Equipment for controlling ship's movements

transcript

EQUIPMENT FOR CONTROLLING THE

A maritime documentary presented by:BSMT 3 - Polaris B GROUP 3

• To effectively control the movement of a ship through water depends on her equipment used for maneuvering. Mainly, the equipment used for this purpose are the ship's main engine(s), propeller(s), rudder(s), thruster(s), anchors and mooring lines. The shiphandler should have the knowledge on how these equipment operate, how to use them, of their advantage, as well as disadvantages.

TYPES OF MAIN ENGINES

• Majority of merchant ships now are fitted with a diesel engine. This type can be started and stopped. Full power is developed faster than the steam turbine engine. However, in reversing there is some delay when the shipis having a headway exceeding 6 knots. The headway must first be reduced before it can be started for astern movement. This is because of the strong resistance of water stream on the blades of the propeller when stopped.

• The steam turbine must be given time to increase revolutions so this type is slow to develop power. It must be completely stoppped before it can be reversed and this takes about five minutes which is rather too long a time when maneuvering in harbor water. For this reason, there is a separate smaller turbine that is used for astern power but provides only 50% or less than ahead power and very slow responce. M

• A gas turbine does not use steam to run it. It uses a high-pressure gas produced in a gasifier that is composed of a combustion chamber and a compressor.

THE PROPELLER

• The purpose of the propeller is to convert the ppower of main engine into thrust (pushing force.) To do this, the principle of the scew is used. Just as a screw works through the wood when turned, similarly the propeller when it rotates also workds through the water driving the ship forward or astern depending on its direction of revolution (rotation.)

• Diameter - The measurement of two times the distance from the centerline of the hub to the tip of one blade of propeller.

• Pitch - The distance a propeller or the ship would advance after one complete revolution of the propeller if it is going through a solid substance, just like a scew going through wood. The average value of the pitch in ordinary propeller designs is 1.2 to 1.4 of the propeller diameter.

• Slip - the difference between the distance a propeller should travel and the distance it actually travels in one complete revolution. It is expressed in percentage.

• Propeller Speed - the speed in miles per hour traveled by the tips of the rotating blades.

• Cavitation occurs under certain conditions during the rotation of propeller wherein cavities (bubbles) are formed in contact with the propeller blades reducing its thrust and thereby, reducing the ship's speed. This occurs when the propeller is rotating excessive speed in rough seas.

How to solve for engine speed:

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PitchRPMdEngineSpee

• Apparent Slip is the difference between the stream projected by the propeller and the speed of the ship, in relation to a fixed point in the water, clear of the wake.

• A right-handed propeller is one that rotates clockwise when going ahead and counterclockwise when going astern facing forward.

• A left-handed propeller rotates counterclockwise when going ahead and clockwise when going astern facing forward.P

• A controllable pitch propeller, also known as variable pitch propeller, has blades that are adjustable. The engine is kept running at a predetermined RPM and constant direction. By adjusting the pitch of the blades the speed of the ship could be increased or decreased or stopped and also for astern movement without reversing the direction or rotation of the propeller.

• Voith Schneider is a special type of propeller fitted to small vessels such as harbor tugs or salvaged boats makes the vessels highly maneuverable. It is not like the above-described propellers that operate like screw.

VORTH SCHNEIDER PROPELLER

• A Kort Nozzle propeller is the same as the fixed-pitch propeller but it is fitted inside a duct.

• The thrust of the propeller blades has two components (parts): a fore-and-aft one and a small arthwatships one. The fore-and-aft component is the force that moves the ship forward ans the arthwatships one is the force that drives the stern of the ship through the water in a direction at right angles to the ship's line of motion. This is also known as transverse thrust. When going ahead from the dead in the water, the bow of the ship will cant (turn) to port as the headway becomes faster, the swing of the bow decreases and may change to starboard.

• When going astern from the dead in the water, the bow of the stern cants strongly to port and continues to do so when the ship gains sternway until the rudder is used to slow down a little by putting it to hard right.

THE RUDDER

• The rudder is a device used for steering (directing the course) and manuevering the ship. In certain manervers it can be used to slow down the ship. The three types of rudders are old-fashioned, balanced or semi-balanced and the active rudder.

• The old-fashioned rudder has all its face area abaft the turning axis (rudder post.)

• The balanced or semi-balanced rudder has about 30%of the face area forward of the turning axis. This type requires less powerto turn it because when it is turned while the ship is moving ahead the water stream strikes the forward area and helps turn the rudder.

• The active rudder is like an old-fashioned rudder but has a propeller driven by asubmerged electric motor fitted at the outer edge of the rudder.

