Stern Unit For Marine Craft

Abstract

A stern unit of marine craft that comprises a casing for housing a shaft, a propeller, and at least one rudder actuated by hydraulic pistons is described. The casing is open at the bottom, with its bottom surface having a parabolic shape in cross-section. The casing has an inlet section near to the hull that diverges rearwardly toward an outlet section. The casing provides a housing to the shaft, propeller, rudders, with a gap located above the trace line of the boat bottom where air is sucked above the water surface line such that an air film extends along the bottom surface of the casing towards the outlet section. The stem unit reduces frictional resistance and supplies additional thrust by providing cavitation between the propeller and the rudders.

1 . A method of reducing water friction on a non-surface drive propeller casing comprising: coupling a movable parabolic and inclined casing having a lower surface to a boat hull over a linear drive propeller, and adjusting the movable parabolic and inclined casing such that an air film is formed between the lower surface and the water when the boat is moving wherein the water friction is reduced.  
   
   
       2 . The method of  claim 1  wherein the movable and inclined casing transversely expands rearwardly from the boat hull.  
   
   
       3 . The method of  claim 1  further comprising adjusting the movable parabolic and inclined casing over the linear drive propeller using a hydraulic piston system.  
   
   
       4 . A method of ventilating a non-surface drive propeller comprising: coupling a movable parabolic and inclined casing having a lower surface to a boat hull over a linear non-surface drive propeller having blades and blade ends, adjusting the movable parabolic and inclined casing such that an air film is formed between the lower surface and the water when the boat is moving and ventilating the non-surface drive propeller using the blade ends to draw air into the propeller.  
   
   
       5 . The method of ventilating a non-surface drive propeller of  claim 4 , wherein adjusting the movable parabolic and inclined casing further comprises the step of moving the casing using a hydraulic piston system.