When operating a displacement vessel, monitoring the running trim is just as important as on a planing vessel.
It is not always possible to arrange a vessel during design and load it when setting sail in such a way as to ensure optimal alignment and optimal trim. As is known, excessive running trim leads to a loss of speed and worsens economic performance.
I encountered this problem when I began testing my displacement boat “Duckling”, converted from a small one (No. 1) lifeboat(length - 4.5 m; width - 1.85 m). As soon as I gave full throttle to the SM-557L engine, the stern trim immediately increased to values clearly exceeding the permissible 5-6°: wave formation increased, but the speed did not increase.
I began to look for a way to reduce the running trim. By analogy with high-speed boats, I decided to use transom plates. I cut out two transom plates of different shapes with variable angles of inclination from bakelized plywood and tested them one by one on the “Duckling”. The very first outputs showed that at small angles of inclination the plates are ineffective, and at large angles the trim is indeed reduced, but at the same time they begin to work as a brake. When sailing on a following wave, strong yaw appears due to the plates; in reverse, the plate blocks the flow of water to the propeller. Be that as it may, but having a power of 13.5 hp. s., it was not possible to reach a speed above 10 km/h either with or without plates. The relative speed - the Froude number along the length - fluctuated somewhere around 0.4.
After unsuccessful trials of transom plates, I decided to try installing a specially profiled ring attachment on the propeller. The nozzle that deflects the jet downwards from the propeller, according to my calculations, was supposed to not only create additional lift on the hull, reducing the running trim, but also at the same time increase the efficiency of the propeller, since the SM-557L engine develops too many revolutions for the possible speed .
The Utenka propeller shaft has an inclination relative to the vertical line of about 8°. The front part of the nozzle - from the nose edge to the plane of the propeller disk - is made coaxially with propeller shaft. In the plane of the propeller disk, the axial line of the nozzle has a kink - it is inclined downward by 8° (here the angle of inclination to the vertical line is already 16°).
As can be seen from the diagram, behind the plane of the screw disk in the upper part of the nozzle, its internal generatrix looks like a straight line. The resulting force P c is decomposed into the thrust force and the lifting force. The thrust force was measured with a dynamometer and turned out to be equal to 200 kgf. The lifting force P p, which directly reduces the running trim, is approximately equal to 57 kgf.
Now about making the nozzle. Trapezoidal slats were cut from polystyrene foam, which were then glued into a cylinder using epoxy glue. Processing was carried out with a sharp knife and rasp and checking the profile using templates. The outside of the finished nozzle was covered with two layers of fiberglass with epoxy glue. The inner surface of the nozzle is covered with epoxy putty, into which flake graphite is rubbed in to reduce friction.
Two aluminum angles are fixed at the top and bottom, tightened with M6 bolts. These bolts and circular slings made of 0 2 mm steel cable securely fasten the nozzle and squares into one piece. The front ends of the squares are attached to the sternpost, the rear ends to the rudder post (ruder post).
The ends of the propeller blades are cut to the inner diameter of the nozzle with an annular gap of 2-3 mm.
I have already successfully completed two navigations with the “Duckling” attachment. During this period the following was established:
- speed increased from 10 to 12 km/h (Froude number approx. 0.5);
- running trim is practically absent;
- even on a steep following wave, the boat obeys the rudder well, and the propeller is almost not exposed;
- The boat moves reliably and satisfactorily obeys the rudder in reverse.
Installing an outboard motor on any displacement boat, be it a fofan, a tuzik or a four-oared yawl, always causes a strong trim to the stern, which increases with increasing speed. In an article about the Pella boat, it was noted that its speed under the Veterok engine (8 hp) is 9.16 km/h when the driver sits on the stern bank, and 11.2 km/h when he sits in the nose. Here is a clear indicator of how the running trim affects speed. But there are other disadvantages of such a landing. It is enough to mentally draw a straight line from the eyes of the helmsman sitting at the stern forward through the top point of the stem to make sure that objects on the water ahead are not visible to him. With such poor visibility along the course, the operation of any vessel is prohibited. Two options can be proposed; lay ballast in the bow of the boat or install an attachment on the propeller.
If factories producing outboard motors master the production of profiled anti-trim nozzles, a lot of gasoline will be saved, and most importantly, the operating conditions of boats will improve and navigation safety will increase; in any case, the risk of collision with floating obstacles will be reduced.
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.
Aft trim shifts the center of hydrodynamic pressure to the stern, increases heading stability 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 has a tendency 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 by the area “l” of the immersed surface of the chine of the heeled side more area“p” cheekbones of a 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.
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 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 heel and trim angles 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.
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)- 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
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 significantly higher than the transverse one, therefore it is most important for safe navigation to ensure proper lateral stability.
Depending on the magnitude of the inclination, stability at small angles of inclination is distinguished ( initial stability) and stability at large inclination angles.
Depending on the nature of the acting forces, static and dynamic stability are distinguished.
Static stability - considered under the action of static forces, that is, the applied force does not change in magnitude.
Dynamic stability - 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 get:
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 vessel is many times greater than the transverse h. therefore, if the vessel has lateral stability, then longitudinal stability is certainly ensured.
2. VESSEL TRIM AND TRIM ANGLE
In the practice of calculating the inclination of a vessel in the longitudinal plane, associated with determining the trim, instead of the angular trim, it is customary to use a linear trim, the value of which is defined as the difference between the draft of the vessel 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 abscissa value 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, it is 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 blow, 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 device 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.
