Tuesday, May 12, 2009

Propeller and ship’s hull

Ship propeller and boat propeller is still considered as the most economical propelling devices for ships and boats. Other propelling devices such as paddle wheel, magnetohydrodynamic propulsion system have efficiencies that are lower than screw propeller.
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Propeller designer and naval architect must understand how propeller, ship's hull, rudder and main engine interact from one to another.
Ship's Hull - The hull form of a ship influences the flow of water from the forward end of the ship to the stern where the propeller is mostly installed. The finer the hull form the faster will be the ship's speed in water. The fullness of ship's hull form is reflected by its block coefficient or CB. Ships with higher block coefficient (0.70 to 0.98) will tend to be slow in the water. Even if the block coefficient is small (0.50-0.70), the speed of ship moving in the water can be slower due to the fouling of hull surface. Hull fouling is mostly caused by barnacles, and tube worms. If the fouling is worse, it can increase the frictional resistance of ship up to 40%.
Another way which naval architects do to reduce the resistance of ship thus increasing the speed or improving the propulsive efficiency is by adding bulbous bow to the ship. By doing this, the naval architect and propeller designer can reduce the wave making resistance of the designed ship.
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Ship, hull and rudder are integral propulsion system of ships. But they will not work without a main engine inside the hull. Today, marine diesel engine is used as the prime mover of ships. The use of steam and gas turbines are only used in small number of ships due to the complexity of the design although they relatively have low vibration.
Marine diesel engine for large vessels is especially designed to have low RPM. Theoretically, the lower the RPM of a propeller, the better will be its efficiency. Modern propellers are designed and manufactured to have high skew to minimize the vibration and maximize the efficiency.
In the last few decades, the development of multi-hull ships has led to the building of catamaran, trimaran and SWATH (small water plane area twin hull) ships. These new hull types are mostly used on passenger ship, ferries, fast patrol boats and ships or boats that need high speed. by Charles Roring in Manokwari of West Papua - Indonesia. Also read: Propeller design methods; Propeller Strength Calculation; Cavitation of Marine Propeller

1 comment:

Charles Roring said...

Thank you for your comment. Your boat design blog is also nice.