Revision as of 09:51, 30 March 2016

How Propellers Work

Like many aspects of ship design, designing propellers is a mixture of science and art in the form of experience. There is a lot going on at the stern of a ship that is poorly understood. This is not surprising. Every hull form, engine-propeller combination, interaction with the rudder wake region, and sea condition is different. Wave action introduces a random vertical moment into this equation.

For these reasons, most ship propellers operate at efficiencies around 56%percent, with the principle design problem being cavitation as rotational speed is increased. In comparison, aircraft propellers operate at 90% efficiency. On a ship a propeller efficiency of less than 50% indicates a poor propeller/hull design.

(Efficiency is measured as a ratio of the propulsive energy produced to the energy consumed in rotating the propeller shaft.)

AAmpere (amp), SI unit of electrical current propeller works by accelerating a mass of water. This generates an equal and opposite reaction in the propeller/shaft/engine assembly. This is the thrust that drives the boat forward. In short, the propeller pushes the engine which is solidly attached to the hull – more about this poor state of affairs later.

When the propeller turns it accelerates a column of water through the screw. The pitch of the screw determines the acceleration and the distance travelled for each turn. In essence, this is like a nut turning on a screw. Increasing the pitch increases the weight of water moved and the horsepower required.

Since for efficiency a displacement boat should not exceed its hull speed (a function of the waterline length), and a propeller cannot exceed the available space under the stern, a propeller must be designed for the hull and the amount of thrust it must impart to overcome friction at the maximum hull speed.

Arbitrarily increasing the speed of the propeller or making it larger without changing the design of the screw will not make the boat go faster. Indeed, at some faster rotational speed, the water around the blades cavitates excessively, and the boat will go slower.

Cavitation is the formation of a vacuum along parts of the blade. This lowers the vapour pressure of the water, causing air bubbles to form, just like in boiling water. This causes slip, loss of efficiency and pitting of the metal. Increasing the pitch of a propeller also increases its propensity for cavitation.

Consistency (accuracy) in pitch and camber are important to reduce the potential for cavitation. This also reduces vibration: as much as 80% of hull vibration can be caused by a propeller. Propellers should be balanced both statically and dynamically.

Fixed propellers are the most efficient, strongest and least expensive. But they are at their most efficient only at their designed rpmRevolutions per minute and hull speed. At other speeds, their efficiency falls off. A fixed propeller is ideal for a boat that cruises long distances at a constant speed. Boats that stop and start a lot or change speed frequently are better with a controllable-pitch propeller.