The concept of the pitching of the ship. Device for reducing the rolling of the vessel in waves Types of anchors

Roll stabilizers are usually called devices that are used to reduce the amplitude of the ship's roll.

The effect of the vibration dampers installed on the ship is that they create a variable stabilizing moment, opposite in sign to the disturbing wave moment. Currently, only rolling dampers are used. It is practically difficult to reduce the amplitude of pitching and heaving with the help of dampers, since dampers have not yet been created that are capable of developing stabilizing moments that are much greater than during rolling.

Roll stabilizers are divided into passive and active. The action of the working bodies of passive dampers is based on the creation of a stabilizing moment due to the oscillatory movements of the vessel during rolling, that is, when using them, there is no need for special energy sources. In active dampers, a variable stabilizing moment is created forcibly by means of special mechanisms controlled by a special regulating device, which, in turn, reacts to the vessel's vibrations. Active dampers are more effective, but they require additional power to work.

Passive pacifiers. The passive roll stabilizers include zygomatic keels and passive stabilizing tanks.

Zygomatic keels are the simplest and most effective means of reducing roll and therefore find the widest application.

Passive sedative tanks can be of two types: closed, not communicating with seawater (type I) and open, communicating with seawater (type II). The tanks are half filled with water (sometimes fuel) and are connected by canals. Passive damping tanks are most effective in resonant pumping. Under certain conditions and modes of irregular waves, such dampers can lead to an increase in the amplitude of the roll. The presence of a free surface of liquid in tanks also adversely affects the stability of the vessel. Due to these reasons, passive tanks are currently practically not used.

Figure: 1
Figure: 2 Composition of the sedimentation tank. 1 - zygomatic keel, 2 - reinforcement, 3 - rolling, 4 - damping resistance of zygomatic keels Figure: 3 Calming tanks. 1 - sedative tanks; 2 - air valve; 3 - connecting air channel; 4 - onboard deep tanks; 5 - overflow channel; b - roll of the vessel; 7 - water in the tank Figure: 4 Marine gyroscope. 1 - moment M of the gyroscope; 2 - heeling moment M; 3 - a pair of forces in the swinging frame bearing; 4 - axis of gyroscope rotation; 5 - precession; 6 - braking torque of the swinging frame bearing; 7 - direction of rotation of the gyroscope (angular velocity); 8 - precession speed

Active sedatives. Active dampers include steering wheels, active damping tanks and gyroscopic dampers - stabilizers.

Figure: five Figure: 6 Active side rudders. 1 - retractable rudders; 2 - overwhelming rudders; 3 - forces acting on the rudders; 4 - direction of the ship's course, 5 - direction of rolling 6 - torque of rudders

Steering rudders are a very effective means of reducing roll and are widely used on transport and especially on passenger ships. They are placed on special drives that provide a change in the angles of attack according to a certain law, their extension from the body and cleaning inside the body.

Practice shows that it is advisable to use on-board rudders at speeds exceeding 10-15 knots. In this case, the side rudders lead to a significant (several times) decrease in the amplitudes of side roll.

Active sedative tanks are usually designed as Type I tanks. To regulate the movement of water, either pumps installed in the water channel or blowers located in the air channel are used.
The pump or blower is controlled using special automation so that it is possible to regulate the water supply from one tank to another and provide the required change in the stabilizing moment. The efficiency of the installation does not depend on the speed of the vessel: the tanks equally moderate the pitching on the move and at rest. Disadvantages of active tanks: the complexity of the design, high cost, the use of complex control equipment, a decrease in the vessel's carrying capacity and the need for additional energy consumption.

The gyroscopic stabilizer is a powerful gyroscope that rotates on an axis in the frame. The gyroscope is placed vertically. The roll of the ship during rolling causes the gyroscope axis to rotate - the so-called gyroscope precession. As a result, a gyroscopic moment arises, which is the stabilizing moment of the damper. Gyroscopic pacifiers can be either passive or active. In a passive damper, precession occurs as a reaction to the rolling of the ship. In active dampers, precession is created forcibly due to the transfer of external energy to an electric motor controlled by an automatic regulator that responds to the ship's rolling mode. Disadvantages: significant weight, high cost, complexity of the device and operation (Fig. 4).

Determination of the metacentric height of the vessel by the rolling period

During operation, the boatmaster often needs to check the values \u200b\u200bof the metacentric height of the vessel under various cases of its loading. Such a need arises, for example, as the reserves of fresh water and fuel are consumed, when the issue of the expediency of receiving ballast is being decided. The inclining experience gives quite reliable results, but it requires a lot of time, certain conditions and special training.

