right prop size for my boat

[jasper60103]

Hi, All.
I was hoping you can help me choose the best prop for my boat. The motor runs fine and the boat planes, but its not reaching the WOT operating RPM range.
The performance data I provide is based on me riding alone and
that will normally be the case.

Do you think I should step down to 10" or 9" pitch,
or leave it alone?

thanks, -jasper

1) Performance issue you are trying to correct.

I'm trying to get my motor in the WOT RPM operating range of 4500-5500.

2) Current prop manufacturer, model, aluminum or stainless and as a minimum.

Johnson, Part Number: 0389944, Aluminum

3) Current prop diameter and pitch (required).

9.25 x 12

4) Wide open throttle RPM and speed with an average load (very helpful)

4100-4200 RPM and 19-20mph

5) Engine/drive make, model, year, and HP

Johnson 1981 25hp

6) Boat make model, year, length and weight

Crestliner 16' Voyager Deluxe, 1977, 400lbs.

 

[steelespike]

Re: right prop size for my boat

Looking at your picture it appears your motor is quite low.
Antivent(cavitation) plate even with the bottom is considered a starting point.
You could raise it by adding 1/4' sticks to the top of the transom.
Sticks simply hold the height as you adjust the clamps.
Raise it to a point just before venting(cavitation) becomes a problem.
Just be sure it pumps water and clamps have a good grip.
With all being ideal could add a couple of mph and some rpm.
Be sure the throttle is opening all the way.And in excellent tune.
It would be best to fine tune your setup before getting a prop.
Your present prop is the biggest listed on iboats
Your motor is listed at 4500 to 5,500 I would shoot for closer to 5,500.
I think I would try a 10" prop.the 2" change should put you much closer
to your max rating. You should check out the Turning Point Rascal I think available for smaller motors.Close to ss. performance at al. prices.
These props usually give an overall improvement and resist venting
allowing higher motor settings. I feel with careful setup and prop selection
you should be close to 27 mph If your not too heavy yourself.

 

[Bifflefan]

Re: right prop size for my boat

The answer above is about the best one ive heard...ever.
Cant get better than that...

 

[jasper60103]

Re: right prop size for my boat

Ok, I'll start with my setup as you recommend. The cavitation plate may be an inch or so below. I have to double check. Thanks, steelespike !

 

[jasper60103]

Re: right prop size for my boat

I checked the service manual and says the 11" pitch prop comes standard with the motor. I assume the previous owner had a light setup and changed to a 12" pitch. I have a second prop, but unfortunately its a 12" pitch as well.

I think a 10" pitch as you recommended seems about right. I'm near 300# and that boat is a good size tinnie.

Also, the angle of the pic is misleading, I don't think my motor is that low, but I will double check.

Thanks again for your analysis.

 

[junior1113]

Re: right prop size for my boat

old rpm x old pitch times divided by new pitch equals new rpm. this only work if both props are similar in geometry.
4200X12/10=5040 new rpm with 10" pitch

 

[steelespike]

Re: right prop size for my boat

If your heavy I would move the tank as far forward as possible.
Maybe rig a seat about where that box is on the side of the boat and use a tiller extension.The boat will plane much easier and stay on plane at lower power.Probably run faster in rough conditions.Just simple plywood attached at the stern seat extending forward
with a simple hinged plywood leg to support it.Might be able to rig it so you can take down easily and store behind the stern seat.Regardless It will be interesting to see how you make out with your improvements.

 

[jasper60103]

Re: right prop size for my boat

steelespike, I will experiment with the setup in the Spring. Unfortunately I had to put the rig put away for winter. Its getting cold up here.

I still want to try a 10" prop too. I think a 12p is too big for this boat hauling my big butt. I called tc outboards and they have some used ones for about $45. That will save me some money.

junior1113, thanks for the calculation. Hopefully a 10p and minor setup changes will get us there.

Oh, here's a clip from the last time out. She runs good and like the way the boat handles. It would be be nice to get up to WOT rpm and gain a few mph out of her though.

http://www.youtube.com/watch?v=NxNhDIXohq4

 

[walleyehed]

Re: right prop size for my boat

How about the tach??? No mention of verifying the tach yet....
If we don't know it to be spot-on we're waisting time guessing.

"4200X12/10=5040 new rpm with 10" pitch".......WoW!!!!
this IS an old formula, but if you had a cupped 12 and a straight 10, this would be close. It would take a lot more lift to get the 10 to turn 850rpm more than a 12, and 5000rpm will kill that engine almost as fast as 41-4200 will if thats the "true" rpm.
Check the tach first...at Idle AND WOT...50 off at Idle may be 500-700 on top.