Surface Piercing Propeller
In recent years the number of Surface Piercing Propellers being used in high-speed vessels has increased considerably. This is due to their higher propulsive efficiencies than conventional propellers, considerably reduced appendage drag as no shaft or support brackets are immersed in the water-flow below the hull and the absence of any cavitating phenomena which can be particularly severe and limiting for conventional propellers.
Surface Piercing Propellers are unique in that they are designed to deliver their thrust off the aft blade face only using a mass/velocity momentum change of the impinging water. Due to this peculiar functioning, the blades are often wedge-section in shape and the presence of an air-sheet on the forward face of the blade prevents the inception of vapour bubbles and thus cavitation. Typical installation of a Surface Piercing Propeller consists of a driveshaft coming out of the transom at about 50 degrees which leads to a propeller immersion of about 50% in dry-transom and planing conditions. One unintended advantage of this installation is the considerable reduction in the draft of the vessel.