a little dumbfounded

[kidneyboy]

I have a 17' 1985 four winns with a 3.8l omc that has been freshly rebuilt. The boat came with 2 original omc 15 1/2 X 15" props and they worked ok but would wind up to 5000+ so changed to a michigan match 15 1/2 X 15" which actually measures 15 3/4" in diameter, and it was a bit sluggish on hole shot but would wind up to around 5000 rpm also and get about 37mph on a gps. Then I bought a solas amita 14.8" x 17" prop and I have a much better hole shot and I also get about 42 mph at 4500 rpm at 5000 ft elevation here in CO. Can anyone explain why I get better performance from the steeper pitch prop.

 

[kidneyboy]

Re: a little dumbfounded

Also this may sound dumb but even though the boat will get up to 42 mph at 4500 rpm I typically cruise with the tach at 3500 - 4000. I guess my question is everyone talks about wot, they don't run all day or extended runs at wot. It just seems that it's probably hard on an engine to run balls to wall for its entire life.

 

[MikDee]

Re: a little dumbfounded

Probably because you don't need a 15 1/2" diameter prop, or the total large area of it, due to your boat not being all that heavy. Thus, freeing up horsepower to spin that higher pitch prop, and get better use of the bite of it.

On my 19' Bayliner outboard, the normal prop on my 125hp Force outboard is a 13" diameter X 17" pitch, (no bigger diameter needed) and it does 40mph.

 

[steelespike]

Re: a little dumbfounded

I have a 17' 1985 four winns with a 3.8l omc that has been freshly rebuilt. The boat came with 2 original omc 15 1/2 X 15" props and they worked ok but would wind up to 5000+ so changed to a michigan match 15 1/2 X 15" which actually measures 15 3/4" in diameter, and it was a bit sluggish on hole shot but would wind up to around 5000 rpm also and get about 37mph on a gps. Then I bought a solas amita 14.8" x 17" prop and I have a much better hole shot and I also get about 42 mph at 4500 rpm at 5000 ft elevation here in CO. Can anyone explain why I get better performance from the steeper pitch prop.

My guess is the prop is almost 1" less in diameter the would produce a noticeable difference in performance.All else being equal a smaller diameter prop is easier to turn.It also appears it is more efficient producing more speed at less rpm.perhaps it is cupped.I'm sure one of the experts can explain Solas and Michigan prop design.
The constant reference to wot is necessary because that is a fixed point to measure engine performance.If the motor operates at its max rpm and speed with a particular prop then it will operate at it's peak rpm for any given speed.Practical cruising speed can be at just over planing speed to as much a 2,000 rpm higher.Assuming that the slowest planing speed produces the best mpg is inaccurate.Constant operation at wot will certainly shorten engine life.