How is the navigation performance of the ship?

In order to ensure the safe and normal navigation of the ship under various conditions, it is required that the ship has good navigation performance, including buoyancy, stability, anti-sinking, rapidity, sway and maneuverability.

Ship buoyancy

The floating ability of a ship under certain load conditions is called buoyancy.

Ships are floating bodies, and the forces that determine the ups and downs of ships are mainly gravity and buoyancy. Its floating bar is that gravity and buoyancy are equal in size and opposite in direction, and the two forces should act on the same vertical line.

The gravity of a ship is the total weight of the ship. The buoyancy of a ship refers to the buoyancy of water on the hull.

According to Archimedes theorem, the buoyancy of a ship is equal to the weight of the same volume of water displaced by the hull.

The ship's center of gravity, usually represented by W, passes through the ship's center of gravity, also called center of gravity (G), and its direction is vertical downward, and the position of the ship's center of gravity G changes with the movement of goods; The buoyancy of a ship, usually represented by B, passes through the geometric center of the ship's underwater volume, also known as the buoyancy center (G), and the direction is vertical upward. The position of the buoyancy center G of the ship changes with the change of the hull volume below the waterline, as shown in figure 1-23.

The gravity (W) and buoyancy (B) of a ship are equal in magnitude and opposite in direction, and act on the same vertical line. At this time, the ship floats on the water in balance.

If more cargo is added, the ship will sink with the increase of gravity, so the draft will increase and the buoyancy will increase, until the buoyancy and gravity are equal, the ship will reach a new equilibrium position; Similarly, if gravity decreases and the ship floats, it will reach another new equilibrium point.

The equilibrium floating state of the ship is referred to as the floating state of the ship for short. The floating state of ships can be divided into four types.

1. Positive floating state

Refers to the situation that the draft of the bow, stern and middle of a ship is equal.

2. Pitch state

Refers to the situation that the left and right draughts are equal but the front and rear draughts are not equal. The draft at the bow is greater than that at the stern.

Water is called the first inclination; The draft of the stern is greater than that of the bow, which is called stern tilt. In order to keep the propeller at a certain depth and improve the efficiency of the propeller, all ships that are not fully loaded on the first voyage must have a certain stern inclination angle.

3. Tilt state

It refers to the situation that the draft of a ship is equal from beginning to end and unequal from left to right, which is not allowed in navigation.

Tilt state.

4. Any state

It refers to the state of lateral inclination and vertical inclination.

When ships are sailing at sea, they often encounter waves hitting the deck, which will become very thick in winter.

Ice is equivalent to adding weight to the ship. In order to ensure the safety of the ship, the ship must maintain a certain reserve buoyancy (also called reserve buoyancy). Reserve buoyancy refers to the buoyancy generated by the watertight space between the ship's main deck and the waterline, as shown in the following figure.

The less cargo, the higher the freeboard of the ship, the greater the reserve buoyancy and the better the buoyancy, which is more conducive to navigation safety. Therefore, in order to ensure the safety of the ship and make full use of the ship's load capacity, it is necessary to carry out reasonable stowage according to different seasons and navigation areas, so that the maximum draft does not exceed the full load waterline specified on the load line sign.

Ship stability

Stability refers to the ability of a ship to tilt under the action of external torque (such as wind and waves). ) and return to its original equilibrium position after the external torque is removed.

Ship stability can be divided into lateral stability and longitudinal stability according to the inclination direction; According to the dip angle, it can be divided into initial stability (dip angle 100 or less) and large dip angle stability; According to the nature of external torque, it can be divided into static stability and dynamic stability. For ships, the possibility of longitudinal capsizing is extremely small, so lateral stability is generally discussed.

When the ship is in a balanced position, the center of gravity (G) is on the center line of the ship, because the structure of the ship is symmetrical, and the weight distribution on the ship is also required to be symmetrical. As mentioned earlier, gravity (W) is vertically downward from the center of gravity (G). The buoyancy center (C) of a ship is the geometric center of the underwater volume of the ship. When the ship is floating, it is also on the center line of the ship, and the buoyancy (b) is vertically upward from the center of buoyancy (c), as shown in figure 1-25.

When the external torque forces the ship to tilt, if the cargo does not shift, the position of the center of gravity remains unchanged. However, due to the change of underwater volume shape, the floating center moved from point C to point C 1. At this time, gravity and buoyancy form a couple resisting inclination, as shown in figure 1-26. When the external torque disappears, the ship returns to the initial position under the action of the torque generated by the above couple. This moment is called recovery moment. A ship is said to be stable when it is in a stable equilibrium state.

If the ship's center of gravity is too high, or the ship's width is narrow, when the ship heels under the action of external torque, because the floating center distance of the narrow ship is short, the moment generated by the couple composed of gravity and buoyancy will make the ship continue to tilt, or even capsize. This moment is called overturning moment, as shown in figure 1-26. When a ship is in an unstable equilibrium, it is said that the ship has no stability.

