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Sometimes when we are sailing on a small motor boat, we feel that the vibration is high. This is probably caused by mistakes done during the selection of propeller. I often see a brochure from the manufacturer of a marine diesel engine which shows that the propeller and the engine is provided as one package.
As a matter of fact the propeller that is supplied by an engine manufacture might not be suitable for the boat's hull when moving at sea. When a boat owner or builder is ordering an engine, the selection of the propeller has to be given to a propeller designer, a naval architect or a marine engineer who has the knowledge of calculating the ship resistance and effective power and translating it into the engine selection and propeller design for the boat.
Usually to reduce the vibration, a propeller designer will add the number of blades from 2 or 3 to 4 or 5. For passenger ships, the number of blades (Z) for the propeller can be around 5 to 7 blades depending on the size of the ship and the rpm of the engine. The higher the number of blades, the lower will be the propulsion efficiency. The propeller designer will have to compromise such contradicting factors to achieve the best dimensions before the design drawing of this propelling unit could be sent to the propeller manufacturer for production stage.
Watch the following Youtube video to have a closer look of how the rotation of a ship propeller creates vibration.