Down in pitch, up in diameter What effect would it have??

[badly bent or broke]

On my 75 HP Force/Mercury I have a 10 3/8" Dia, 14 Pitch aluminum and am keeping an eye out for a deal on a spare prop. I'm a newbie so I don't have anything to compare performance to but the boat gets up on plane and will do 36 mph (GPS) at 5100 RPM (within spec range).
On EBay I see a 11 1/4" Dia, 10 Pitch aluminum and wonder what the net result would be if I went to this prop? I understand that decreasing pitch will increase RPM but wouldn't increasing diameter offset the increase in RPM?
Would the lower pitch/larger diameter improve hole shot, top end or equal itself out?

 

[jerryjerry05]

Re: Down in pitch, up in diameter What effect would it have??

You might have clearance problems with a bigger prop and performance problems too.Your hole shot would be better but probably no top end.
It sounds like what you have is about as good as it's gonna get.If you really want to reprop go to a dealer and ask about stainless they will tell you what size and kind to buy.
Aluminum is more forgiving in mud and rocks.They only loose ears when a stainless might take out propshafts and everything.Jerry

 

[racer-x]

Re: Down in pitch, up in diameter What effect would it have??

From what I have read, generally speaking the lower the pitch, your rpm's will increase with each change.

The blade shape and size will determine the amount of drag/resistance the prop will encounter as it pushes against the water.

I would guess that you might get a little better bottom end push with the larger blade/smaller pitch but the top end might suffer as it tries to push the larger blade through the water.

If your boat is only turning 5100 rpm now, it's in kind of a "sweet spot" for that engine. If you kept the same size diameter, and went down in pitch you might gain a little top end if the rpm's come up a few hundred.

I think that's why people go with stainless though. Not as much prop deformation, and more push at any given rpm.

At least that's the impression I was left with after reading about SS vs. aluminum..... and it's a real science because not all boats seem to respond well to the same prop/change in size. I have never seen a test where they inceased the diameter, and generally the prop for the application was within a half inch from the different manufacturers.

 

[steelespike]

Re: Down in pitch, up in diameter What effect would it have??

If you go down to a 10" pitch you will gain about 800 rpm(way too much!)
Generally Diameter is a product of prop design and only considered regarding clearance.If you got to a 12" aluminum it will give you about 400 rpm putting you at 5,500.This would do a better job staying within rpm range with a load.What boat are you pushing?
You may want to consider your setup before making a prop change.If you can raise the motor Vertically you can gain some rpm and speed how much depends on where it is at now.

 

[badly bent or broke]

Re: Down in pitch, up in diameter What effect would it have??

Thanks for the responses guys. It sounds like even if I got a good deal on this prop it wouldn't be very satisfactory. I guess I suspected that but your input confirms it.