From the above two situations, it can be seen that in Figure 1-26, point M (the intersection of the new buoyancy action line after the ship inclines and the center line of the ship) is above point G of the center of gravity, and the ship is stable, and point M is called the center of stability. In figure 1-27, point M is below point G, and the ship is unstable. Through analysis and research, whether the ship is stable depends on the relative position between G and M and the distance between G and M, that is, GM value is the standard to measure the stability of the ship, which is called the initial stable height. Its relationship with stability is as follows: when the M point is above the G point and GM > 0, the ship has stability, and the greater the GM value, the better the stability, but the ship's sway will be aggravated; When the M point is below the G point, GM < 0, the ship is unstable, and it is easy to capsize once it is subjected to external torque; When point M and point G coincide, GM=0, and the ship is unstable, because once subjected to external force torque, the ship is in a neutral equilibrium state, which is extremely unsafe for the ship.

Ship sinking resistance

Independence refers to the ability of a ship to remain unsinkable and capsized when one or several cabins are flooded.

In order to ensure the anti-sinking, in addition to sufficient reserve buoyancy, generally effective measures are to set up double bottoms and a certain number of watertight bulkheads. In the event of collision or grounding, a cabin loses buoyancy due to water inflow, and the watertight bulkhead can limit the water inflow to a small range as far as possible to prevent the water inflow from spreading to other cabins without causing excessive buoyancy loss. In this way, the lost buoyancy can be compensated by the reserve buoyancy, which ensures the unsinkability of the ship and creates favorable conditions for plugging and emergency rescue.

For ships with different uses, sizes and navigation areas, the requirements for anti-sinking are different. It is divided into "No.1" ships, "No.2" ships and "No.3" ships. "No.1 cabin" refers to any ship whose cabin is damaged and flooded without sinking. Generally, ocean-going cargo ships belong to the "one-cabin system". A "two-cabin" ship refers to a ship whose adjacent two cabins are damaged and flooded, so as not to cause sinking. "Three-cabin system" ships and so on. General chemical carriers and liquid bulk carriers belong to "two-cabin" or "three-cabin" ships. For a "one-cabin" ship, the water in the first cabin is not impossible to sink under any loading conditions, because the cabin designed according to the anti-sinking principle is calculated according to the cabin water inflow under the average permeability. The so-called permeability refers to the ratio of air intake to cabin space. Therefore, when the cargo hold of an ordinary cargo ship full of steel is flooded, the water inflow will greatly exceed the reserve buoyancy, which may not guarantee that the ship will not sink.

It should also be pointed out that whether the damaged ship will capsize or sink after entering the water is also related to the anti-sinking measures taken by the crew on board to some extent. After the damaged ship enters the water, there are many measures, such as pumping water, injecting water, plugging, strengthening, abandoning the ship's load, shifting or transferring ballast water, etc. Pumping, water injection, plugging, reinforcement, abandoning ship load and shifting load are all to ensure the buoyancy of the ship. Sometimes, in order to reduce the inclination of the ship and improve the floating state and stability of the ship, it is often achieved by injecting water or transferring it to the corresponding cabin.

Ship speed and resistance

The ability of a ship to increase its speed as much as possible under the condition of a certain output power of the main engine is called ship rapidity. Speed includes two meanings: energy saving and speed, so improving the speed of a ship should also start from these two aspects, that is, increasing the thrust of the propeller and reducing the resistance of the ship's navigation.

Ship resistance includes water resistance and air resistance. Because the density of water is more than 800 times higher than that of air, the ship's hull water resistance is mainly considered when sailing at sea. Hull water resistance consists of friction resistance, eddy current resistance (shape resistance) and wave-making resistance. Their sum is the total water resistance of the hull. Namely:

Friction resistance is caused by the viscosity of water. When a ship moves in the water, there is always a layer of water attached to the hull surface and moves with the hull. The energy consumed by the ship's movement to drive the water molecules to move is the energy consumed by the ship to overcome the friction resistance. The magnitude of friction resistance is related to the underwater surface area, smoothness and speed of the hull surface. Therefore, it is an important measure to reduce friction and resistance to remove the dirty bottom by regular berthing.

In addition to frictional resistance, the ship will also produce eddy current resistance when moving forward. When the ship moves forward, it will produce relative water flow. Because of the viscosity of water, the relative current velocity near the surface of the ship is small. When it reaches the stern, its cross-section is enlarged, and the flow speed drops rapidly, reaching zero or countercurrent, which causes the vortex motion at the stern, reduces the pressure at the stern, and forms pressure drag on the ship, which is called vortex resistance or shape resistance. The part with large curvature of the hull is easy to produce eddy current, and the eddy current resistance of the ship with sharp contraction of the stern cross section is more serious, while the streamlined hull does not produce eddy current resistance or only produces minimal eddy current resistance. Therefore, improving the underwater hull line shape has great influence on the rapidity of the ship.

