Bow trim is the position of the ship when the draft of the bow is greater than the draft of the stern. Bow trim reduces the speed of the vessel. Longitudinal stability and trim Safe trim aft in meters

INTRODUCTION. 2

1. CONCEPT OF LONGITUDINAL STABILITY OF A VESSEL.. 3

2. VESSEL TRIM AND TRIM ANGLE... 6

CONCLUSION. 9

REFERENCES.. 10

INTRODUCTION

Stability is the ability of a floating craft to withstand external forces that cause it to roll or trim and return to a state of equilibrium after the end of the influence of external forces (External influence can be caused by a wave blow, a gust of wind, a change in course, etc.). This is one of the most important seaworthiness qualities of a floating craft.

The stability margin is the degree of protection of a floating craft from capsizing.

Depending on the plane of inclination, a distinction is made between lateral stability during roll and longitudinal stability during trim. In relation to surface vessels, due to the elongated shape of the ship's hull, its longitudinal stability is much higher than transverse stability, therefore, for navigation safety, it is most important to ensure proper lateral stability.

Depending on the magnitude of the inclination, a distinction is made between stability at small angles of inclination (initial stability) and stability at large angles of inclination.

Depending on the character active forces distinguish between static and dynamic stability.

Static stability - considered under the action of static forces, that is, the applied force does not change in magnitude.

Dynamic stability - is considered under the action of changing (i.e. dynamic) forces, for example wind, sea waves, load movement, etc.

The most important factors affecting stability are the location of the center of gravity and the center of magnitude of the vessel (CV).

1. CONCEPT OF LONGITUDINAL STABILITY OF A VESSEL

Stability, which manifests itself during longitudinal inclinations of the ship, i.e., during trim, is called longitudinal.

Despite the fact that the trim angles of the vessel rarely reach 10 degrees, and are usually 2-3 degrees, the longitudinal inclination leads to significant linear trims with a large length of the vessel. So, a ship 150 m long has an inclination angle of 1 degree. corresponds to a linear trim equal to 2.67 m. In this regard, in the practice of operating ships, issues related to trim are more important than issues of longitudinal stability, since in transport vessels with normal ratios of the main dimensions, longitudinal stability is always positive.

When the ship is tilted longitudinally at an angle ψ around the transverse axis of the center of gravity, the water will move from point C to point C1 and the supporting force, the direction of which is normal to the existing waterline, will act at an angle ψ to the original direction. The lines of action of the original and new direction of the support forces intersect at a point.
The point of intersection of the line of action of the supporting forces at an infinitesimal inclination in the longitudinal plane is called longitudinal metacenter M.

The radius of curvature of the movement curve of the central wheel in the longitudinal plane is called longitudinal metacentric radius R, which is determined by the distance from the longitudinal metacenter to the C.V.
The formula for calculating the longitudinal metacentric radius R is similar to the transverse metacentric radius;

where IF is the moment of inertia of the waterline area relative to the transverse axis passing through its center of gravity (point F); V is the volumetric displacement of the vessel.

The longitudinal moment of inertia of the waterline area IF is significantly greater than the transverse moment of inertia IX. Therefore, the longitudinal metacentric radius R is always significantly larger than the transverse radius r. It is tentatively believed that the longitudinal metacentric radius R is approximately equal to the length of the vessel.

The basic principle of stability is that the righting moment is the moment of the pair formed by the force of the weight of the vessel and the supporting force. As can be seen from the figure, as a result of the application of an external moment acting in the DP, called trim moment Mdif, the ship has tilted at a small trim angle ψ. Simultaneously with the appearance of the trim angle, a restoring moment Mψ occurs, acting in the direction opposite to the action of the trim moment.

The longitudinal inclination of the ship will continue until the algebraic sum of both moments becomes equal to zero. Since both moments act in opposite directions, the equilibrium condition can be written as an equality:

Mdif = Mψ.

The restoring moment in this case will be:

Мψ = D" × GK1 (1)

where GK1 is the shoulder of this moment, called shoulder of longitudinal stability.

From the right triangle G M K1 we obtain:

GK1 = MG × sinψ = H × sinψ (2)

The value MG = H included in the last expression determines the elevation of the longitudinal metacenter above the center of gravity of the vessel and is called longitudinal metacentric height.

Substituting expression (2) into formula (1), we obtain:

Мψ = D" × H × sinψ (3)


where the product D" × H is the longitudinal stability coefficient. Bearing in mind that the longitudinal metacentric height H = R - a, formula (3) can be written as:

Мψ = D" × (R - a) × sinψ (4)

where a is the elevation of the ship’s center of gravity above its center of elevation.

Formulas (3), (4) are metacentric formulas for longitudinal stability.

Due to the smallness of the trim angle in the indicated formulas, instead of sin ψ, you can substitute the angle ψ (in radians) and then:

Мψ = D" × H × ψ or Мψ = D" × (R - a) × ψ.

Since the longitudinal metacentric radius R is many times greater than the transverse r, the longitudinal metacentric height H of any ship is many times greater than the transverse h. therefore, if the ship has secured lateral stability, then longitudinal stability is guaranteed.

