When it comes to designing propulsion systems for fast craft, the type of propeller becomes important: Different types of propulsion have different levels of efficiency associated with them.
Surface drives use supercavitating surface piercing propellers, that are more efficient at very high speeds. Indeed, on vessels that will operate primarily below 35 knots, a surface drive is not a good choice of propulsion, since surface piercing propellers are less efficient at low speeds. Indeed, for slower vessels, water jets might be a better choice. The chart below lists several types of propeller: (Allison, J. (1978). Propellers for high-performance craft)
Looking at the graph, it is immediately apparent that water jets are far less efficient at higher speeds, and the efficiency of fully submerged propellers drops precipitously at speeds over 55 knots.
This is why surface drives are the only technology that has the capability to deliver performance at high speeds.
One other consideration that impacts efficiency is that of propeller slip… The loss of efficiency (thus, speed) as a result of drag. This, in particular, is one are in which surface drives excel. Since drag is minimized, very little slip is experienced. Typically, slip % on sirface piercing propellers averages 7% – 12%.
