Propeller Design

Pitch

The design of a propeller is very much like that of a screw. So much so that propellers are casually referred to as screws in speech, with a single-screw vessel having a single propeller and a twin-screw vessel having two propellers. Like a screw turning in wood, a propeller advances through the water with a helical twisting motion. How far the propeller theoretically advances in one full revolution describes its pitch described in linear measurement. If a propeller has four feet of pitch, it should move the vessel forward four feet with every revolution.

Slip

Because a propeller is attempting to move a vessel through a liquid medium instead of a solid medium there will be some degree of inefficiency. Slip is a measurement of that inefficiency expressed in percentage. If a propeller with four feet of pitch only moves the vessel two feet forward in one revolution it has a 50% slip.

Cavitation

One of the causes of slip is cavitation. As the propeller turns voids are formed on the trailing and leading edges of the blades causing a loss of propulsive efficiency, pitting of the blades, and vibration. These voids are known as cavitation. Over time cavitation will damage the propeller.

Sideways Force

When a propeller spins it has the effect of walking sideways. This is caused by torque from the velocity and angle at which the surrounding water impinges upon the propeller blades. This affects boat handling by literally walking the rear end of the boat left or right, depending on which direction the propeller is spinning.

A right handed propeller turns clockwise when viewed from behind a vessel in forward gear. When a vessel with a right handed propeller goes astern the propeller spins in reverse, or counterclockwise. The effect this has is walking the stern of the vessel to port when she backs down. The opposite is true of a vessel with a left handed propeller.

The transverse effect of this sideways force is always acting on the vessel but it is much more noticeable when operating astern propulsion. When a boat operator uses this characteristic intentionally, it is called “prop walk.” This technique makes it easier for a right handed single-screw vessel to dock port-side to and a left handed single-screw vessel to dock starboard-side to. This can also be used in getting away from the dock.

Kort Nozzles

Often found on tugs as well as larger ships, a Kort nozzle is a hollow tube surrounding the propeller used to improve thrust. This also has the effect of reducing sideways force which will reduce maneuverability alongside docks.

Controllable Pitch Propellers

Usually only found on medium sized ships or greater. A controllable pitch propeller—or CPP—operates on the principle of having a constant speed of rotation while changing the thrust by adjusting the angle at which the blades attach to the propeller hub. It does this using a complex internal mechanism. This results in some unusual handling characteristics.

Due to how steerage is only made possible by passage of water over the rudder, when making large speed changes with a CPP there may be moments where there is a loss of steering control. Another difficulty presented by CPP design is finding true neutral. Because the hub is always rotating, if the propeller blades are off even slightly this will result in slight headway or sternway.