2. VESSEL TRIM AND TRIM ANGLE

In the practice of calculating the inclination of a ship in the longitudinal plane associated with determining trim, it is customary to use instead of angular trim linear trim, the value of which is determined as the difference between the draft of the vessel at the bow and stern, i.e. d = TN - TC.

The trim is considered positive if the vessel's draft at the bow is greater than at the stern; trim aft aft is considered negative. In most cases, ships sail with trim to the stern.
Let us assume that a ship floating on an even keel along the VL waterline, under the influence of a certain moment, received a trim and its new effective waterline took the position V1L1. From the formula for the restoring moment we have:

ψ = Мψ / (D" × H).

Let us draw a dotted line AB, parallel to VL, through the point of intersection of the stern perpendicular with V1L1. Trim d is determined by leg BE of triangle ABE. From here:

tg ψ ≈ ψ = d / L

Comparing the last two expressions, we get:

d / L = Mψ / (D" × H), hence Mψ = (d / L) × D" × H.

Let us consider methods for determining the draft of a vessel under the influence of a differential moment resulting from the movement of cargo in the longitudinal-horizontal direction.

Let us assume that the load p is moved along the ship to a distance lx. The movement of the load, as already indicated, can be replaced by the application of a couple of forces to the vessel. In our case, this moment will be differentiating and equal: Mdiff = P × lx × cos ψ the equilibrium equation for longitudinal movement of the load (equality of the trimming and restoring moments) has the form:

P × lx × cosψ = ​​D" × H × sinψ

whence tanψ = (P × lx) / (D" × H)

Since small inclinations of the ship occur around an axis passing through the C. T. F of the waterline area, the following expressions can be obtained for the change in draft bow and stern:

Consequently, the drafts bow and stern when moving cargo along the ship will be:

If we take into account that tanψ = d/L and that D" × H × sinψ = Mψ, we can write:

where T is the draft of the vessel when positioned on an even keel;

M1cm is the moment that trims the ship by 1 cm.

The value of the abscissa XF is found from the “curves of the elements of the theoretical drawing”, and it is necessary to strictly take into account the sign in front of XF: when point F is located forward of the midsection, the value of XF is considered positive, and when point F is located aft of the midsection - negative.

Leverage lx is also considered positive if the load is transferred towards the bow of the vessel; when transferring the load to the stern, the lx arm is considered negative.

CONCLUSION

Stability is one of the most important seaworthiness qualities of a floating craft. In relation to ships, the clarifying characteristic of the stability of the vessel is used. The stability margin is the degree of protection of a floating craft from capsizing.

External impact can be caused by a wave strike, a gust of wind, a change in course, etc.

In the practice of calculating the inclination of a ship in the longitudinal plane, associated with determining the trim, it is customary to use a linear trim instead of an angular trim.

BIBLIOGRAPHY

1. I., A., S. Control of landing, stability and stresses of the ship’s hull: Textbook. manual - Vladivostok, Moscow State University. adm. G.I. Nevelskoy, 2003. - 136 p.

2. N. Operational calculations of the seaworthiness of a vessel - M.: Transport, 1990, 142 p.

3. K., S. General structure of ships. - Leningrad: "Shipbuilding". - 1987. - 160 p.

4. G. Theory and structure of the vessel. - Textbook for river schools and technical schools. M.: Transport, 1992. - 248 p.

5. G. Vessel structure: Textbook. - 5th ed., stereotype: - L.: Shipbuilding, 1989. - 344 p.

When a submarine floats, the equality between its weight and the supporting force (buoyancy) is gradually violated. The weight of the bow and stern relative to each other also changes, which leads to the appearance of trim.

The supporting force is equal to the product of the density of water and the submerged waterproof volume of the submarine's pressure hull. The density of sea water depends on salinity, temperature and pressure. The volume of the pressure hull also changes and depends on the depth of immersion and the temperature of the sea water, the weight of the submarine depends on the consumption of variable cargo: fuel, oil, ammunition, fresh water, provisions, etc. Most of these cargo are replaced by sea water, including fuel.

The difference in the densities of fuel and water leads to an imbalance. As a result, the equality between the weight of the submarine and the supporting force is violated, which leads to the emergence of so-called residual buoyancy. If the supporting force is greater than the weight of the submarine, then the residual buoyancy will be positive; if less, it will be negative. With positive residual buoyancy, the submarine tends to float, with negative residual buoyancy, it tends to sink.

Uneven consumption of variable loads in the bow and stern parts of the boat leads to the formation of trims.

Bringing residual buoyancy and trim to specified values ​​by receiving (removing) water from overboard into the surge tank and moving water between trim tanks is called trimming.

The above and other reasons make it necessary to periodically trim the submarine.

Trimming can be done without moving or while moving.

Trim without travel

Trimming without stroke is performed:

When the submarine has not dived for a long time;

In areas where it is difficult to maneuver underwater;

At the sign;

For educational purposes.

When the sea state is no more than 3-4 points, trim without running is usually performed at periscope depth, and when the sea state is over 4 points - at safe depths.

