Thursday, October 23, 2008

Lesson 37: Twin Screw Vessels and Bank Effects

Turning in a Limited Space

Single-screw vessels can be turned easily in restricted waters. To start the swing, the engine speed is set at full ahead and the rudder is put full right; then the engine is reversed to full astern until way is lost. When way is lost, the rudder is shifted; after sternway has started, the rudder is again shifted and the engine put full ahead. This procedure is repeated until the vessel is on the desired heading. This maneuver makes use of the tendency of right-handed propellers to back to port. In strong winds, it is wise to turn in such a way that the tendency to back into the wind can be used to increase the turn.

A twin-screw vessel with a single rudder can be turned by going ahead on one engine and astern on the other, using the rudder only when headway or sternway has been gained. When the vessel is fitted with twin rudders that are directly behind the propellers, the rudder is placed hard over in the direction of the turn before the maneuver is begun and one engine is backed at the speed necessary to prevent headway.

Twin Screw Vessels

The twin-screw vessel has two propellers -- one on each side of the centerline. These propellers are maneuvered by separate throttle controls. Generally the propellers are outturning; that is, the starboard propeller is right-handed and the port, left-handed. This balances the sidewise pressure of the propellers and makes it possible to keep the ship on a straight course with no rudder. Discounting outside influences, the twin-screw vessel backs with equal facility to port or starboard.

The various forces affecting the action of the single-screw ship are still present, but normally a twin-screw vessel is not affected by these forces as much as a single-screw vessel. This is because the forces from one screw balance the forces from the other screw.

One powerful force is the momentum of the ship, ahead or astern, acting through the center of gravity. When a twin-screw ship is going ahead and one screw is backed, two opposing forces are set in motion; namely, the force of the backing screw acting in one direction and the weight of the ship acting in the opposite direction. This is in addition to the forces from the action of the pressure on the rudder if it is put over.

Bank Suction and Bank Cushion

Bank suction involves the tendency of a vessel to veer toward the bank as it displaces the water along the shore. The opposite affect, known as bank cushion is often encountered at slower speed when the bow wave bounces off of the shore and back against the vessel. The effects of these phenomena are compounded the nearer the vessel is to the shoreline and varies with speed.

Mooring Lines

The lines used to secure the ship to a wharf or to another ship are called "mooring lines." They must be as light as possible for easy handling and, at the same time, be strong enough to take considerable strain when coming alongside and holding a ship in place. Nylon line (about 1” in diameter) is the customary cordage for mooring lines on small passenger and commercial vessels. Figure shows the locations and names of the lines. Lines should be neatly coiled or arranged to prevent fouling, to eliminate hazards, and to keep the working area clear.

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