It is much easier to estimate the transverse metacentric height h if the rolling period T θ and the coefficient C are known according to the formula obtained from the captain's formula:

h \u003d 4 C 2 B 2 T θ 2

The period of pitching T θ can be determined by recording the damped free vibrations of the ship with gyroscopic inclinographs or inclinographs equipped with time markers.

In practice, the rolling period T θ can be determined as follows. When the boat is in one of the extreme inclined positions, start the stopwatch. Having counted 10 full oscillations, stop the stopwatch at the moment when the vessel comes to the initial inclined position. The period T θ is determined by dividing the time counted by the stopwatch by 10.

The described approximate method gives satisfactory results in the absence of free surfaces of liquid cargo on the ship, as well as in the case when the correction for their effect is no more than 5% of the metacentric height for a given load.

The result of calculating the metacentric height h also depends on the successful choice of the value of the coefficient C included in the expression for h. For this, it is necessary to take its values \u200b\u200baccording to the known values \u200b\u200bof the coefficient C, for ships of the same type or similar in design. Coefficient C \u003d 0.36 ± 0.43 depending on the type of vessel.

Suggested reading:

Swing refers to the vibrational motion that the vessel makes about its equilibrium position.

Oscillations are called free(on calm water), if they are made by the vessel after the cessation of the action of the forces that caused these vibrations (a squall of wind, a jerk of the towing rope). Due to the presence of resistance forces (air resistance, water friction), free oscillations gradually damp and stop. Oscillations are called forced,if they occur under the influence of periodic disturbing forces (incident waves).

The pitching is characterized by the following parameters (Fig. 8): amplitude θ - the greatest deviation from the equilibrium position; sweep - the sum of two consecutive amplitudes; period T - time to complete two full swing; acceleration.

Swaying complicates the operation of machines, mechanisms and devices due to the effect of the arising inertial forces, creates additional loads on the strong connections of the ship's hull, and has a harmful physical effect on people.

Figure: 8. Pitching parameters: θ 1 and θ 2 amplitudes; θ 1 + θ 2 span.

Distinguish between side, pitching and heaving. When rollingvibrations are performed around the longitudinal axis passing through the center of gravity of the vessel, when keel - around the transverse. The roll at a short period and large amplitudes becomes gusty, which is dangerous for mechanisms and is difficult for people to tolerate.

The period of free vibrations of a vessel in calm water can be determined by the formula T \u003d c (B / √h,where IN - the width of the vessel, m; h - transverse metacentric height, m; from - coefficient equal to 0.78 - 0.81 for cargo ships.

The formula shows that with an increase in the metacentric height, the rolling period decreases. When designing a vessel, strive to achieve sufficient stability with moderate smoothness of rolling. When sailing on rough seas, the boatmaster must know the period of natural oscillations of the vessel and the period of the wave (the time between the running onto the vessel of two adjacent ridges). If the period of natural vibrations of the vessel is equal to or close to the period of the wave, then a resonance phenomenon occurs, which can lead to overturning of the vessel.

When pitching, it is possible to either flood the deck, or when the bow or stern is exposed, they hit the water (slamming). In addition, the acceleration that occurs during pitching is much greater than during rolling. This circumstance must be taken into account when choosing mechanisms installed in the bow or in the stern.

Heaving caused by a change in the supporting forces when a wave passes under the vessel. The period of heave is equal to the period of the wave.

To prevent undesirable consequences from the action of pitching, shipbuilders use means that contribute, if not to a complete cessation of pitching, then at least to moderate its swing. This problem is especially acute for passenger ships.

To moderate the pitching and flooding of the deck with water, a number of modern ships make a significant rise of the deck in the bow and stern (sheerness), increase the camber of the bow frames, design ships with a tank and poop. At the same time, water deflectors are installed in the nose on the tank.

To moderate the roll, passive uncontrolled or active controlled roll dampers are used.

Passive sedatives include zygomatic keels, which are steel plates installed over 30 - 50% of the ship's length in the cheekbone area along the line of water flow (Fig. 9). 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%.

Figure: 9. Scheme of action of the zygomatic (lateral) keels.

Passive tanks - these are tanks installed on the sides of the vessel and connected to each other at the bottom by overflow pipes, at the top - by an air channel with an isolation valve that regulates the transfer of water from side to side. It is possible to adjust the cross-section of the air channel in such a way that the fluid, when rolling, will overflow from side to side with a delay and thus create a heeling moment that counteracts inclination. These tanks are effective in pumping modes with a long period. In all other cases, they do not moderate, but even increase its amplitude.

