USCG Masters Captains License 25/50/100 Ton Upgrade
SEAMANSHIP - Ship Handling
Seamanship - Module 1
Ship Handling
Many forces affect ship handling, such as propeller thrust, rudder force, and side force. High pressure and low-pressure areas beneath a ship are created by the propeller and rudder. As a propeller revolves to go forward, the shape and pitch of each blade produce a low-pressure area on the forward face of the blades and a high-pressure area on the opposite side. As a ship moves in the direction toward the low-pressure area, the force set up by this displacement of water is transmitted along the propeller shaft to thrust the ship ahead, thus propeller thrust.
When the rudder is set at an angle on a moving ship, a high-pressure area is built up on the leading surface and a low-pressure area forms on the trailing surface. Through this difference in pressure areas, the water exerts a force against the leading surface of the rudder. In turn, the rudder forces the stern in the direction opposite that which the rudder is set, resulting in the rudder thrust.
Side force is a force that moves (walks) the stern of a ship in the direction of propeller rotation. The effect is as though the lower blades of the propeller were touching the bottom and pushing the stern to the side.
Side force is at its maximum when the ship starts moving, and decreases rapidly with an increase in ship speed. Side force is greater when a ship is backing than when moving ahead.
If a right-hand propeller is turning ahead, it would tend to push the stern to starboard and the bow to port. If this is same propeller were turning to move the ship astern, the side force would move the stern to port and the bow to starboard. Side force is reversed from right hand to left hand propellers.
A ship’s pivot point is usually one-third the ship’s length from her bow when moving ahead, and at or near her stern when moving astern. Location of a pivot point varies with the ship’s speed and loading. In close waters, keep in mind the position of the pivot point before starting a turn. This is especially important when moving ahead in order to prevent the stern from swinging to an undesirable location.
A ship’s rudder is used to attain or maintain a desired heading. The force necessary to accomplish this heading is created by pressure against the flat surface of the rudder. Factors that have a bearing on rudder effect include the rudder’s size, rudder angle, headway or sternway, propeller direction, and side force.
The term avast means stop.
Wind and currents are factors in ship handling. A current is the horizontal flow or movement of water. The speed of a current is greater in deep water than in shallow water. When rounding a bend in a channel, the deepest water can be found on the outside of the bend, while the shallower water is found on the inside of the bend. In shallow water, the water may boil up at the stern, creating pressure aft. A common occurrence when a vessel is running into shallow water is that squat will cause a decrease in bottom clearance and an increase in draft. If the current is from ahead and there is plenty of room, the bow can be eased alongside, and the forward bow spring line put out. The current will bring the vessel in to the dock.
When the current is from astern, putting out the after-quarter spring line can produce good results. Going alongside with a current from astern is more difficult because the following current reduces the effectiveness of the rudder.
Wind effect is another important factor when going alongside. The wind effect is determined by the superstructure and the amount of freeboard. With a current from astern, slacking the stern lines will carry the stern off.
In a narrow channel, bank cushion and bank suction can affect a vessel. Bank cushion effect refers to a vessel being pushed away from the nearest river bank. This occurs when the vessel is operating close to a bank. As it moves ahead, the water between the bow and the near river bank builds up on the side of the vessel, causing the bow to move away from the bank. Bank suction refers to the stern of the vessel being pulled towards the bank of the channel. As the vessel moves ahead while near a bank, the unbalanced pressure of the water on the aft quarter lowers the water level between the vessel and the bank, forcing the stern to move towards the bank. Bank suction occurs predominantly with twin screw vessels. The combined effect of bank suction and bank cushion may cause a boat to take a sudden sheer toward the opposite bank. A single-screw vessel with the starboard side near the right bank may actually lose control. To bring the boat under control, increase the speed and add a small amount of right rudder. A twin-screw vessel usually recovers from sheer by increasing the speed on the port engine and adding right rudder.
