Define what a Surface-Piercing propeller is

[sandhammaren05]

Originally Posted by baeckmo

Oh, its quite simple, the supercavitating and ventilating propeller has a nonsteady thrust performance, while a fully wetted rotor has a more or less steadily falling character. If you plot kt versus Ja for supercavitating operation, kt starts at a low value, slowly increasing until Ja ~0.4 times P/D. There the flow quite suddenly changes from a full cavity on the blade suction side, to baseventilated flow. The phenomenon resembles what is happening when you spill your coffe along the cup's side; increase the inclination and suddenly the flow breaks away.

This characteristic has been well known since Pozdunin presented the supercavitating propeller in the mid-forties and has to be catered for in the design of pumps, inducers and propellers. Unfortunately, few of the racing enthusiasts take the effort to learn the science behind the heating and beating, thus the "black magic" attitude. Supercavitating/superventilating flows, ie multiphase flows in turbomachines behave completely different compared to singlephase flow.

I'll send a typical propeller diagram for a sc prop as soon as my scanner has been replaced, ok?

Sorry to take so long in getting around to this, and thanks for "On the Operating Principles ...". With J_A ≈.4P/D that is not the transition I'm describing. In that gtransition the parameters I stated have J_A>1. Also, I doubt that there can be a cavity after transition, the form drag would be enormous (we gain considerably in racing by running blades so thin that cracks often develop). So I guess it's time to stick my neck out and post the guess that I sent you before you posted your solution. Here's my uneducated guess:

I think the sudden transition is like liftoff of a wing. Extra circulation is created about the blade by the cup, the shedding of the trailing vortex is the transition. That is, by the asymmetry of the surfacing prop the blade under water is a bit like a hydrofoil running upside down. Naturally, a trailing vortex was shed when the prop made the boat move from rest in the first place, but if you remove the cup then the transition under consideration will disappear. That's why I asked if your answer holds when there's no cup. I'm not convinced that supercavitation occurs, I would guess that the water is simply aerated and always wets the blade that's under water. A supercavitating prop could not be used to push an outboard to top performance, too much form drag from the cavity.

Note added later: One could also speculate that the transition is due to collapse of a cavity. In any case, airlift acting on the bow plays a big role in speeding up the transition.

Well, as I said I'm speculating. When in 1978 I built racing props trying to match the LE to the inflow I wasn't thinking about cavities. Any sharp edge generates an eddy, so there's eddy drag generated by a sharp prop blade moving relative to the inflow. I was naively trying to reduce the drag by getting rid of that eddy and then used camber to make up for the lost angle of attack. Every racer knows about 'cup', which is an extreme for of camber at the trailing edge but when racers discovered leading edge 'cup' they simply bent the blades without a pitch gauge and had no idea what they were doing. They gained acceleration and lost top speed. I used exactly the same prop to run le Mans starts and run closed course races that I used for my straightaway record run, because the leading edge camber increased both the acceleration and top speed.