The advantage of trim without running is that this method allows you to trim a submarine in an area with shallow depths. Disadvantages include: the need for trim when setting off and ensuring external security in areas that are difficult to maneuver.

It is advisable to trim at periscope depth with a obviously lightweight submarine, for which, before immersing in the surge tank, it is necessary to take in water that is 5-10 tf less than the calculated value (depending on the design of the submarine). The main ballast is received first in the end groups, then in the middle. If, after filling the end groups of the main ballast tanks, the submarine has a trim of more than 0.5°, the trim moment should be extinguished by distilling water from one trim tank to another. After filling the middle group of main ballast tanks, trim begins.

Positive buoyancy, depending on the value, is extinguished by the intake of water from overboard into the equalization tank through the kingston or precise filling valve. To remove air bubbles from the end groups of the main ballast tanks and from the superstructure, the submarine must be “rocked,” that is, the trim must be moved from one end to the other, distilling water between the trim tanks, and then the ventilation valves of these tanks must be closed. With the removal of air bubbles from the tanks of the end groups, the tanks of the middle group are ventilated in the same way. It is recommended to stop distilling water from one trim tank to another when the trim does not reach the specified value by 1.5-2°.

In a submerged position, the nature of the residual buoyancy is judged by the readings of depth gauges. If a submarine sinks, it has negative residual buoyancy. To bring the boat to zero buoyancy, water from the surge tank is pumped overboard. If a submarine floats, it has positive residual buoyancy. To bring it to zero buoyancy, water is taken into the surge tank from overboard. Trimming without progress is considered completed if the submarine maintains a constant depth with a given trim for some time. At the end of the trim, the actual amount of water in the auxiliary ballast tanks is measured and recorded, as well as the personnel available in each compartment and conning tower are checked and recorded.

Trim on the move

Performed in areas that allow the submarine to maneuver freely underwater. In calm sea conditions, trimming can be done at periscope depth, and in rough conditions - at safe depth.

To understand the essence of trim and control of a submarine in an underwater position, you need to know the principle of operation of horizontal rudders and the forces acting on the submarine.

When repositioning the horizontal rudders while moving (Fig. 3.1), hydrodynamic forces of the stern Rк and bow Rн horizontal rudders arise.

Rice. 3.1. Forces arising when shifting horizontal rudders


These forces are proportional to the square of the submarine's speed and the rudder angles. The forces Rк and Rн can be replaced by their components parallel to the GX and GY axes. The forces Rxk and Rxh increase the resistance of water to the movement of the submarine. The forces Ruk and Ryn change the trim and direction of the submarine in vertical plane.

According to the well-known theorem of theoretical mechanics, the forces RyK and RyH can be represented as applied at the center of gravity of the submarine with the simultaneous action of hydrodynamic moments of the horizontal rudders Mk and Mn. Shifting the stern horizontal rudders to dive gives a moment - Mk, which trims the submarine to the bow, and a lifting force +Ruk. repositioning the bow horizontal rudders for ascent gives a moment +Mn, which trims the submarine aft, and a lifting force +Ryn

Shifting the stern horizontal rudders for ascent gives a trimming moment at the stern +Mk and a sinking force _RyK, and shifting the bow horizontal rudders for a dive gives a trimming moment at the stern - Mn and a sinking force -Rk.


Rice. 3.2. Forces acting on a submarine while moving underwater


The joint use of horizontal rudders creates a trim moment and force applied to the center of gravity of the submarine, which are the resultant trim moments and forces created separately by the stern and bow horizontal rudders.

A submarine having a steady speed Vpl in a submerged position is subject to static and dynamic forces (Fig. 3.2). Static forces include the weight force, the supporting force and their moments, which act on the submarine constantly. These forces are usually replaced by the resultant - residual buoyancy Q and its moment Mq. With longitudinal inclinations (trim φ), a restoring moment Mψ occurs, which tends to return the submarine to its original position.

Dynamic forces and moments include thrust force, thrust moment of propellers and hydrodynamic forces and moments. The thrust force of the propellers Tt is proportional to the speed of rotation of the propeller. During steady motion, the thrust force of the propeller is balanced by the drag. The thrust moment of the propellers Mt arises due to the fact that the axes of the shaft line on a submarine usually do not coincide in height with the center of gravity and are located below it. Therefore, the moment of thrust force of the propellers trims the submarine to the stern.

Hydrodynamic forces arise when a submarine moves. For practical trim, it can be assumed that at a constant depth the resultant of the hydrodynamic forces Rm acting on the hull is proportional to the speed and trim angle. Point K, applied to the resultant Rm, is called the center of pressure. The center of pressure does not coincide with the submarine's center of gravity and is usually located forward of it.

Based on the theorem of theoretical mechanics mentioned above, the effect on the submarine of the resultant hydrodynamic forces can be represented as a force Rm applied to the center of gravity G of the submarine and a moment MR. The force Rm can be broken down into its components. The component Rmх (drag) characterizes the resistance of water to the movement of a submarine. The Rm component plays an important role in the controllability of a submarine in the vertical plane. At a constant diving depth with a trim near zero or at the stern, the lifting force Rmu, and the moment MR trims the submarine to the stern; with a trim to the bow, the force Rtu is sinking, and the moment MR trims the submarine to the bow.

