Low Speed Surfacing Propeller

[sandhammaren05]

Having raced both V-bottoms and tunnels, and doing my own prop work, I can tell you that you cannot replace cup by larger diameter, the result would be a much lower top speed. There's a sudden transition to higher speeds that occurs with raceboats/surfacing props that's not understood.

Originally Posted by bananas

The U.S. Navy developed some high speed surface propellers that were tested at low speeds in a cavitation tunnel. They achieved efficiencies of about 80-Percent at pitch ratio of about 1.5. The tunnel tests were in the speed range of 11 to 25-Feet per Second.

These propellers had nothing in common with current surface propeller designs. A half wet surface propeller needs about the square root of 2 more diameter to match the thrust of a fully submerged propeller. In most current surface propeller designs I have seen, this thrust deficit is made up with extreme cup. The cup kills low speed performance. So current surface propeller designs perform best with very fast, low disk loading installations.

A surface propeller will also produce much bow down force, with peak force at about 50-Percent wet. This often gives significant trim problems.

If the design is executed well there will be efficiency gains. But it is easy to loose the advantage with small design errors.

[sandhammaren05]

PS When the outfit is set up with a surfacing prop the cup certainly does not kill low speed performance. An uncupped prop will run very inefficiently at all planning speeds, low and high.

[brunello]

Originally Posted by dgruenwald

We have a need for a very low speed surfacing propeller design. The application will be in the top speed range of 23-30 mph. Does anyone know where we might go for expertise in this area?

Do have a look at the "Surface propeller and low speed displacement boat " thread, you might find a lot of interesting (and different) opinions.
We at Flexitab feel that surface propulsion is perfectly suitable for low speed vessels, this is why we tested a ship (clearly not the one mentioned by others in this thread) in the naples tank test, were surface props outperformed well-tuned submerged ones. (see pictures attached in the "Surface propeller and low speed displacement boat " thread).

Regards, Brunello

[sandhammaren05]

Originally Posted by bananas

The U.S. Navy developed some high speed surface propellers that were tested at low speeds in a cavitation tunnel. They achieved efficiencies of about 80-Percent at pitch ratio of about 1.5. The tunnel tests were in the speed range of 11 to 25-Feet per Second.

These propellers had nothing in common with current surface propeller designs. A half wet surface propeller needs about the square root of 2 more diameter to match the thrust of a fully submerged propeller. In most current surface propeller designs I have seen, this thrust deficit is made up with extreme cup. The cup kills low speed performance. So current surface propeller designs perform best with very fast, low disk loading installations.

A surface propeller will also produce much bow down force, with peak force at about 50-Percent wet. This often gives significant trim problems.

If the design is executed well there will be efficiency gains. But it is easy to loose the advantage with small design errors.

PS
Cup increases bow lift. Contrary to theory, which is pretty piecemeal, we do not run sq rt 2 more diameter with surfacing props in racing, we run the same diameter or less. I've wondered (naive question): for top speed with light load, is thrust the right parameter to maximize? I'm not sure. Thanks for the post!

[Simdrive]

Yes, go to WWW.simplicity-marine.com they have been using there surface drives for tow boats and parasail boat for years.

Alan