Laboratory work 2.1 (2 hours) is being conducted on this topic
When the ship is moving in a straight course and the rudder is in the center plane, in the absence of wind and current, the thrust force of the propellers is balanced by the forces of water resistance to the movement of the ship's hull. The rudder and hull symmetrically flow around the oncoming jets of water and there are no forces deflecting the vessel. When shifting the steering wheel to a certain angle α , on the side facing the flow around, an increased pressure occurs on the steering wheel, and reduced pressure on the opposite side of the steering wheel. The pressure difference on the sides of the rudder creates a force R, pressing on the rudder blade and depending on the speed of water flowing onto the rudder blade, the angle of the shift, the shape and area of the rudder blade. After the rudder shift, the ship continues to move straight ahead due to inertia, and then turns in the direction of the rudder shift. Consider the action of the force R on the ship at the first moment after the rudder is shifted.
Let's decompose the force R according to the parallelogram rule into two components of force: RU- perpendicular to the ship's DP steering force, and Rx - directed by DP braking force. We apply two equal and oppositely directed forces to the CG of the ship R 1 and R 2, parallel and equal to the force RU. Forces RU and R 2 form a pair of forces, and their turning moment Mr called steering momentMr = Ru 0.5L where 0.5L is the arm of the pair of forces Ru and P 2 . Force RU when moving in a straight course is determined by the formula:
Ru \u003d k 1 k 2 s p 0.5pSp (k υ υ) 2 (α + β s) where:
k 1 - coefficient, taking into account the increase in steering force from the setting of steering washers (1.15-1.2);
k 2 - coefficient, taking into account the influence of the proximity of the rudder blade to the ship's hull (1.05-1.3 with a smaller gap, a larger coefficient);
with p - angular coefficient. 5.15/1+(2S p / h p 2) where h p is the height of the rudder, m;
ρ is the mass density of water (for fresh water 102 kgf s 2 / m 4);
Sp - area of the rudder blade, m 2;
k υ - coefficient. taking into account the change in the speed of water flow on the rudder blade, from the action of the propeller and the ship's hull (1.1-1.55, more when pushing, less for single ships);
υ is the speed of water flow onto the rudder blade, m/s;
α – rudder angle, deg;
β c is the angle of the sloping water flow behind the stern, caused by the contours of the vessel. (for one and three screw ships β c = 2-4 0 , for twin-screw ships with 2 rudders β c =0 0).
It can be seen from the figure that when the rudder is shifted onto the ship, they begin to act: turning moment Mr , directed towards the deflection of the rudder blade; force RU , displacing the ship in the direction opposite to the turn and the force Rx increasing resistance to movement. An increase in resistance when shifting the rudder reduces the speed of the ship (when moving in a straight line and keeping the ship on course with 5 degree rudder shifts, up to 2% of the speed is lost), so the rudder shift should not exceed 1 0 .
Displacement and drift of the vessel in the direction opposite to the turn of the rudder reaches its greatest value in the aft part of the vessel, which should be taken into account when making turns and revolutions near hazards.
After overcoming the forces of inertia, the vessel begins to move along a curved trajectory - circulation. At this time, the ship, like any physical body moving along a curve, is affected by centrifugal force With , directed in the direction opposite to the turn. It is applied to the ship's center of gravity and is proportional to its mass. m , square of speed υ s of translational motion and is inversely proportional to the radius of curvature of the trajectory of motion r . C \u003d mυ with 2 / r .
This strength with a shoulder h (distance between the center of gravity and the ship's center of magnitude) creates a heeling moment Мcr = Сh, causing the ship to roll in the direction opposite to the ship’s turn, which should also be taken into account when performing a sharp turn and turn (reduce speed and rudder angle). Circulation, its periods and elements, see above.After the propellers stop, the water pressure on the rudder sharply decreases. With a decrease in speed, the vessel obeys the helm worse and may lose control. When the propeller is operated “backward”, a reduced pressure is created on the side of the rudder facing the propeller, therefore, when the rudder is shifted “to the right”, the bow of the vessel deviates to the left and vice versa, i.e. the ship's stern leans towards the rudder.
