In The fall of 2004 I purchased a boat and outboard.  The company that built the boat also had the responsibility of mounting the out board.  Unfortunately the company built the transom of the boat to high and this had a major effect on the boats performance.  With the prop so close to the water’s surface the out board would ventilate ever time I tried to get the boat up on step.  It would also ventilate on tight turns at high speeds.  Unless I wanted to ship the boat back down to Seattle for modifications I would have to find a propeller that would operate effectively near the surface.  The follow is some of the things I learned while solving this problem.

As the propeller rotates (fig 3-1) it forces water down and back as this is happening water must move into the void created by the spiraling blades.  This creates a pressure differential across the blade- Low pressure on the back side and high pressure on the front side.  This causes water to be sucked into the propeller and accelerated out the back (fig 3-3) much like a house-hold fan (fig 3-2).  This action creates the thrust that drives a boat.