Marine Propeller Geometry Pitch    Pitch of propeller is same as that of a screw. If a propeller moves 5 cm forward in one full rotation its nominal pitch would be 5 cm. The actual distance traveled by the ship does not equal the pitch of propeller and the difference between nominal pitch and the actual distance traveled is called slip. Blades are twisted to have the same pitch along the blade from root(where they are connected to the hub) to the tip. Pitch converts the torque of the shaft into thrust by accelerating the water in the opposite direction in which the ship is moving.  Rake    Rake is the angle of propellers to the hub. We can either have positive or negative rake angle. The rake angle is measured from line perpendicular to the propeller. Rake improves performance by keeping the water between blades and hub longer. Rake does not have to be constant along the whole length of edge. As we can see in the picture below rake can start at 10 degrees at the root of the blade all the way to 30 degrees at the tip of the blade. Skew    Skew makes the blade asymmetrical when viewed from fore. Skew ensures that the radial sections of the blade does not pass through a section at the same time. Skew delays the entry of each section. Blades can have positive or negative skew. Skew of a blade reduces vibrations. 3a - Positive skew 3b - Negative skew Cup    Cup is a small curl located at the tip of a blade and. It is always bend towards the aft of the boat. Cupping on the tip of blade increases rake and when added to the trailing edge it increses pitch therefore improving the performance of the propeller. Propeller Cross Section Generally propeller blades get thinner towards the tip.

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