What to expect from changing from prop diam 9 to 8.25 ?

[pecheux]

Regarding 1997 Force 15hp. Presently uses 9 diag X 8 pitch prop. what would I expect in performance difference if I used a 8.25 dia X 6 pitch ? (one available for purchase)

Tx

 

[steelespike]

Re: What to expect from changing from prop diam 9 to 8.25 ?

Generally the 200 rpm per inch doesn't apply to small outboards.
I couldn't hazard a guess except very likely in excess of 200 rpm per inch.
I wouldn't be surprised to see 700 or 800 rpm increase.
Final results would depend on the present max rpm.
While I'm a little surprised to see the diameter go down as the pitch goes down
it isn't likely to affect the rpm much.

 

[pecheux]

Re: What to expect from changing from prop diam 9 to 8.25 ?

Generally the 200 rpm per inch doesn't apply to small outboards.
I couldn't hazard a guess except very likely in excess of 200 rpm per inch.
I wouldn't be surprised to see 700 or 800 rpm increase.
Final results would depend on the present max rpm.
While I'm a little surprised to see the diameter go down as the pitch goes down
it isn't likely to affect the rpm much.

My typo error ... the prop available is a 8 diam X 9.25 pitch. I just found out the standard prop for that motor would be a 9 X 9 So either I will order that one or the 8 X 9.25 depending on results gain or loss evalutations from my actual 9 X 8 to one of the two available.

I dont think this motor could use higher rpm but I could use a little faster top speed conditionaly I dont loose much on the hole shot. TX

 

[steelespike]

Re: What to expect from changing from prop diam 9 to 8.25 ?

If we don't have any present max rpm and speed we don't even know for sure if you should go
either up or down.Refresh our memory what boat,dry weight? and typical load.Do we have a rpm and speed?Your going up an inch and 1/4 from your present prop.I would expect 200 to 400 lower rpm.If its doing really well on the 8" then the reduction may be toward the lesser side.
If it strugglres with the 8 then very likely toward the larger reduction.

 

[pecheux]

Re: What to expect from changing from prop diam 9 to 8.25 ?

Tx Steelespike, I guess I dont fully understand the logistik or outboard propellers. I had figured out that with a 1 inch reduction on diameter (from 9 to 8) and an increase of 1.25 inch on the pitch (8 X 9 1/4) would have resulted in a 1/4 inch total prop gain only from the 9 X 8 presently mounted.

 

[steelespike]

Re: What to expect from changing from prop diam 9 to 8.25 ?

Generally as pitch goes up daimeter goes down A 1 1/4" increase is signifigant in a small outboard
and the decrease in diameter 1" while goodsized pretty much follows the norm. And I wouldn't expect more than 50-100 rpm related to diameter.
Diameter is seldom figured directly in prop selection and usually involves extreme loads like pontoons, house boats or large cruisers. The 9 1/2 is only an inch less than the largest prop listed at iboats.

 

[pecheux]

Re: What to expect from changing from prop diam 9 to 8.25 ?

I think I got it from your explanations. Pitch is what I have to keep in mind, not diameter. Therefore that 9 1/4 pitch prop available on Ebay would be too much of an increase from the 8" pitch prop already in use. In a perfect world I would find one that would be 8 1/2 pitch to get that little bit more speed considering the 9" pitch is the standard pitch (as far as I found on the net)
Thank you and have a great day.

 

[pecheux]

Re: What to expect from changing from prop diam 9 to 8.25 ?

I finaly got the data on the prop mounted on my 15 hp Force (1997) It's a model # P715 8 3/8 X 6 Great hole shot, good power, but lacks top speed on a 14 footer fiberglass boat (avatar) I would then ask experts what prop model would be just one step up ? Iboat advertises 9" diam props but I can see there is no room for a prop having 9" diameter since the actual one is only 1/4 of an inch from the top plate. Tx

 

[worx]

Re: What to expect from changing from prop diam 9 to 8.25 ?

I am having the opposite problem...I am try ing to push a 14 fiberglass with a 15 Evinrude & it has a 9.5 x10 & it wont even plane,tries but cant get over the hump.:facepalm:

 

[pecheux]

Re: What to expect from changing from prop diam 9 to 8.25 ?

I am having the opposite problem...I am try ing to push a 14 fiberglass with a 15 Evinrude & it has a 9.5 x10 & it wont even plane,tries but cant get over the hump.:facepalm:

I also own a Johnson 9.9 hp with the same prop you have ... and it planes good and fast when I am alone in the boat. Back in 1981 or so had a similar 14 footer (same as my avatar) with a Johnson 15 hp and it would plane with the wife & I + 2 kids in the boat ... cant understand why yours does not ???

 

[pecheux]

Re: What to expect from changing from prop diam 9 to 8.25 ?

My appologies to everbody trying to help with prop size exchange since I came up with the wrong prop size to start with. Written on the prop was what I thought was 9x8 which later turned out to be 8x6 (reversed reading) after taking the prop out and reading part number P 715 (8.25 X 6)

Still would like an upgrade to a higher pitch and my options would be a 9x7 on Iboat or a 8.25 x 8 used. Cant figure out what would be best yet.
Thank you.

 

[steelespike]

Re: What to expect from changing from prop diam 9 to 8.25 ?

I am having the opposite problem...I am try ing to push a 14 fiberglass with a 15 Evinrude & it has a 9.5 x10 & it wont even plane,tries but cant get over the hump.:facepalm:

Are you sure it is running on both cylinders.You wouldn't be the first to not know except for the lack of performance.

 

[worx]

Re: What to expect from changing from prop diam 9 to 8.25 ?

Are you sure it is running on both cylinders.You wouldn't be the first to not know except for the lack of performance.

Comp 120 each cyl & new NGK plugs,with spark checker light it is showing spark on both cyls..... Runs strong but wont break plane.