The basis for trim while moving is the movement of the submarine at a constant depth and on a straight course, as this makes it possible to determine the direction of forces and moments. Determining the direction of forces and moments in practice is facilitated by knowledge of the following characteristic positions of an undifferentiated submarine sailing at a constant depth, depending on the angles of the horizontal rudders and trim:

Trim 0° - the stern horizontal rudders are shifted to float;

Trim 0° - the stern horizontal rudders are shifted to submersion;

The trim is on the bow - the stern horizontal rudders are shifted to dive;

The trim is on the bow - the stern horizontal rudders are shifted to float;

Trim to the stern - the stern horizontal rudders are shifted to float;

Trim to the stern - the stern horizontal rudders are shifted to submersion.

Examples of trim on the move

Example 1. The submarine on a direct course moves at low speed, maintains a constant depth with a trim of 0°.


Rice. 3.3. The submarine has a heavy bow


The stern horizontal rudders are shifted to float 12°, the bow rudders are at zero. It is possible to differentiate the submarine (Fig. 6.6).

The stern horizontal rudders create a trimming moment at the stern +MK and a sinking force - RyK. The +MK moment seeks to create a trim to the stern, but the submarine has zero trim. It follows from this that there is some moment that counteracts the +MK moment to create trim aft. Such a moment may arise due to the fact that the bow of the submarine is heavier than the stern or, which is the same thing, the stern is light, i.e. the submarine has an excess trimming moment on the bow - Mid. To trim a submarine by moment, you should move water from the bow trim tank to the stern tank and at the same time move the stern horizontal rudders to zero.

It is impossible to determine in practice the nature of the residual buoyancy in this case, since the direction of the force Q, the resultant of the forces of weight and buoyancy, is unknown. Since the submarine maintains a given depth, the residual buoyancy can be:

Zero when the forces Rmy and Ryк are equal in magnitude;

Negative if Rmу > Rvк;

Positive if Rmu
Residual buoyancy in this case can only be revealed later in the process of differentiating the submarine according to new instrument readings.

Example 2. The submarine on a direct course moves at low speed, maintains a constant depth with a trim of 5° on the bow. The stern horizontal rudders are shifted to float 12° to the bow, the bow rudders are in the plane of the frame (at zero). It is necessary to trim the submarine (Fig. 3.4).

The stern horizontal rudders create a trimming moment at the stern + MK and a sinking force - RyK. The trim to the bow creates a sinking force - Rm, and a moment -MR, which trims the submarine to the bow. The submarine maintains a constant depth, but under the influence of sinking forces it must sink, therefore, there is a force that prevents it from sinking. In this case, such a force can only be residual positive buoyancy, i.e. the submarine is light. The +MK moment, as in example 1, is prevented from creating a trim at the stern by the excess trim moment at the bow - Mid, i.e. the submarine has a heavy bow.

With this characteristic position of an undifferentiated submarine, it is necessary to first move water from the bow to the stern, while moving the stern horizontal rudders to submerge to keep the submarine at a constant depth, and then take water from overboard into the surge tank for trimming by buoyancy.


Rice. 3.4. The submarine is light, the bow is heavy


An attempt to trim the submarine first by buoyancy and then level the trim may lead to the fact that it will not be possible to maintain it at a given depth. In fact, with the start of receiving water from overboard, the submarine will begin to sink due to an increase in its weight. To maintain a given depth, you will have to reduce the trim on the bow, i.e., reduce the sinking force -Rm, for which it is necessary to shift the horizontal rudders to ascent. But, since the horizontal rudders are shifted only to a limited angle and already have 12° for ascent, shifting them to the full angle for ascent (up to the limiter) may not reduce the trim on the nose to the required value. Consequently, the submarine will submerge.

In the same way, forces and moments are analyzed and trim is performed on the move in other characteristic positions of an untrimmed submarine.

In practice, trim on the move is performed as follows. After the personnel occupy the places according to the dive schedule, the conning hatch is battened down, the electric motors are given a low speed and the main ballast is received, after which the command is given: “Trim the submarine at a depth of so many meters, at such a speed, with a trim of so many meters.” degrees bow (aft).” The main ballast is received, as during trimming, without stroke. The ventilation of the middle group of main ballast tanks is closed at a depth of 5-7 m. The specified trim depth is maintained by the stroke and trim. When going to depth, significant trim should not be created. The ventilation of the end tanks of the main ballast is closed immediately upon the arrival of the submarine at a given depth (after transferring the trim from bow to stern).

If, after filling the middle group of main ballast tanks, the submarine receives negative buoyancy, you should create a trim to the stern with horizontal rudders and stroke and, holding the boat at a given depth, simultaneously pump out water from the surge tank.

If this turns out to be insufficient, give a bubble to the middle group of tanks or blow it out, pump out the required amount of water from the surge tank and, having removed the bubble from the middle group of tanks, continue trimming. These measures are taken depending on the speed of the submarine's dive.