The pressure force on the rudder blade when reversing at the first moment is determined by the formula: Ru \u003d s y 0.5S ρ υ 2
, consider the action of the force R
on the ship in reverse. Rudder deflection causes a turning moment from a pair of forces R and R 1, an increase in water resistance to the movement of the ship's hull and a decrease in speed from the action of force R x, and the ship's drift towards the rudder. When circulating in reverse, under the action of the steering force, the water pressure on the aft part of the side increases (force R 1 y), towards which the steering wheel is shifted. This force creates a turning moment opposite to the turning moment of the rudder and the total turning moment at the initial moment of circulation in reverse gear is equal to the difference between the steering moments and the resistance of the water to the ship's hull. Therefore, even with equal pressure on the steering wheel, agility in forward gear is better than in reverse. However, some time after the start of the turn, the angular velocity begins to increase and the hydrodynamic forces from the outer side become greater than the dynamic force R 1 y caused by rudder shift. At this time, the turning moment of the vessel is the sum of the rudder moment and the positional moment, which causes an increase in the rate of turn. The magnitude of the positional moment is close to the magnitude of the rudder moment, so shifting the rudder in the opposite direction may not give the desired effect and will not take the vessel out of circulation. Considering this phenomenon, high turning and driving speeds should not be allowed when reversing.
To take the vessel out of circulation, reverse "forward" and steer the vessel in forward gear.
§ 24. Forces acting on the hull of a floating ship
The hull of a vessel floating on water is subject to permanent and temporary forces. Constants include static forces, such as the weight of the vessel and the pressure of water on the submerged part of the hull - the supporting forces. The forces that appear when the ship rolls on a rough water surface should be attributed to the temporary ones: the inertia forces of the ship's masses and the forces of water resistance.Forces acting on a ship floating on still water, despite the equality of their resultants, are unevenly distributed along the length of the hull. Support forces, as is known, are distributed along the length corresponding to the volume of the hull submerged in water and are characterized by the shape of the drill along the frames. The forces of weight are distributed along the length of the hull depending on the location of its elements, such as bulkheads, superstructures, masts, mechanisms, installations, loads, etc. In fact, it turns out that in one section along the length of the hull, the weight forces prevail over the supporting forces and vice versa on the other.
Rice. 39. Bending of the ship's hull caused by uneven distribution of forces acting on it. 1 - weight force curve; 2 - curve of forces of maintenance.
From the disproportionate distribution along the length of the body of the forces of weight and support forces arises overall buckling ship hull (Fig. 39).
When a ship is navigating on a rough surface, support forces act on its hull, constantly changing its value in certain sections of the ship's length. These forces reach their maximum value when the ship is heading perpendicular to the direction of the wave, the length of which is equal to the length of the ship. When passing the top of the wave near the midsection, in the middle part of the hull, excess support forces are formed with a lack of them at the extremities. From the uneven distribution of support forces in this case, it turns out hull bend(Fig. 40, a). After a short period of time, the vessel moves to the bottom of the wave, while the excess support forces move to the ends, which causes hull deflection(Fig. 40, b).
Due to the rolling of the vessel that has arisen in waves, inertia forces act on the hull, which have an additional effect on it, and while sailing at high speed against a large oncoming wave, when the bottom part of the bow end hits the water (slamming phenomenon), additional shock or dynamic loads occur.
INERTIA-BRAKING CHARACTERISTICS OF THE SHIP
Forces and moments acting on the ship.
The system of ship motion equations in
horizontal plane.
Maneuvering characteristics of the vessel.
Requirements for the content of information about
Maneuvering characteristics of the vessel.
General information about inertia-braking
ship properties.
7. Features of reversing various types
Propulsion systems for ships.
Vessel deceleration.
The ship as an object of control.
A transport marine vessel moves on the border of two media: water and air, while experiencing hydrodynamic and aerodynamic effects.