IN active tanks (fig. 10) water is pumped by special pumps. However, the installation of a pump and an automatic device that controls the operation of the pump greatly complicates and increases the cost of the design.

Figure: 10. Active sedative tanks.

Currently, on passenger and research ships, the most commonly used active side rudders (Fig. 11), which are conventional rudders, installed in the widest part of the vessel slightly above the bilge in an almost horizontal plane. With the help of electro-hydraulic machines, controlled by signals from sensors that react to the direction and speed of the ship's inclination, it is possible to change their angle of attack. So, when the vessel is tilted to the starboard side, the angle of attack is set on the rudders so that the lifting forces that arise in this case create moments opposite to the inclination. The efficiency of the rudders on the move is quite high. In the absence of pitching, the rudders are removed into special niches in the body so as not to create additional resistance. The disadvantages of rudders include their low efficiency at low speeds (below 10-15 knots) and the complexity of the automatic control system.

Figure: 11. Active side rudders: a - general view; b - scheme of action; c - forces acting on the side rudder.

There are no dampers to control pitching.

It should be noted that submarines and vessels with a small waterline area practically do not experience rolling and, therefore, there is no need to equip these vessels with devices for its control.

Swinging is called oscillatory motion about a position

free floating equilibria

water by ship. Distinguish between rolling, pitching and heaving. Roll is called oscillatory motion, passing in the longitudinal axis of the DP. Pitching refers to the vibrational movements made by the vessel around the transverse axis. Rolling is called the oscillatory movements made by the vessel in the vertical plane up and down and caused by a change in the support forces during the passage of the WAVE.

131. Period and amplitude of rolling

amplitude - the greatest deviation from the mean to the extreme position of the swinging body; period - the time of the two full swing;

132. Relationship between pitching and vessel stability

The shorter the period, the faster the pitching, the longer the period, the longer the pitching. Cb 0.6 to 0.8 for medium and large vessels

133. Roll stabilizers.

To prevent unpleasant consequences from the action of pitching, pitching dampers are used on ships, which, by the nature of the action, are divided into passive - uncontrollable, and active - controlled. The zygomatic (lateral) keels are the simplest roll stabilizers used on almost all ships. More significant reduction in rolling amplitudes

can be obtained by installing active side rudders. Calming pitching can, in principle, be achieved by installing controlled horizontal rudders (such as lateral) in

the ends of the ship, but so far such dampers are practically not used.

134. List the elements of the steering device

The rudder consists of a feather and stock. A feather is a flat or, more often, a two-layer streamlined shield with internal reinforcing ribs, an area

which have sea \u200b\u200bvessels is 1 / 40-1 / 60 of the submerged area

parts of the DP. Ball is a pivot with which

turn the rudder feather.

135. Types of rudders

Depending on the position of the steering wheel relative to the axis

rotations distinguish between ordinary rudders, which

the feather is completely located aft from the axis of rotation; balance rudders, in which the feather is divided by the axis of rotation into two unequal parts: the large one is aft from the axis, the smaller one is in the bow; semi-balanced rudders differ from balance ones in that the balance part is not made along the entire height of the rudder.

136. List the elements of the anchor device

The anchoring device is used to provide secure anchorage

in the sea, in the roadstead and in other places remote from the coast, by

fastening to the ground with an anchor and an anchor chain. It includes: anchors, anchor chains (ropes), anchor machines, anchor haws and stoppers.

137. Types of anchors. Anchor chains

Anchors, depending on their purpose, are divided into stationary, designed to hold the vessel in a given place, and auxiliary - to keep the vessel in a given position while mooring at the main anchor. The auxiliary includes

aft anchor - stop anchor, the mass of which is 1/3 of the mass. The anchor chain is used to attach the anchor to the ship's hull. It consists of links (Fig. 7.11), forming bows 25-27 m long, connected to one another using special detachable links. Bows form an anchor chain with a length of 50 to 300 m. Depending on the location in the anchor chain, anchor (attached to the anchor), intermediate and root bows are distinguished. Anchors are attached to the anchor chain using anchor brackets. To prevent twisting of the chain, swivels are included in it. For fastening and emergency recoil of the root end of the anchor chain, special devices with a folding hook, the so-called verb-hook, are used, which makes it easy to free the vessel from the etched anchor chain.