Prop for 91 Evinrude 15HP

[Dizz]

My small bass boat came with 15 HP,and from what I could tell it has the power prop,OEM 9.5 x 10.​It planes easily with 2 big boys.
It will do between 13-15 mph.
I bought a Solas 9.25x11".From what I see on the new prop page this is the most speed prop available,and it gains about 1mph.
Would a 4 blade do any better,or same?
I know you can expect just so much from this small motor,and it is fast enough,but trying to learn about props here.
Back in 90 I had a new 14 Smoker Kraft jon,and a new Johnson 15 HP,and it would do about 23mph,but,I know it is much lighter than the 15' SeaNymph I`m pushing now with same basic motor.
Many Thanks for ideas!

 

[jimmbo]

I doubt very much that you will see any increase in speed. The higher pitch prop will place a larger load on the engine and will reduce the WOT rpm. Unless your engine is revving more than 6000 rpms with the 10 inch you will not see any benefit from the 11 inch. The 11 will be a slower accellerator

 

[steelespike]

What year is the motor?Is the speed by gps?
You over looked one key prop fact. When you increase the pitch it makes it harder for the motor to turn.
11" is like being in overdrive and trying to climb a hill.
Depending on your wot(wide open throttle) rpm rating you need a lower pitch prop.I think a 9" is a good starting point.
It's critical that the motor be able to operate freely within its wot rpm rating.

 

[steelespike]

1991, 15 is rated for 5500 to 7000 rpm.

 

[Dizz]

Standard prop for this 91 model is 9.25x10.On this 15' plywood floor bass boat with swivel bases,casting front deck,450# of people,all gear,it planes in seconds.Motor sounds like it is running fast,no tach.GPS speed.When I put the 11" prop on,it sounds the same to me,but not sure of the speed,I think last year I found it gained 1mph,but the 10" is OMC,the 11" is Solas,but Solas is .25" less diameter.The 11" does not make it sluggish to plane.
I was thinking of also putting this motor on my 12' wide jon boat,hence the higher pitch prop.
Those 9.9/15hp 2 stroke OMC outboards are studs,I think,and I have a hard time using even 2gal of gas a day,unless I try to WOT all day.
On the other hand,I have a LN 2000 Erude 6hp,put the 8hp carb on it,and it is very weak I think.
Point is the OMC 9.9 is much more planing power than the 8hp.

 

[steelespike]

I should explain about prop selection.
Marine engines work very hard like a car pulling a trailer up a never ending hill.
The motor must be able to operate within its wot rpm range.Your 91 is 5500 to 7000.
At wot it must be within 5500- 7000 rpm.To much pitch and the motor will lug and perform
poorly.It may develop premature repair problems. To low pitch will result in over reving which could result
in innards coming outside unless you throttle back.Once you establish it makes the required wot rpm you can operate at what ever rpm you like.
Using a prop calculator we can guess your wot rpm. Not the best idea but its a start. Your at about 4500. Your 11" prop is likely even worse.
Your motor setup may need some help.
An induction tach would help get the best setting.Motor needs to be in excellent tune.
The antivent plate(just above the prop)needs to be about even with the bottom of the transom.

 

[Dizz]

Sure sounds like it is running in it`s happy place,with either prop.
I will check the height of the antivent plate,but this engine uses the standard motor transom clamps,but I can see where it could be adjusted.
I`ve had this rig at least 15 years,motor runs great,and has very high compression,electric start.It will troll for hours,I just run it WOT now&again.Has had the same spark plugs for over 5 years,it does not foul them,but I clean them if needed.
I will figure out a tach set-up and get back to you.
Many thanks,Dizz

 

[Dizz]

I see looking at available props for this 91 15 hp motor that much lower pitch props are available,7,8 and 9",so I`m in the high pitch range,with a heavy boat to boot.​when I`m running WOT for longer trips,I usually back off the throttle a tad,thinking I`m gutting my motor full rpm`s.Have to get that tach!

 

[steelespike]

You will be amazed what the motor can do with the right prop.
Its good you ease off but with the 10 and 11 your still going up hill in over drive.
That reminds me I think a 8" might be a better starting point.
If it turns out too light you can still control rpm with the throttle.
It should have very strong hole shot stay on plane easier at reduced throttle.
If we go for a 9" and its too much you shouldn't use it for extended periods.
A careful lightly loaded wot run with the 10" and a tach.may help determine a prop choice.

 

[ttankmoran]

That 15 needs to be able to use its RPM. You went the complete opposite direction in prop pitch, you should have gone down to a 8 or 9 pitch. These engines really come alive 5500 rpm and higher!

 

[Sea Rider]

Are you sure that OB wot revs 5500 - 7000 rpm, better check 91 Owners Manual if you happen to have one. If that's a 250 CC OB should rev 5000-6000 revs.

Happy Boating

 

[steelespike]

My info indicates 81 - 93 15hp Evinrude 5500 -7000. 2 separate sources

 

[Sea Rider]

OK, just for the record, how many CC is that 91 model ? Check if possible to achieve max 7K wot rpm playing with prop pitches along an induction tach while boating lightly loaded to get the max water performance out of that combo.

Happy Boating

 

[Dizz]

OK,maybe I did go the wrong direction.The manual that came with this motor listed 2 props,the 10" mine came with,and a 11",I assumed was for lighter 14' basic jon boats. ​Having trouble finding the manual,but google search looks like this is a 216cc,imfo sketchy.
If my 15' bass boat with this 15hp,with this 10" prop,2 people and some gear planes off in seconds,had me thinking I`m OK,as my 8hp 96 ERude 2 stroke takes forever to plane off in my Alumnicraft 1236 jon with 2 people,and with much gear forget it.
What tach do I need to be looking for,where to get?

 

[steelespike]

There are a number of induction tachs out there I think some one will help.
It is important that you run the wires so they don't pickup stray pulses.

 

[Sea Rider]

Would recommend a Hardline Hour/Tach model

https://www.hardlineproducts.com/pro...ourtach-meter/

Check Amazon, probably bit cheaper

Easy to install, no grounding needed, can cut single wire excess once conveniently routed inside OB

Fire sequence programmable

Can be installed on tiller near throttle grip.

Happy Boating