To achieve the specified movement parameters, the vessel must be controlled. In this sense the ship is a controlled system. Each the controlled system consists of three parts: the control object, the control means and the control device (machine or human)
Control – this is such an organization of the process that ensures the achievement of a specific goal corresponding to the management task.
When a ship is sailing on the high seas, management task is in ensuring its transition from one point to another along a rectilinear trajectory, keeping a given course and periodically correcting it after receiving observations. In this case heading is a controlled coordinate, and the process of maintaining its constant value is purpose of management.
The instantaneous value of a series of coordinates determines the state of the ship in this moment. These coordinates are: course, speed, drift angle, lateral offset relative to the general course and etc. They are output coordinates. In contrast, the coordinates, which are causes of controlled movement are called input . This rudder angle and propeller speed . When choosing the values of input coordinates, the control device (autopilot, navigator) is guided by the values of output coordinates. This relationship between effect and cause is called feedback.
The considered control system is closed, because a control device (navigator) operates in it. If the control device ceases to function, then the system becomes open and the behavior of the control object (vessel) will be determined by the state in which the controls are fixed (rudder angle, frequency and direction of rotation of the propeller).
In the discipline "Ship control" the tasks of controlling a ship are studied, the movement of which occurs in the immediate vicinity of obstacles, i.e. at distances comparable to the size of the control object itself, which excludes the possibility of considering it as a point (for example, as in the course "Navigation").
Forces and moments acting on the ship
All forces acting on the ship are usually divided into three groups: driving, external and reactive.
To the driving refers to the forces created by the controls to give the vessel linear and angular motion. These forces include: propeller thrust, rudder side force, forces generated by active control means (ACS), etc.
to externalinclude the forces of wind pressure, sea waves, currents. These forces in most cases interfere with maneuvering.
To jetinclude the forces and moments resulting from the movement of the vessel. Reactive forces depend on the linear and angular velocities of the vessel. By their nature, reactive forces and moments are divided into inertial and non-inertial. Inertial forces and moments are due to the inertia of the vessel and the attached fluid masses. These forces only arise when the presence of accelerations - linear, angular, centripetal. The inertial force is always directed in the direction opposite to the acceleration. With uniform rectilinear motion of the vessel, inertial forces do not arise.
Non-inertial forces and their moments are due to the viscosity of sea water, therefore, they are hydrodynamic forces and moments. When considering controllability problems, a moving coordinate system associated with the ship is used with the origin at its center of gravity. The positive direction of the axes: X - in the nose; Y - towards the starboard side; Z - down. Positive reading of angles is taken clockwise, however, with reservations regarding the angle of shifting, the drift angle and the heading angle of the wind.
For the positive direction of the rudder shift, a shift is taken that causes circulation in a clockwise direction, i.e. shifting to the starboard side (the rudder blade turns counterclockwise).
A positive drift angle is taken to be one at which the water flow runs from the port side and, therefore, creates a positive transverse hydrodynamic force on the ship's hull. Such a drift angle occurs on the right circulation of the ship.
The general case of vessel motion is described by a system of three differential equations: two equations of forces along the longitudinal X and transverse Y axes and an equation of moments around the vertical Z axis.
Vessel strength- the ability of its body not to collapse and not change its shape under the influence of permanent and temporary forces
Forces acting on the hull of a floating vessel
Temporary and permanent forces act on the ship's hull. It is necessary to include the forces arising during the motion of the vessel on the rough surface of the water as temporary forces: the inertia forces of the masses of the vessel and the forces of water resistance. The constants include static forces, the weight of the vessel and the pressure of water on the submerged part of the hull - the supporting forces. The forces acting on a vessel floating on calm water, despite their equal effects, are unevenly distributed along the length of the hull. The support forces are distributed along the length according to the volume of the hull submerged in water and are characterized by the shape of the drill along the frames. The forces of weight are distributed along the length of the hull depending on the location of its elements, such as masts, bulkheads, mechanisms, superstructures, installations, loads, etc. It turns out that in one section along the length of the hull, the support forces prevail over the weight forces, and on the other - vice versa.