If the submarine does not submerge, water should be taken into the surge tank through the seacock or precision filling valve. As soon as the depth gauge shows a change in depth, water intake is suspended.

To remove air bubbles from the end tanks of the main ballast and from the superstructure, it is necessary to alternately trim the submarine to the bow and stern (“rock” the submarine), and then close the ventilation valves of the end groups of the main ballast tanks.

In order to correctly and quickly differentiate the submarine by the position of the horizontal rudders and trim, the residual buoyancy and excess trim moment are determined, after which they begin trimming.

If the trimming officer does not have sufficient experience, the following rules must be followed:

1. If the submarine maintains a given depth and its trimming moment from the horizontal rudders coincides with the trim, you should first trim it by buoyancy, and then by trim.

2. If the submarine maintains a given depth, but the trim does not coincide with the trimming moment of the horizontal rudders, you should first trim it by trim, and then by buoyancy.

By draining or receiving water into the equalization tank and pumping auxiliary ballast between the trim tanks, a position is achieved so that the bow horizontal rudders are at zero, and the stern ones are with a slight deviation from the plane of the frame. In this case, the submarine with a slight trim to the bow should maintain depth. In this position it is considered differentiated.

At the end of the trim, the ventilation valves of the main ballast tanks are opened and closed (“slammed”) to bleed the remaining air cushion. Having made sure that at a given speed the submarine maintains a constant depth on a straight course with zero or a given trim, the shift of the stern horizontal rudders does not exceed ±5°, and the bow rudders lie at zero, the command “Trim is complete” is given. The compartment commanders report to the central post about the presence of personnel in the compartments and the amount of water in the auxiliary ballast tanks. This data is recorded in the log and trim logs.

On the stability of a cargo ship when moving big influence loading it has. Steering a boat is much easier when it is not fully loaded. A vessel that has no cargo at all is more easily controlled by the rudder, but since the vessel's propeller is located close to the surface of the water, it has increased yaw.

When accepting cargo, and therefore increasing draft, the vessel becomes less sensitive to the interaction of wind and waves and is more steadily maintained on course. The position of the hull relative to the surface of the water also depends on the load. (i.e. the ship has a list or trim)

The moment of inertia of the ship's mass depends on the distribution of the cargo along the length of the vessel relative to the vertical axis. If most of the cargo is concentrated in the aft holds, the moment of inertia becomes large and the ship becomes less sensitive to the disturbing influences of external forces, i.e. more stable on the course, but at the same time more difficult to follow the course.

Improved agility can be achieved by concentrating the heaviest loads in the middle part of the body, but at the same time deteriorating motion stability.

Placing cargo, especially heavy weights, on top causes the vessel to roll and roll, which negatively affects stability. In particular, the presence of water under the bilge slats has a negative impact on controllability. This water will move from side to side even when the rudder is tilted.

The trim of the vessel worsens the streamlining of the hull, reduces the speed and leads to a displacement of the point of application of the lateral hydrodynamic force on the hull to the bow or stern, depending on the difference in draft. The effect of this displacement is similar to a change in the center plane due to a change in the area of ​​the bow valance or stern deadwood.

Trim to the stern shifts the center of hydrodynamic pressure to the stern, increases stability on the course and reduces agility. On the contrary, bow trim, while improving agility, worsens course stability.

When trimming, the effectiveness of the rudders may worsen or improve. When trimming to the stern, the center of gravity shifts to the stern (Fig. 36, a), the steering moment arm and the moment itself decrease, agility worsens, and motion stability increases. When the trim is on the bow, on the contrary, when the “steering forces” and are equal, the shoulder and moment increase, so agility improves, but course stability becomes worse (Fig. 36, b).

When the ship is trimmed to the bow, the maneuverability of the ship improves, the stability of movement on an oncoming wave increases, and vice versa, strong rumbles of the stern appear on a passing wave. In addition, when the ship is trimmed to the bow, there is a tendency to go into the wind in forward motion and the bow stops falling into the wind in reverse.

When trimming aft, the ship becomes less agile. When moving forward, the ship is stable on course, but in oncoming waves it easily veers off course.

With a strong trim to the stern, the ship tends to fall with its bow into the wind. When going astern, the ship is difficult to control; it constantly strives to bring its stern to the wind, especially when it is directed sideways.

With a slight trim to the stern, the efficiency of the propulsors increases and the speed of most vessels increases. However, further increase in trim leads to a decrease in speed. Bow trim, due to increased water resistance to movement, usually leads to a loss of forward speed.

In navigation practice, trim to the stern is sometimes specially created when towing, when sailing in ice, to reduce the possibility of damage to propellers and rudders, to increase stability when moving in the direction of waves and wind, and in other cases.

Sometimes a ship makes a voyage with some list on one side. The list can be caused by the following reasons: improper placement of cargo, uneven consumption of fuel and water, design flaws, lateral wind pressure, accumulation of passengers on one side, etc.

Fig.36 Effect of trim Fig. 37 Influence of roll

Roll has a different effect on the stability of a single-screw and a twin-screw vessel. When heeling, a single-rotor ship does not go straight, but tends to deviate from course in the direction opposite to the heel. This is explained by the peculiarities of the distribution of water resistance forces to the movement of the vessel.

