Underwater Vehicle Propulsion Tech: Tail Shape And
Successful underwater vehicle (UV) performance is all about a properly functioning system. The components of a UV’s Vehicle-Propulsor-Drive system are not individual performers. They must be in harmony.
Often overlooked during system design is the relationship between the propeller and the vehicle body, particularly how the water reaches the propeller and – and perhaps even more importantly – how the local pressures between the vehicle and propeller can affect development of useful thrust. This article is intended to give UV vehicle designers and builders a little insight into this critical hydrodynamic interaction.
Let’s begin our story by focusing on the propeller. There are two principal pressure zones on either side of a propeller blade that coexist and develop the thrust that moves the vehicle. On the aft side of the propeller is a “positive-pressure” zone (noted as P+ in the graphic). Hold your hand out your car’s window. Slightly rotate your hand to bring your thumb up. As your car moves, air is captured under your hand creating a “positive-pressure”. This is equivalent to the propeller’s aft-most side (its “face”), and it pushes the blades forward.
Simultaneously, there is a suction that is also developed on your hand due to the curvature of the flow that wraps around your thumb and along the back of your hand. (You probably cannot feel this as our hands are not great airfoils, but it is there.) This suction is a “negative-pressure” zone that pulls the....