• When the propeller is turning for ahead movement it produces a slipstream (discharge current) flowing astern. If the rudder is put over to one side, say to starboard, the force of the slipstream strikes the face area of the rudder producing a transverse thrust that pushes the stern to port causing the ship to turn on its pivot point and turning the bow to starboard. The Bernoulli's effect also explains the rudder movement as follows: The rudder's amidships side are curved slightly outwards. Therfore, the flow of water speeds up slighly over the rudder surface thereby causing a slightly lower pressure.

• When the rudder is moved to one side, the pressure is lower on one side than the other, creating a lift. There is also a force called drag which tends to slow the vessel down. The resultant of these two forces will be the direction of movement of the rudder and the bow will go the opposite way. If you want to turn the ship to port, then turn the helm to port. Except for any active rudder, the rudder can be turned to a maximum angle of 45 degrees either side but it may stall (get stuck) when the ship is at full ahead. An angle of 35 degrees is considered to have maximum effect.

SIDE THRUSTERS

• The equipment for manuevering a ship, that is, the propeller and rudder, are all situated at the stern to propel and steer the ship. These equipments can move the bow sideways but a longitudinal motion accompanies it. Some ships are fitted with a device near the bow canned bow thruster to give the forward end of the ship a lateral thrust when needed without necessarily gicing the ship a forward motion.S

ORDERS TO THE HELM AND ENGINE ORDER TELEGRAPH AND BOW THRUSTER

• During maneuvers for berthing or unberthing, or other occasions when the rudder and engine are used in various orders are given to the helmsman, engine order telegraph and bow thruster cotrol operators. Orders must be given firmly and clearly, and should be repeated by the perso to whom the order is directed in the exact words given.

Examples:

1. "RIGHT (LEFT) 10° RUDDER". The wheel is turned to the right (left) until rudder indicator shows 10° to the right (left.)

2. "RIGHT (LEFT) FULL RUDDER". The wheel is turned to the right (left) until rudder indicator shows 35°.

3. "RUDDER AMIDSHIPS". The rudder is brought to the amidships position. This is to allow down the swing of the bow and is a warning that the new course or heading is being approached.

4. "STEADY" or "STEADY AS SHE GOES". At this order the helmsman should keep the ship on the heading she has at that instant by manipulating the wheelas required, and reports "NOW STEADY ON _______° SIR".

5. "MEET HER". The rudder is brought up to the other side about 10° in order to stop the swing of the bow.

6. "SHIFT THE RUDDER". Change the position of the rudder from right to left, or vice versa at the same number of degrees.

7. "NOTHING TO THE RIGHT (LEFT)". The Helmsman should keep all small variations in steering to the right (left) of the compass course. This is often given when the ship is being affected by wind or current.

All commands to the helmsman in regard to the course refer to the compass by which he is steering and should be given in three digits. Example: "COURSE, ZERO, NINE, FIVE". After accomplishing an order the helsman should report it, as for example, "RIGHT 10° RUDDER NOW, SIR." The correct acknowledgment to any report by the helmsman should be "VERY WELL."

• Order given to the engine room telegraph operator.

When going to use the engines, the first order given i: "STAND BY ENGINE(s)". At this command the operator repeats the order and puts the handle or pointer of the instrument to Stand-By poisiton. Orders that follow are given in three parts:

1. The first part designate which engine is referred to as STARBOARD (PORT) ENGINE or ALL ENGINE. This alerts the operator. O

2. The second part of the order refers to the direction in which the telegraph handle/pointer should be moved, as "AHEAD" or "ASTERN".

3. The third part gives the speed at which the engines be run.

Example: When the order "PORT ENGINE ASTERN SLOW" is given, the operator repeats and at the same time rings up the telegraph on the engine order telegraph then reports "PORT ENGINE ASTERN SLOW, SIR."

• Order to the Bow Thruster control operator:

Orders given should also be repeated word for word by the operator before executing it. The order given is, first in which direction the bow of the ship should be followed by the speed, as for example: "BOW THRUSTER TO STARBOARD SLOW?"

SUMMARYThe power to move the ship is generated by the main engine. By applying the principle of screw, the power from the main engine is converted by the propeller into thrust in order to propel the ship through water. The rudder is the device that steers or guides the ship to the direction that the shiphandler wants the ship to go. Bow thrusters provide means to move the ship's bow especially when berthing or unberthing. Standard commands to the helmsman, operator of the engine order telegraph and bow thruster control should be used and the correct reply to the orders should be observed.

Fin.PREPARED BY:JARQUEO, RodenLAPASTORA, ArvyLORAEZ, Victor AllanMATA, Martin ValeMALLARI, Daniel

Equipment for controlling ship's movements

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