Wave-making resistance is due to the traveling wave of the ship, which produces resistance opposite to the direction of the ship. Ship traveling waves are divided into bow waves and stern waves. In the propagation of ship traveling waves, if the bow wave and stern wave overlap each other at the stern, the wave-making resistance will be great. If the bow wave and stern wave cancel each other at the stern, the wave-making resistance is small. So the wave-making resistance depends on the size. , mainly related to speed and captain. The faster the speed, the greater the wave resistance. At a certain design speed, the proper selection of the captain can reduce the wave-making resistance. Ocean-going ships often adopt the bulbous bow type, just to adjust the captain to achieve the purpose of reducing wave resistance.

As for improving the propeller thrust, it is mainly used in seagoing ships at present. When the output power and speed of the main engine are fixed, the correct design or selection of propeller geometry has a great relationship with thrust. Therefore, the running ship should: properly select the pitch of the adjustable pitch propeller, adjust the proper draft and draft difference, and keep the propeller in a deep position under water when sailing.

Ship rocking

Under the action of external force, the ship makes periodic horizontal and vertical swaying and yawing movements, which is called ship yawing. This is a harmful performance. Severe shaking will reduce the speed, cause cargo damage, damage the hull and machinery, make passengers seasick, and affect the life and work of the crew.

Ship rolling can be divided into four forms: rolling, pitching, vertical rolling and vertical lifting. Rolling is the rocking motion of the ship around the longitudinal axis; Pitching is the rocking motion of the ship around the horizontal axis; Vertical rolling is the rocking motion of the ship around the vertical axis; Vertical lifting means that the ship moves up and down with the waves. When a ship encounters wind and waves at sea, it is often a compound movement of the above four kinds of swaying. Because scrolling is obvious and has great influence, we only introduce scrolling to understand its regularity.

The severity of ship rolling is related to the size of wind and waves from the external conditions, but it is also related to the stability from the ship's own conditions.

Under the action of external force, the ship leans to one side from the original equilibrium position. When the external force stops, due to the stability of the ship, it will produce a restoring moment, which will make the ship move to the original equilibrium position. When the ship returns to the equilibrium position, it will continue to lean to the other side due to inertia. When the inertia force is offset by the corresponding restoring torque, the ship will move to the original equilibrium position under the action of restoring torque. According to this law of motion, the ship swings from side to side repeatedly. Only when all external forces on the ship are exhausted by water resistance can the ship stop at the original equilibrium position. This swing in still water is called "free swing". When the ship passes through a complete rocking cycle from the inclined side, the ship rocks violently; When the period of free swing of the ship is long, the ship swings slowly. The length of free swing is related to the stable height GM of the ship. If the GM value of the ship is too large, the restoring torque is very strong. When the recovery speed is very fast, the swing period is short, forming a violent swing; On the contrary, the swing period is long and the ship swings slowly. When the ship sails in the waves, it is necessary to add the forced swing caused by the waves. The time required for the peak to move a wavelength distance is called "wave period". For a moving ship, the time from the first peak to the second peak is called "wave apparent period". The magnitude of wave apparent period depends on the wave period and the course and speed of the ship.

When the free swing period of the ship is greater than the apparent wave period, the ship's swing in the waves will be weakened; When the free swing period is less than the apparent period of the wave, the swing will be enhanced. If the period of a ship's free swing is similar to the apparent period of waves, the ship's swing will increase sharply, which is called "harmonic swing". Harmonic sway is a dangerous phenomenon for ships, which will have adverse effects on crew, passengers, cargo, hull structure and machinery, and even endanger the safety of ships in serious cases.

If the ship is found to have harmonic rolling, measures should be taken immediately to change the phenomenon of harmonic rolling. You can change the course and speed, so as to change the included angle between the course and the wave or change the apparent motion speed of the wave, so as to avoid harmonic sloshing.

In order to reduce the ship's rolling, the stern keel is generally installed at the stern outside the hull, which is simple in structure and does not occupy the internal position of the hull, and the rolling reduction effect is obvious. Practice shows that the stern keel can reduce the sway by about 20%~25%, and the disadvantage of the stern keel is to increase the water resistance and affect the speed. Large passenger ships also use anti-rolling tanks, fin stabilizers and gyro balance anti-rolling devices to reduce the ship's sway in the wind and waves.

Ship maneuverability

The ability of a ship to maintain and change its state of motion is called maneuverability.

The so-called motion state refers to the course and speed, so the maneuverability should include the ship's ability to change the course quickly and keep the specified course stable, as well as the ship's performance of changing and keeping the speed, as well as the inertia when the ship stops and reverses.

The maneuverability of the ship is mainly realized by the steering wheel, but it is also supplemented by anchors, cables and tugboats during berthing and berthing operations to improve the maneuverability of the ship.