Resistance and Propulsion Final

1859 Words Feb 21st, 2018 7 Pages
Resistance and Propulsion Final

1.

a) There are two main reasons that ships use a foil shaped rudder as opposed to a flat, plate type rudder, and they are both related to each other. First, a foil shaped rudder creates a much greater “lifting” force at smaller angles of attack compared to a flat plate. Second, a foil shaped rudder achieves a higher angle of attack before the rudder “stalls”. The greater lifting, or in this case turning force induced by a foil shaped rudder at smaller angles is a product of the smooth, high velocity, laminar flow across the low pressure side of the rudder more so than the flow across the high pressure side of the rudder. The high-pressure side would cause a greater lift on the flat plate as opposed to the foil using a similar angle of attack because the water is traveling on a straight path, as opposed to a curved path on a foil type rudder, and should move at a lower velocity, which translates into a higher pressure. Therefore, it is the low-pressure side that is pulling the rudder creating the turning moment that turns the ship. The higher rudder angles achievable by a foil shaped rudder are a direct result of the smooth, rounded entry of the foil shaped rudder. This ensures a smooth laminar flow, which causes the separation point to occur much further back on the foil shaped rudder than the flat plate style rudder at higher angles of attack. Because the separation point is held further back along the rudder, the lift associated with…
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