When a single-screw vessel moves without heeling, two forces and , equal to each other in magnitude and direction, will exert resistance on the cheekbones of both sides (Fig. 37, a). If we decompose these forces into their components, then the forces will be directed perpendicular to the sides of the cheekbones and they will be equal to each other. Consequently, the ship will sail exactly on course.

When the ship rolls, the area “l” of the immersed surface of the chine of the heeled side is greater than the area “p” of the chine of the raised side. Consequently, the chine of a heeled side will experience greater resistance to oncoming water and less resistance will be experienced by the cheekbone of a raised side (Fig. 37, b)

In the second case, the water resistance forces and applied to one and the other cheekbone are parallel to each other, but different in magnitude (Fig. 37, b). When decomposing these forces according to the parallelogram rule into components (so that one of them is parallel and the other is perpendicular to the side), we make sure that the component perpendicular to the side is greater than the corresponding component of the opposite side.

As a result of this, we can conclude that the bow of a single-rotor vessel, when heeling, tilts towards the raised side (opposite to the heel), i.e. in the direction of least water resistance. Therefore, in order to keep a single-rotor vessel on course, the rudder has to be shifted in the direction of the roll. If on a heeled single-rotor vessel the rudder is in the “straight” position, the vessel will circulate in the direction opposite to the heel. Consequently, when making revolutions, the circulation diameter in the direction of roll increases, in the opposite direction it decreases.

In twin-screw ships, yaw is caused by the combined effect of unequal frontal resistance of water to the movement of the hull from the sides of the ship, as well as by the different magnitude of the impact of the turning forces of the left and right engines at the same number of revolutions.

For a vessel without heel, the point of application of water resistance forces to movement is in the center plane, so resistance on both sides has an equal effect on the vessel (see Fig. 37, a). In addition, for a vessel that does not have a roll, the turning moments relative to the center of gravity of the vessel, created by the thrust of the screws and , are practically the same, since the arms of the thrusts are equal, and therefore .

If, for example, the ship has a constant list to port, then the recess of the starboard propeller will decrease and the recess of the propellers on the starboard side will increase. The center of water resistance to movement will shift towards the heeled side and take a position (see Fig. 37, b) on a vertical plane relative to which the thrusters with unequal application arms will act. those. Then< .

Despite the fact that the right propeller, due to its smaller depth, will work less efficiently compared to the left one, however, with an increase in the arm, the total turning moment from the right machine will become significantly greater than from the left one, i.e. Then< .

Under the influence of a greater moment from the right car, the ship will tend to evade towards the left one, i.e. tilted side. On the other hand, an increase in water resistance to the movement of the vessel from the side of the chines will predetermine the desire to tilt the vessel in the direction of higher, i.e. starboard.

These moments are comparable in magnitude to each other. Practice shows that each type of vessel, depending on various factors, tilts in a certain direction when heeling. In addition, it was found that the magnitudes of the evasive moments are very small and can be easily compensated by shifting the rudder 2-3° towards the side opposite to the side of the evasion.

Displacement completeness coefficient. Its increase leads to a decrease in force and a decrease in damping moment, and therefore to an improvement in course stability.

Stern shape. The shape of the stern is characterized by the area of ​​the stern clearance (undercut) of the stern (i.e., the area that complements the stern to a rectangle)

Fig.38. To determine the area of ​​the feed cut:

a) stern with suspended or semi-suspended rudder;

b) stern with a rudder located behind the rudder post

The area is limited by the stern perpendicular, the keel line (baseline) and the contour of the stern (shaded in Fig. 38). As a criterion for cutting the stern, you can use the coefficient:

Where - average draft, m.

The parameter is the coefficient of completeness of the DP area.

Constructive increase in undercut area aft end by 2.5 times can reduce the circulation diameter by 2 times. However, this will sharply deteriorate course stability.

Handlebar area. The increase increases the lateral force of the steering wheel, but at the same time the damping effect of the steering wheel also increases. In practice, it turns out that an increase in the steering wheel area leads to an improvement in turning ability only at large steering angles.

Relative elongation of the steering wheel. An increase, while its area remains unchanged, leads to an increase in the lateral force of the steering wheel, which leads to a slight improvement in agility.

Steering wheel location. If the rudder is located in the screw stream, then the speed of water flowing onto the rudder increases due to the additional flow speed caused by the screw, which provides a significant improvement in agility. This effect is especially noticeable on single-rotor vessels in the acceleration mode, and decreases as the speed approaches the steady-state value.

On twin-screw ships, the rudder located in the DP has relatively low efficiency. If on such vessels two rudder blades are installed behind each propeller, then agility increases sharply.

