15hp high thrust prop ? Johnson 2 stroke

[elkhunter338]

I have a 2001 15hp Johnson 2 stroke I use as a kicker on my 21' boat.
At WOT the current prop gets the engine to about 4600 rpms, wot should be above 5000 rpms.
The current prop is a 9.25x9 4 blade, this prop works great on my 14' boat with 2-3 people.

My prop choice to go to is a 10x5 or 10x7 4 blade, gaining diameter. do I go with a 5 or 7 pitch.

Any real world advise on changing props on kicker motors welcome.
Thanks,

 

[steelespike]

Re: 15hp high thrust prop ? Johnson 2 stroke

I would think a 7" would get you into the 5000 area. I wouldn't think you will get much increase in speed but the motor will be happier.

 

[elkhunter338]

Re: 15hp high thrust prop ? Johnson 2 stroke

I would think a 7" would get you into the 5000 area. I wouldn't think you will get much increase in speed but the motor will be happier.

So you think the gain in diameter from 9.25 to 10 won't fully off set going from a 9 pitch to 7 pitch, to were my rpms essentially stay the same. I figure gps speed is going to stay the same, but won't be lugging the motor at wot.

My fear is the 10x5 will be too shallow of a pitch.

 

[steelespike]

Re: 15hp high thrust prop ? Johnson 2 stroke

Generally as pitch decreases the diameter increases. Don't worry about the diameter.
If the diameter increase fully affected the pitch decrease you could never achieve your required rpm.
I think the fact that the boat is never going to plane will affect the rpm result more than the diameter.

 

[elkhunter338]

Re: 15hp high thrust prop ? Johnson 2 stroke

I found 2 propellers.
10x5 4 blade and a 10x7 3 blade, or go on iboats for the 10x7 4 blade.

the other two are about 20-30 cheaper than the 10x7 4 blade off iboats.
I need to gain a good 600 rpms, my current prop is the 9.25x9 4 blade

 

[steelespike]

Re: 15hp high thrust prop ? Johnson 2 stroke

Being the lowest pitch available the 5" probably assures the fullest rpm increase.
The Solas props have a good reputation for predictable performance.
I would stick with them.I do feel a 4 blade will grip better and give a little better control.

 

[trendsetter240]

Re: 15hp high thrust prop ? Johnson 2 stroke

I'm thinking the 5" pitch will be too small. You will have to rev the motor quite a bit higher to maintain trolling speed. 7" should be about right for the setup you described.

Remember, when you are dealing with props that are 5, 7 and 9 inch pitch the changes are double what you would see with a 15, 17 or 19" pitch prop change.

Going from 9" to 7" is a 23% reduction in pitch.

Going from a 19" to a 17" is a 11% reduction in pitch.

The change from 9" to 7" should increase your top RPM by 400-500.

 

[elkhunter338]

Re: 15hp high thrust prop ? Johnson 2 stroke

I'm thinking the 5" pitch will be too small. You will have to rev the motor quite a bit higher to maintain trolling speed. 7" should be about right for the setup you described.

Remember, when you are dealing with props that are 5, 7 and 9 inch pitch the changes are double what you would see with a 15, 17 or 19" pitch prop change.

Going from 9" to 7" is a 23% reduction in pitch.

Going from a 19" to a 17" is a 11% reduction in pitch.

The change from 9" to 7" should increase your top RPM by 400-500.

Good point, I should have thought of that. I am an engineer type and that type of calculation should have been at my finger tips. I too am thinking the 5 pitch is too shallow.

Now Narrowed down to a Michigan Wheel 10x7 3 blade $60
Or the solas 10x7 4 blade $76 plus shipping
I am thinking the 4 blade is better, but the high thrust yamaha use a 3 blade prop, honda uses 4 blade 10x7
looking at 2 stroke vs. 4 stroke.

 

[trendsetter240]

Re: 15hp high thrust prop ? Johnson 2 stroke

Good point, I should have thought of that. I am an engineer type and that type of calculation should have been at my finger tips. I too am thinking the 5 pitch is too shallow.

Now Narrowed down to a Michigan Wheel 10x7 3 blade $60
Or the solas 10x7 4 blade $76 plus shipping
I am thinking the 4 blade is better, but the high thrust yamaha use a 3 blade prop, honda uses 4 blade 10x7
looking at 2 stroke vs. 4 stroke.

I'd recommend the 10x7 4 blade solas. It is designed for high thrust applications like yours.

 

[elkhunter338]

Re: 15hp high thrust prop ? Johnson 2 stroke

I plan to go with the 10x7 solas and will post my results after I get the chance to run the motor. Now just have to order a prop from iboats, they appear to have the best deal.