Tuesday, March 16, 2010

Boat and Ship Propellers

In general the working principles of boat propellers and ship propellers are the same. They convert the rotational movements of the blades into thrust. Boat propellers have sizes between 30 centimeters to around one meter in diameter depending on the size of the boat and its stern draft whereas for ship's propeller, a open hatch bulk carrier can have a diameter of  up to seven meters.
Because of its large diameter, the rpm of a ship's propeller will be much lower compared to the rpm of a boat's propeller. In addition, the position of the boat propeller is located near the sea surface, this is the reason its efficiency is lower. Propeller that is closer to water surface can easily attract air into its rotational motion. Its high rpm also makes the boat's propeller always works in cavitation condition.
For ship's propeller that is operating in fully loaded condition, i.e. at its summer load water line, the blades will receive huge loads or pressure. From this operating condition, the propeller material used for a small boat is usually aluminum. Propellers for large ships are manufactured  from such materials as Ni-Al-Bronze; or Cu-Ni-Al. Propeller designer knows that the design of a marine propeller will always be done for service speed of the ship. It is the speed that main engine usually runs at its CSR. CSR stands for Continues Service Rating. There is also similar other term called NCR or Normal Continues Rating. When designing a marine screw propeller, I usually use the Bp delta charts of B-Series propellers from Wageningen.
Well, I will discuss more about marine propellers and their design characteristics in my later posts.
As an illustration of how a large ship's propeller will look like, please watch the following video of how divers polish its blades under water.