The influence of the ship's speed on its controllability appears ambiguous. Hydrodynamic forces and moments on the rudder and hull of the vessel are proportional to the square of the oncoming flow velocity, therefore, when the vessel moves at a steady speed, regardless of its absolute value, the ratios between these forces and moments remain constant. Consequently, at different steady-state speeds, the trajectories (at the same rudder angles) retain their shape and dimensions. This circumstance has been repeatedly confirmed by field tests. The longitudinal size of the circulation (extension) significantly depends on the initial speed of movement (when maneuvering at low speed, the run-out is 30% less than the run-out at full speed). Therefore, in order to make a turn in a limited water area in the absence of wind and current, it is advisable to slow down before starting the maneuver and perform the turn at a reduced speed. The smaller the water area in which the vessel circulates, the lower its initial speed should be. But if during the maneuver you change the speed of rotation of the propeller, then the speed of the flow flowing onto the rudder located behind the propeller will change. In this case, the moment created by the steering wheel. will change immediately, and the hydrodynamic moment on the ship’s hull will change slowly as the speed of the ship itself changes, so the previous relationship between these moments will be temporarily disrupted, which will lead to a change in the curvature of the trajectory. As the propeller rotation speed increases, the curvature of the trajectory increases (the radius of curvature decreases), and vice versa. When the ship's speed comes into line with the bow speed of the propeller, the curvature of the trajectory will again become equal to the original value.

All of the above is true for the case of calm weather. If the vessel is exposed to wind of a certain strength, then in this case controllability significantly depends on the speed of the vessel: the lower the speed, the greater the influence of the wind on controllability.

When for some reason it is not possible to allow an increase in speed, but it is necessary to reduce the angular speed of rotation, it is better to quickly reduce the speed of the propulsors. This is more effective than moving the steering gear to the opposite side.

Vessel trim (from Latin differens, genitive case differentis - difference)

tilt of the ship in the longitudinal plane. D. s. characterizes the landing of the vessel and is measured by the difference between its draft (deepening) stern and bow. If the difference is zero, the ship is said to be “sitting on an even keel”; if the difference is positive, the ship is trimmed to the stern; if it is negative, the ship is trimmed to the bow. D. s. affects the maneuverability of the vessel, operating conditions of the propeller, maneuverability in ice, etc. D.s. There are static and running, which occurs at high speeds. D. s. usually regulated by the intake or removal of water ballast.


Big Soviet encyclopedia. - M.: Soviet Encyclopedia. 1969-1978 .

See what “ship trim” is in other dictionaries:

    TRIM of the vessel- Origin: from lat. differens, differentis the difference in the inclination of the vessel in the longitudinal plane (around the transverse axis passing through the center of gravity of the waterline area) ... Marine encyclopedic reference book

    - (Trim difference) the angle of longitudinal inclination of the vessel, causing a difference in drafts of the bow and stern. If the depth of the bow and stern is the same, then the ship sits on an even keel. If the recess of the stern (bow) is larger than the bow (stern), then the ship has... ... Marine dictionary

    - (Latin, from differe to distinguish). The difference in the depth of immersion in water between the stern and bow of a ship. Dictionary of foreign words included in the Russian language. Chudinov A.N., 1910. DIFFERENT lat., from differre, to distinguish. Difference in stern immersion in water... ... Dictionary of foreign words of the Russian language

    - (ship) the inclination of the ship in the longitudinal vertical plane relative to the surface of the sea. It is measured by trim meters in degrees for a submarine or the difference between the recesses of the stern and bow for surface ships. Affects agility... ...Nautical Dictionary

    - (from Latin differens difference) the difference in the draft (deepening) of the vessel bow and stern... Big Encyclopedic Dictionary

    Marine term, the angle of deviation of the ship's hull from the horizontal position in the longitudinal direction, the difference in the draft of the stern and bow of the ship. In aviation, to denote the same angle that defines the orientation aircraft, the term is used ... ... Wikipedia

    A; m. [lat. differens] 1. Special. The difference in the draft of the bow and stern of the ship. 2. Finance. The difference in the price of a product when ordering and receiving it during trading operations. * * * trim (from the Latin differens difference), the difference in the draft (deepening) of the vessel... ... encyclopedic Dictionary

    Trim- DIFFERENT, the difference in the depth (landing) of the vessel bow and stern; if, for example, the stern is deepened by 1 ft. more than the bow, then they say: the ship has a depth of 1 ft at the stern. D. had a special meaning in the sail. fleet, where a good sailing ship d.b. have D. on… … Military encyclopedia

    - [from lat. differens (differentia) difference] of the vessel, the inclination of the vessel in the longitudinal plane. D. determines the landing of the ship and is measured by the difference between the drafts of the stern and bow. If the difference is zero, the ship is said to be sitting on an even keel; if the difference... Big Encyclopedic Polytechnic Dictionary

    Trim of the ship (vessel)- tilt of the ship (vessel) in the longitudinal plane. It is measured using a trim meter as the difference between the draft of the ship and the stern in meters (for submarines in degrees). Occurs when rooms or compartments at the ends of a ship are flooded, unevenly... ... Glossary of military terms

After obtaining the value of the average MMM draft, corrections for trim are calculated.

1st trim correction(correction for the displacement of the center of gravity of the current waterline - Longitudinal Center of Flotation (LCF).

1st Trim Correction (tons) = (Trim*LCF*TPC*100)/LBP

Trim - ship trim

LCF - displacement of the center of gravity of the effective waterline from the midships

TRS - number of tons per centimeter of sediment

LBP - distance between perpendiculars.

