Negative trim. 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. What is the displacement and the coefficient of completeness of the vessel

How is the draft and trim of a ship determined?

To determine the draft and trim in the bow and stern parts, recess marks in decimeters in Arabic numerals are applied on both sides. The lower edges of the numbers correspond to the draft they indicate. If the draft by the stern is greater than the draft by the bow, then the ship has a trim to the stern and, conversely, when the draft by the stern is less than the draft by the bow, the ship has a trim to the bow.

When the bow draft is equal to the stern draft, they say: “the ship is on an even keel.” The average draft is the sum of the bow and stern drafts.

What is the displacement and fullness coefficient of the vessel?

The main quantity characterizing the size of a vessel is the volume of water displaced by it, called volumetric displacement. The same amount of water expressed in units of mass is called mass displacement. For the pontoon shown in Fig. 5, the volumetric displacement V will be 10 x 5 x 2 = 100 cubic meters. However, the underwater volume of the vast majority of ships differs significantly from the volume of the parallelepiped (Fig. 6). As a result, the vessel’s displacement is less than the volume of the parallelepiped built on its main dimensions and draft.

Fig.5

To estimate the degree of completeness of the underwater surface, the concept of the overall completeness coefficient g was introduced into the theory of the vessel, showing what proportion of the volume of the specified parallelepiped is the volumetric displacement of the vessel V. Therefore: V = g x L x B x T

Limits for changing the overall completeness coefficient g

To determine the mass displacement, it is enough to multiply the value of V by the value of the specific gravity of water (fresh - 1000 kgcub.m, in the World Ocean - from 1023 to 1028 kgcub.m. The extreme values ​​​​of the displacement of a vessel during its normal operation are displacement when fully loaded and displacement when empty The difference between them is called deadweight. It represents the mass of transported cargo, fuel supplies, lubricating oils, water, provisions, crew and passengers with luggage, i.e.

Net payload is the mass of transported cargo that can be taken on board.

In some cases, concepts such as standard displacement, full, normal and maximum displacement are used.

Standard displacement is the displacement of a completely finished vessel, fully crewed, equipped with all mechanisms and devices and ready for departure. This displacement includes the mass of SPP equipment ready for action, food and fresh water, excluding fuel reserves, lubricants and boiler water.

The total displacement is equal to the standard displacement, with reserves of fuel, lubricants and boiler water in quantities that provide a given cruising range at full and economical speeds.

The normal displacement is equal to the standard displacement, plus reserves of fuel, lubricants and boiler water in the amount of half the reserves provided for the full displacement.

The largest displacement is equal to the standard displacement plus full reserves of fuel, lubricants and boiler water in tanks (tanks) specially equipped for this purpose.

The stability of a cargo ship when moving is greatly influenced by its loading. 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 of movement 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 speed 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 cheekbone of a heeled side will experience greater resistance to oncoming water and less resistance to 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 deepening of the starboard propeller will decrease and the deepening 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 is the average draft, m.

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

A constructive increase in the undercut area of ​​the 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 the 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.

Bank And trim can be formed as a result of the movement of people, cargo, pitching, turns. The appearance of running trim small vessels bow or stern occurs as a result of an incorrect position (angle) of the outboard motor on the transom of the boat. The angles of heel and trim can reach dangerously critical angles, especially if there is water in the ship’s hull and its overflow. Pouring water towards the slightest inclination of the vessel contributes to the formation of an even greater list or trim and can cause the vessel to capsize. There should be no water in the housing.

When heeling, the resistance on the side of the heeled side is greater and the ship tends to evade in the opposite direction, that is, less resistance. Therefore, in order to keep the ship on course, you have to shift the rudder towards the heeled side, which increases the drag force and accordingly reduces the speed.

During sharp turns of displacement vessels, the roll is especially large and directed outward. People on board, during a sudden maneuver, can move towards the list and thereby further aggravate the position of the ship. There may be a real danger of capsizing. The navigator needs to know the relationship between the speed of his vessel and the maximum possible, from a safety point of view, rudder angle. Before maneuvering, you need to make sure that people are in their places and there are no prerequisites for moving them and cargo.

Planing ships, due to the shape of the hull contours, heel to the inside of the turn. This is safer because the inertial force is directed in the opposite direction of the turn and tends to reduce the roll. It should be remembered that people in the cockpit, especially when standing, may fall or fall overboard. It is necessary to avoid sharp turns, and if necessary, be sure to warn people on board.

For a small displacement vessel, a stern trim of no more than 5 cm or the “Even Keel” position is considered normal. When the stern trim is more than 5 cm, the speed decreases, since a significant immersion of the stern increases the entrained mass of water and the drag of the vessel. Trim to the stern causes increased stability of the vessel on course. If it is necessary to change the direction of movement, it reacts poorly to shifting the steering wheel and tends to fall into the wind.

When trimming to the bow, water resistance also increases and speed decreases. Bow trim worsens the ship's stability on course and causes increased sensitivity to rudder shifts. At the slightest shift, the ship begins to deviate from the straight course and becomes difficult to control on straight sections of the route. These phenomena are explained by the fact that, in the presence of trim, the hydrodynamic effect on the ship's hull along its length differs significantly from normal operating conditions.

When trimming to the bow, the stern of the ship, which has less resistance from the surrounding water, becomes more mobile and overly sensitive to the shifting of the rudders, and when trimming to the stern - vice versa.

On planing vessels, the stern trim makes it difficult to get on plane. The vessel may not get over the resistance hump. When planing, the phenomenon of “dolphining”, periodic vertical movements of the bow, is possible.

This phenomenon can be easily stopped by moving part of the weight to the nose. If it is difficult to plan a vessel with an overloaded stern, even temporarily moving part of the cargo to the bow is sufficient. When trimming to the bow of a planing vessel, the stem almost does not rise above the water. This increases the wetted surface of the vessel, hence the speed decreases. In addition, on a course at an angle to the wave, a sharp yaw of the vessel is possible. This occurs as a result of the fact that if there is a large part of the wave on the port side when entering a wave, then the ship will yaw to the right and vice versa.

It should be remembered that when towing the towed vessel, bow trim is not allowed. In this case, the ship will constantly yaw, and when it returns to its original course, it may capsize. At the same time, trim to the stern allows the vessel to go strictly in the wake of the towing vehicle.

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

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 corrected for 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 performed using 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 - 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.

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.

Great 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, the term is used to denote the same angle that specifies the orientation of the aircraft... ... 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)- the 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