Monday, July 13, 2009

The effect of turbulence on propeller and safety of cruise ship

by Charles Roring

Naval architects or ship designers when designing a marine vehicle will always have to design the propulsion system according to the designed speed stipulated or required by ship owner. The most common form of propelling device in ships or boats is propeller.


When designing the propeller, one of the propulsion factors which they have to determine is the blade area ratio which is the ratio of the blades' area compared to a disc area with the same radius as the propeller blades. The developed area ratio (DAR) of a propeller is determined from cavitation chart, commonly Burril chart, obtained from experiments of a number of propeller series conducted by various cavitation tunnels around the world. Cavitation occurs when the flow of fluid, or sea water, is not steady. In other words, when there is turbulence, cavitation will occur in ship's propeller.


Now the question is will it be harmful to passenger? My answer is No. If there is turbulence in the propeller of airplane, my answer will be different. Turbulence which causes the cavitation of propellers in ships is not dangerous for passenger ships but will be very dangerous to airplanes. The safety of passengers in cruise ships will not be jeopardized if there is turbulence or cavitation in the propellers.

The effect of cavitation is the loss of lifting force on propeller blades. If this term is confusing then the simple explanation for the occurrence of turbulence and cavitation is the loss of thrust that is required both in ships and airplanes. The loss of thrust is the same as the loss of speed. Ships will remain afloat whereas for airplanes, they will fall. This is the most common cause of plane crash. In bad weather, the flow of the fluid is not steady. It is not good both for ships and airplanes.

Cavitation causes erosion on blade surface of the propellers. It also increases the rpm of propeller which in turn, will cause vibration, and overheating in the main engine. The most common solution which naval architects do to minimize the cavitation of the propeller is increasing the blade area ratio. Also read: Ship Displacement and Capacity Limitation and Traveling with ship; Propeller Rake and Blade Thickness Diameter Ratio

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