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Author Topic:   Slow Prop?
Dr T posted 09-29-2001 09:38 PM ET (US)   Profile for Dr T   Send Email to Dr T  
This thread orignally started in a discussion of the right prop for a Sport 13 with a 1982 Johnson 35 HP. (See http://continuouswave.com/ubb/Forum4/HTML/000361.html

I have recently tested three props on this boat: a OMC OEM 11x9 aluminum, a Quicksilver 10.5x11 aluminum, and an old OMC OEM (?) 10x13. The tests were made at Boyd Lake (about 30 miles north of Denver) at 5400 ft. altitude with winds less than 5 MPH. The trim position for the 11x9 was the middle hole only, for the other two I used both the middle and next to top holes. Speed was indicated by a paddlewheel. Boat load was me (270 lbs) with one fairly full fuel tank.

Results:

11x9 22 mph 5800 rpm Pull the kids.
10.5x11
Middle Hole 24 mph 5000 rpm
2nd Hole 28 mph 5600 rpm Best all around
10x13
Middle hole 24 mph 4000 rpm
2nd hole 28 mph 4600 rpm Bow-wow

Given other comments on props, I would have expected the 10x13 to turn at at least 5100 rpm at the best trim angle. Also, I experienced mild porpoising with this prop that I did not experience with the other two.

Clearly, the 10x13 is a real bowser on the boat. (I must say it never did perform that well close to sea level).

The question is why. Are some 10x13 props that inferior in design or did I just manage to pass a critical point of non-linearity in motor performance at this altitude?

I am very interested in your comments on this one.

Dick posted 09-29-2001 10:29 PM ET (US)     Profile for Dick  Send Email to Dick     
Lots of things to consider here. At your altitude it will take a lower pitch prop than at sea level. All 13, 11, etc pitch props are not the same some are cupped and some are not. In most applications a cupped prop will give better performance. OMCs prop technology is a long way behind Mercurys. They built some great motors but didn't do their homework in the prop department.
Looks like that Merc 11 pitch is the way to go.

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