Bending of a ship's hull caused by uneven distribution of forces acting on it. 1 - weight force curve; 2 - curve of forces of maintenance.
From the uneven distribution along the length of the hull of the forces of weight and support forces, a general longitudinal bending of the ship's hull arises. These forces reach their maximum value when the ship is heading perpendicular to the direction of the wave, the length of which is equal to the length of the ship. When passing the top of the wave near the midsection, in the middle part of the hull, excess support forces are formed with a lack of them at the extremities.
From the uneven distribution of the forces of support in this case, the kink of the body (a) is obtained. After a short period of time, the ship moves to the bottom of the wave, while the excess support forces move to the extremities, which causes a deflection of the hull (b). Due to the rolling of the vessel that has arisen in waves, inertia forces act on the hull, which have an additional effect on it, and while sailing at high speed against a large oncoming wave, when the bottom part of the bow end hits the water (slamming phenomenon), additional shock or dynamic loads occur.
The concept of ship strength
The strength of the vessel is the ability of its hull not to change its shape and not collapse under the influence of temporary and permanent forces. Distinguish between the general and local strength of the vessel.
The overall longitudinal strength of the ship's hull is its ability to withstand the action of external forces applied along the length.
The overall strength of the ship is provided by a watertight shell, which is the shell and the upper deck, the flooring of other decks, longitudinal bulkheads with their supporting structures and all structural ties that have a length greater than the height of the side.
The local strength of the hull is the ability of its individual structures to withstand additional forces: mainly sea water pressure and concentrated loads.
To ensure the local strength of individual structures, their special local reinforcement is provided.
In addition to strength, ship structures must also be stable, i.e. they must not change their shape under the action of compressive forces (for example, decks should not bulge, bulkheads bend, etc.). To ensure the necessary stability of structures, additional stiffeners or any other reinforcements are installed on them.
The calculation of the overall strength of the vessel is reduced to determining the dimensions of its strong bonds and calculating the internal stresses that arise in them under the action of applied forces. If the resulting stresses do not exceed those allowed for a given material, then the strength of the vessel is ensured; if it is the other way around, then you should increase the size of the bonds and re-calculate the strength. For such a calculation, it is necessary to know the moment of resistance of the cross section in the middle of the length of the ship's hull.
In structural mechanics, the body is taken as a hollow composite beam of complex design. The calculation of such a beam is reduced to the calculation of the moment of resistance of the so-called equivalent beam, which is a conditional composite beam, the individual parts of which have an area and height arrangement similar to the corresponding elements of strong hull bonds involved in ensuring the longitudinal strength of the vessel. Approximately the smallest value of the moment of resistance is determined by the formula
where η is the utilization factor of the cross-sectional area, equal to 0.5-0.55;
F is the cross-sectional area of the longitudinal bonds;
H is the height of the vessel. As is known, internal stresses bvn during beam bending are found by the formula
where M is the largest bending moment along the length of the vessel. The bending moment depends on the displacement and length of the vessel and is expressed by the dependence
where k is a proportionality factor varying from 20 to 40 depending on the type of ship.
All forces acting on the ship are divided into three groups:
driving;
external;
Reactive.
To driving Forces include forces generated by controls: propeller thrust, rudder side force, forces generated by active controls.
To external forces include the forces of wind pressure, sea waves, current pressure.
To reactive forces include the forces resulting from the movement of the vessel under the action of driving and external forces. They are divided into inertial- due to the inertia of the vessel and the attached masses of water and arising only in the presence of accelerations. The direction of action of inertial forces is always opposite to the acting acceleration. Non-inertial forces are due to the viscosity of water and air and are hydrodynamic and aerodynamic forces.
PROPELLER DRIVE AND RESISTANCE TO SHIP MOVEMENT.
In order for the ship to move at a certain speed, it is necessary to apply a driving force to it, overcoming the resistance to movement. The useful power required to overcome the resistance is given by
where R is the resistance force; V is the speed of movement.
