Degrees of Vessel Motion (Freedom)
A vessel on water has the following six (6) motions or degrees of freedom while underway: roll, pitch, heave, yaw, sway, and surge.
Forces Affecting Boat Handling
Before attempting to handle a boat, it is important to understand the forces that affect a boat under various conditions. A watercraft operator who thoroughly understands these forces can use them to maneuver his boat. Therefore, he will not have to fall back on the often painful, trial-and-error method of learning boat handling. The following vessel characteristics influence the control of single-screw boats having right-hand propellers.
The design of a ship includes the size and shape of the hull, draft, trim, weight, and amount of superstructure. Ships with shallow draft, low superstructure, and slim design normally handle more easily than ships with high superstructure, deep draft, and wide beam because they are less affected by wind and current and respond more rapidly to the rudder.
Each phase of motive force as it reacts on the vessel has its own peculiarities. No set of rules can be devised to cover all types. Every vessel has its own power characteristics, which the operator must learn to determine their effect upon handling of the vessel.
A propeller draws its supply of water from every direction forward and around the blades, forcing it in a powerful stream toward the stern. This moving current which provides the power for propulsion is called "screw current." The water flowing into the propeller is called "suction screw current," that being ejected is called "discharge current." The below illustration shows this water-pressure effect of the suction current vaporizing off the tips of the blades and spiraling back in a helical pattern. The factors that affect propeller action are:
The pitch of a propeller is the distance the propeller would advance in one revolution if the water was a solid medium.
The difference between the speed of the ship and the speed of the propeller is known as the "slip". Slip is caused by the yield of the water against the propeller thrust. In other words, it is the percentage of distance lost because water is a yielding substance.
When the blade-tip speed is excessive for the size and shape of the propeller, the vessel rides high in the water. There is also an unequal pressure on the lower and upper blade surfaces. This condition produces cavities or bubbles around the propeller known as "cavitation." The result is an increase in revolutions per minute without an equivalent increase in thrust. This results in loss of efficiency. When cavitation is fully developed, it limits a vessel’s speed regardless of the available engine power.
The rudder acts the same on a large vessel as on a small craft. The rudder is placed directly behind the propeller to use the powerful discharge current to turn the boat. Moving the rudder to the right deflects the discharge current to the right, which forces the stern to the left. This action is reversed when the left rudder is applied. At very slow propeller speed and with very little way on, there may not be enough control over a boat to maneuver it, especially if other forces are acting upon it at the same time. When this condition prevails, the propeller may be speeded up enough to give it a more powerful thrust against the rudder. Using sudden thrusts of power to kick (move) the stern in this manner is one of the fundamental principles of vessel handling. A vessel can often be turned in twice its length by kicking the stern.
The pivot point for a power driven vessel is about one-third of the vessel’s length from the bow when going ahead and between one-quarter and one-third the vessel’s length from the stern when going astern.
Other Factors Affecting Control
Wind, tidal, ocean currents (waves or sea), and depth of water must be considered when handling a vessel. Shallow water particularly affects deep draft vessels because of the cushion effect similar to that encountered when navigating in narrow channels.
èBoat Handling Characteristics
Characteristics or factors, such as the power, propeller, rudder, and design of a ship affect handling in various ways. For illustrating the effects of these factors, it will be assumed that the sea is calm, there is neither wind nor current, and the ship has a right-handed propeller.
Single Screw Vessels
The single-screw vessel has only one propeller. The operation of this vessel is described below.
With the ship and propeller going forward and the rudder amidships, the ship tends to move on a straight course. The sidewise pressure of the propeller is offset by the canting of the engine and shaft. When the rudder is put over (either to the right or left) the water through which the ship is moving strikes the rudder face, forcing the stern in the opposite direction. At the same time, discharge current strikes the rudder face and pushes the stern over farther. As a result of these forces, the bow moves in the direction in which the rudder has been thrown.
Vessel With Sternway, Propeller Backing
When backing, the sidewise pressure is opposite to that exerted when the ship is moving forward. The discharge current from the propeller reacts against the hull. This current is rotary; therefore, when the propeller is backing, the current strikes the hull high on the starboard side and low on the port side. This current exerts a greater force on the starboard side and tends to throw the stern of the vessel to port.
With rudder amidships, the vessel will back to port from the force of the sidewise pressure and the discharge current. When the rudder is put over to starboard, the action of the suction current against the face of the rudder will tend to throw the stern to starboard. Unless the ship is making sternway, this force will not be strong enough to overcome the effect of the sidewise pressure and the discharge current, and the stern will back to port.
When the rudder is put over to port, the force of the suction current on the face of the rudder intensifies the effect of the sidewise pressure of the propeller and the discharge current and will force the stern rapidly to port. Because of these forces, all right-handed, single-screw vessels tend to back to port.