The sign of the correction is determined by the rule: the first trim correction is positive if the LCF and the greater of the bow and stern draft are on the same side of the midsection, which can be illustrated by Table 3.3:

Table 3.3. LCF correction signs

Trim LCF nose LCF feed
Stern - +
Nose + -

Note - It is important to remember the principle: when loading (increasing draft) the LCF always moves aft.

2nd trim correction(Nemoto correction, the sign is always positive). It compensates for the error arising from the displacement of the LCF position when the trim changes (18).

2nd Trim Correction (tons) =(50*Trim*Trim*(Dm/Dz))/LBP

(Dm/Dz) - the difference in the moment that changes the ship's trim by 1 cm at two drafts: one 50 cm above the average recorded draft, the other 50 cm below the recorded draft.

If the ship has hydrostatic tables in the IMPERIAL system, the formulas take the following form:

1 st Trim Correction =(Trim*LCF*TPI*12)/LBP

2nd Trim Correction =(6*Trim*Trim*(Dm/Dz))/LBP

Correction for sea water density

Ship hydrostatic tables are compiled for a certain fixed density of sea water - at sea ​​vessels usually by 1.025, on river-sea vessels either by 1.025, or by 1.000, or by both density values ​​at the same time. It happens that tables are compiled for some intermediate density value - for example, 1.020. In this case, it becomes necessary to bring the data selected from the tables for calculation into line with the actual density of sea water. This is done by introducing a correction for the difference between the tabulated and actual densities of water:

Amendment=Displacement table *(Density measured - Density table)/Density table

Without correction, you can immediately obtain the displacement value adjusted to the actual density of sea water:

Displacement fact = Displacement table * Density measured / Density table

Displacement calculation

After calculating the values ​​of the average vessel draft and trim, the following is performed:

Based on the ship's hydrostatic data, the vessel's displacement corresponding to the average MMM draft is determined. If necessary, linear interpolation is used;


The first and second corrections “for trim” to the displacement are calculated;

The displacement is calculated taking into account corrections for trim and corrections for the density of sea water.

Calculation of displacement taking into account the first and second corrections for trim is carried out according to the formula:

D2 = D1 + ?1 + ?2

D1 - displacement from hydrostatic tables corresponding to the average draft, t;

1 - first correction for trim (can be positive or negative), t;

2 - second correction for trim (always positive), t;

D2 - displacement taking into account the first and second corrections for trim, i.e.

The first correction for trim in the metric system is calculated using formula (20):

1 = TRIM × LCF × TPC × 100 / LBP (20)

TRIM - trim, m;

LCF - abscissa value of the center of gravity of the waterline area, m;

TPC is the number of tons by which the displacement changes when the average draft changes by 1 cm, t;

1 - first amendment, ie.

The first correction for trim in the imperial system is calculated using formula (21):

1 = TRIM × LCF × TPI × 12 / LBP (21)

TRIM - trim, ft;

LCF - abscissa value of the center of gravity of the waterline area, ft;

TPI - the number of tons by which the displacement changes when the average draft changes by 1 inch, LT/in;

1 - first amendment (can be positive or negative), LT.

The TRIM and LCF values ​​are taken without taking into account the sign, modulo.

All calculations in the imperial system are performed in imperial units (inches (in), feet (ft), long tons (LT), etc.). The final results are converted to metric units (MT).

The sign of the correction?1 (positive or negative) is determined depending on the location of the LCF relative to the midsection and the trim position (bow or stern) in accordance with Table 4.1

Table 4.1 - Correction signs?1 depending on the position of the LCF relative to the midsection and trim direction

where: T AP - draft at the perpendicular, at the stern;

T FP - draft at the perpendicular, at the bow;

LCF is the abscissa value of the center of gravity of the waterline area.

The second amendment in the metric system is calculated using formula (22):

2 = 50 × TRIM 2 × ?MTC / LBP (22)

TRIM - trim, m;

MTS - the difference between MCT 50 cm above the average draft and MCT 50 cm below the average draft, tm/cm;

LBP is the distance between the bow and stern perpendiculars of the vessel, m;

The second amendment in the imperial system is calculated using formula (23):

2 = 6 × TRIM 2 × ?MTI / LBP (23)

TRIM - trim, ft;

LBP - the distance between the bow and stern perpendiculars of the vessel, ft;

MTI - difference between MTI 6 inches above average draft and MTI 6 inches below average draft, LTm/in;

LBP - the distance between the bow and stern perpendiculars of the vessel, ft.

All calculations in the imperial system are performed in imperial units (inches (in), feet (ft), long tons (LT), etc.). The final results are converted to metric units.

The displacement, taking into account the correction for the density of sea water, is calculated using formula (24):

D = D 2 × g1 / g2 (24)

D 2 - displacement of the vessel taking into account the first and second corrections for trim, t;

g1 - density of sea water, t/m 3;

g2 - tabular density (for which displacement D 2 is indicated in hydrostatic tables), t/m3;

D - displacement taking into account corrections for trim and density of sea water, m.

 

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