The driving force is created by a working screw, which, like any mechanism, wastes part of the energy unproductively.
The ratio of useful power to the consumed power is called the propulsion coefficient of the hull-propulsion complex. The propulsion coefficient characterizes the ship's need for energy needed to maintain a given speed.
The maximum propeller thrust develops in the mooring mode (in the case when the ship is on the mooring lines, and its car was given full forward speed). This force is approximately 10% greater than the propeller thrust at full speed. The propeller thrust force when working in reverse for various ships is approximately 70-80% of the propeller thrust in full speed mode.
PITCHING.
Rolling is called the oscillatory movements that the ship makes near the position of its equilibrium.
The fluctuations are called free(on calm water), if they are committed by the vessel after the cessation of the forces that caused these oscillations (wind squall, jerk of the towline). Due to the presence of resistance forces (air resistance, water friction), free oscillations gradually damp out and stop. The fluctuations are called forced if they are performed under the action of periodic perturbing forces (incoming waves).
Rolling is characterized by the following parameters (Fig. 179):
amplitude θ- the greatest deviation from the equilibrium position;
on a grand scale- the sum of two successive amplitudes;
period T- the time of making two full swings;
acceleration.
Rolling complicates the operation of machines, mechanisms and instruments due to the impact of emerging inertia forces, creates additional loads on the strong bonds of the ship's hull, and has a harmful physical effect on people.
Rice. 179. .Pitching parameters: θ 1 and θ 2 amplitudes; θ 1 + θ 2 span.
Distinguish side, keel and vertical pitching. At rolling vibrations are made around the longitudinal axis passing through the center of gravity of the vessel, with keel- around the transverse. Rolling with a short period and large amplitudes becomes gusty, which is dangerous for mechanisms and is hard to bear by people.
The period of free oscillations of a ship in calm water can be determined by the formula T = c(B/√h, where AT- width of the vessel, m; h- transverse metacentric height, m; with- coefficient equal to 0.78 - 0.81 for cargo ships.
It can be seen from the formula that with an increase in the metacentric height, the pitching period decreases. When designing a vessel, they strive to achieve sufficient stability with moderate rolling smoothness. When sailing in waves, the navigator must know the period of the vessel's own oscillations and the period of the wave (the time between two neighboring crests running on the vessel). If the period of the vessel's natural oscillations is equal to or close to the period of the wave, then a resonance phenomenon occurs, which can lead to the capsizing of the vessel.
When pitching, it is possible either to flood the deck, or when the bow or stern is exposed, they hit the water (slamming). In addition, the accelerations that occur during pitching are much greater than when onboard. This circumstance should be taken into account when choosing mechanisms installed in the bow or stern.
Heave caused by a change in the supporting forces as the wave passes under the vessel. The heave period is equal to the wave period.
To prevent undesirable consequences from the action of rolling, shipbuilders use means that contribute, if not to a complete cessation of rolling, then at least to moderate its scope. This problem is especially acute for passenger ships.
To moderate pitching and flood the deck with water, a number of modern ships make a significant rise in the deck in the bow and stern (sheer), increase the collapse of the bow frames, and design ships with a forecastle and poop. At the same time, water-breaking visors are installed in the bow on the tank.
To moderate the roll, passive uncontrolled or active controlled roll stabilizers are used.
Passive sedatives include keels, which are steel plates installed over 30 - 50% of the length of the vessel in the cheekbone area along the line of water flow (Fig. 180). They are simple in design, reduce the pitching amplitude by 15 - 20%, but provide significant additional water resistance to the movement of the vessel, reducing the speed by 2-3%.
Rice. 181. Onboard passive tanks and the position of the liquid in them when the ship is rolling in resonance with the wave.
These tanks are effective in long-period pitching regimes. In all other cases, they do not moderate, but even increase its amplitude.
AT active tanks(Fig. 182) water is pumped by special pumps. However, the installation of a pump and an automatic device that controls the operation of the pump significantly complicates and increases the cost of the design.
