinal impenetrable bulkheads into separate compartments - tanks, and in their upper part so-called fender bulkheads are placed, which additionally “break up” the free surface (breaking it into 2 parts reduces the harmful effect on stability by 4 times). Secondly, the tanks are completely filled.

For the same reasons, it is better to have two narrower fuel tanks on a boat than one wide one. All spare tanks must be completely filled before a storm passage (as the sailors say - pressed in). Liquids must be consumed one at a time - first from one tank to the end, then from the next, so that the level is free in only one of them.

The terrible enemy of small ships is water in the hold, even if its total weight is small. One day a new work boat went out for testing. At the very first turn, it was noted that during the circulation the boat received an unusually large list and was very “reluctant” to come out of it. We opened the aft hatch and saw that water was flowing in the afterpeak, having gotten there through a barely noticeable crack in the seam.

It is very important to drain the hulls of small ships in a timely manner and take measures to ensure that in fresh weather water does not get inside through various holes and leaks.

We started this conversation about stability with the danger from unorganized passengers. Now that we are armed with some basic theory, let us once again emphasize the need to strictly adhere to the established rules of behavior on board any small vessels. After all, due to an oversight, a passenger who gets on board a light motorboat creates a huge heeling force, amounting to almost 1/5 of the vessel’s displacement! And two passengers who decide to walk simultaneously on board the Progress-4 with the wheelhouse are a real threat of capsizing the ship (two such incidents with tragic outcomes occurred in Kalinin last summer).

When inviting guests onto your “cruiser,” politely but firmly instruct them and familiarize them with the existing safety rules. On the smallest ships you may not be able to stand up to your full height and move from place to place, but people may not know this!

Until now it has been said that the position of the CG should not change. There is, however, a large class of sports vessels for which the full movement of the CG in the direction opposite to the roll is the most important condition for achieving high results. We are talking about heeling light racing dinghies and catamarans, and sometimes cruising and racing yachts. Hanging overboard with the help of a trapeze, the athlete moves the CG with his weight and increases the stability arm, which allows him to reduce the roll, or even avoid capsizing...

Finally, it should be borne in mind that even a vessel that is stable in some conditions may not be stable enough in others. Stability may vary, particularly when parked and while driving. Therefore, we also have to take into account running stability. For example, a displacement boat, which when parked does not even react to a passenger sitting at the side, when sailing on the waves, suddenly begins to heel towards him. It turns out that the boat seems to “hang”, resting its stern and bow on the crests of two adjacent waves, and due to the fact that its entire middle part, the widest, ends up in the wave trough, the already known fullness of the waterline has decreased and stability has immediately decreased .

On planing motorboats, the significant hydrodynamic forces that arise during movement to maintain stability, as a rule, increase. However, they can also cause a capsize: for example, if a turn is too sharp, a change in the direction of the propeller stop and a sharp increase (due to drift) in the pressure at the chine outer to the turn create a dangerous pair of forces, which often turns the boat over the side outer to the turn.

Finally, shipbuilders separately analyze cases of dynamic application of heeling forces (there is also a special concept - dynamic stability): with the sudden and short-term application of large external loads, the behavior of the vessel can be completely different from the classical schemes of static stability. That is why, in stormy conditions, under the unfavorable dynamic impact of a squall and wave shock, seemingly absolutely stable yachts, specially designed for sailing in the harshest ocean conditions, capsize. (The yachts of Chichester, Baranovsky, Lewis and other lone daredevils capsized! The subtlety here is that the shipbuilders also foresaw this: the yachts immediately stood on an even keel and became stable again.)

Of course, engineers are not satisfied with assessments like “this ship is stable, but that ship is not very stable”; shipbuilders characterize stability with exact values, which will be discussed in the next article.

When designing any vessel, be it a supertanker or a rowing boat, designers make special stability calculations, and when the vessel is tested, the first thing they do is check whether the actual stability matches the design. To ensure that the stability of any new vessel during normal, competent operation in the conditions for which it is designed is sufficient, monitoring organizations such as the USSR Register specially issue Stability standards and then monitor their compliance. The designers creating the vessel design carry out all calculations, guided by these stability standards, and check whether the future vessel will capsize under the influence of waves and wind. Naturally, additional requirements are imposed on certain types of vessels. Thus, passenger ships are now checked for cases of accumulation of all passengers at one side, and even when heeling in circulation (in this case, the angle of heel should not exceed the angle at which the deck enters the water and a value of 12°). Towing vessels are tested for the jerk effect of the towing rope, and river tugs are also tested for the static effect of the towing rope.

The results of the calculations, together with instructions to the captain of the ship, are documented in one of the most important ship documents, called “Information on Vessel Stability”.

For small vessels, the River Register also recognizes full-scale tests of the lead vessel, carried out according to a special program. These tests can, in doubtful cases, replace the corresponding calculations.

The small recreational fleet, controlled by navigation and technical inspections, does not yet have sufficiently clear and simple stability standards. The seaworthiness of such vessels is standardized mainly by establishing a minimum freeboard height and a length-to-width ratio (from 2.3 to 1). Depending on the height of the freeboard, HTI (now GIMS) divides small vessels into three classes: the first - with a freeboard of at least 250 mm; the second - at least 350 mm; third - at least 500 mm.

The instructions accompanying small boats produced by industry usually contain basic recommendations for maintaining stability. Every amateur boater is introduced to the safety rules before he is issued a certificate for the right to operate a vessel.

E. A. Morozov, “KiYa”, 1978