propeller calculations

[Waltthesalt]

Anyone with experince with using a Michigan Wheel prop recommendations? I'm doing a Atomic-4 to diesel repower. I've gotten Michigan Wheel's calculation for the prop needed. When I check thier recommendation against calculations from Gerr's "Propeller Handbook" I find that the prop will be well over the cavitation limit at fuil load. I've rechecked the calcs several times.
Question: Have you found the MWC recommendations to be reliable in this area?

[tommays]

Props are as much art as science

For example on my powerboat we use 17" 19" 20" and 21" in 3 and 4 blade depending on whats going on that day in terms of boat use and load

I thing your best bet is finding and owners grope for your boat and see what sizes are working for them

[roline]

You want to use the largest diameter with adaquate hull clearance. The best that I could do was 60% effieiency with a 40% prop slip. 14D by 12P with 2GM20, Cal 9.2. You just have to calculate shaft RPM and pitch against hull speed to calculate it.

[Maine Sail]

Originally Posted by Waltthesalt

Anyone with experince with using a Michigan Wheel prop recommendations? I'm doing a Atomic-4 to diesel repower. I've gotten Michigan Wheel's calculation for the prop needed. When I check thier recommendation against calculations from Gerr's "Propeller Handbook" I find that the prop will be well over the cavitation limit at fuil load. I've rechecked the calcs several times.
Question: Have you found the MWC recommendations to be reliable in this area?

Walt,

Contrary to popular beliefs and misconceptions and Dave Gerr prop sizing is NOT an exact science especially on a displacement hull. In order to hit max RPM, and size the prop to do that without going over or under by much, is not easy and often takes two or more attempts to get it spot on. I can remember working with my old friend Brian, a marine surveyor, who always checked the prop sizing against max rated RPM. We found that close to half the boats had the wrong size prop.

You want be sure that you get a prop that will allow your motor to turn max rated RPM.

Universal says this;

"Universal recommends a propeller that will allow the engine to turn 3000 RPM underway at full throttle."

That specs is for the M-25 and can be seen here: http://www.westerbeke.com/ProductBro...%20M-25XPB.pdf


If you are looking for a fixed blade prop I would highly recommend the Campbell Sailor Prop, sold and manufactured by Norm at West By North. He will calculate the sizing for you and he is usually fairly close. You'll ideally want to be within 50+/- RPM of max rated RPM with a clean bottom and prop.