1997 Johnson 25hp to 35hp 2 stroke 3 cyl. conversion - prop decision

**speedfreak** · 08-01-2011

OK, got the air (or lack of) thing worked out (opened up the air box). I took the boat out this evening with a GPS and my tiny tach, here are the results. 13 pitch, stainless stock OMC prop, 6300 RPM (WOT), 32.5 MPH.

Then tried a 15 pitch, aluminum stock OMC prop, 5850 RPM (WOT), 32.0 MPH. I guess the 15p aluminum was slipping, not quit sure what to make out of those numbers.

This little motor calls for 5200 - 5800 as the WOT RPM range. I'm thinking a 14 stainless with a fairly aggressive cup, might be the sweet spot. I would like to keep the R's in the 6's, that way when I load it up hunting (weight wise) I still have some power.

Boat is a 1548 aluminum flat bottom modified V (duck hunting boat). Was really hoping to get this boat up to the 35 MPH range with the conversion, not sure it's going to happen. This was a 25hp conversion to a 35 HP project. Running a 25:1 mix instead of the recommended 50:1.

Any thoughts!!!!!!!!!!! Am I thinking about this right???????

**JohnsonM50** · 12-31-2011

Originally Posted by speedfreak

OK, got the air (or lack of) thing worked out (opened up the air box). I took the boat out this evening with a GPS and my tiny tach, here are the results. 13 pitch, stainless stock OMC prop, 6300 RPM (WOT), 32.5 MPH.

Then tried a 15 pitch, aluminum stock OMC prop, 5850 RPM (WOT), 32.0 MPH. I guess the 15p aluminum was slipping, not quit sure what to make out of those numbers.

This little motor calls for 5200 - 5800 as the WOT RPM range. I'm thinking a 14 stainless with a fairly aggressive cup, might be the sweet spot. I would like to keep the R's in the 6's, that way when I load it up hunting (weight wise) I still have some power.

Boat is a 1548 aluminum flat bottom modified V (duck hunting boat). Was really hoping to get this boat up to the 35 MPH range with the conversion, not sure it's going to happen. This was a 25hp conversion to a 35 HP project. Running a 25:1 mix instead of the recommended 50:1.

Any thoughts!!!!!!!!!!! Am I thinking about this right???????

Seems to me you should be able to get 35 or a little more being that close. Look at weight & its distribution. The hull, absolutely straight bottom? Are there strakes causing drag? If all good there the probable reason for the AL-prop's performance is the thickness of the blades, SS being about 5x stronger can be made thinner with very good results. Maybe a custom cut prop, your guess at 14 or even a 15SS?, testing alone will tell. Good Luck.

**speedfreak** · 12-31-2011

Thanks JohnsonM50,

Going to go up to a 14p SS after duck season is over and see where that puts me. Weight distribution is good. I moved the 6 gallon fuel tank to the front, and have a Shorai lithium iron battery in the back. It is a 540CCA battery that weighs only 4.96 lbs. Not sure what it is going to take to get it to 35mph. May not be able to. The gear ratio is what hurts me a little 2.15 to 1 on those Johnsons vs. Suzukis and Tohatsu's gear ratios.

**JohnsonM50** · 12-31-2011

Originally Posted by speedfreak

Thanks JohnsonM50,

Going to go up to a 14p SS after duck season is over and see where that puts me. Weight distribution is good. I moved the 6 gallon fuel tank to the front, and have a Shorai lithium iron battery in the back. It is a 540CCA battery that weighs only 4.96 lbs. Not sure what it is going to take to get it to 35mph. May not be able to. The gear ratio is what hurts me a little 2.15 to 1 on those Johnsons vs. Suzukis and Tohatsu's gear ratios.

That's true as well as your motor is designed to push a varied load at a moderate speed for a really long time. That being said the lower units on these are big, the gears & shafts big, the pump large, etc etc. That all uses up energy but still as close as you are 'the' right prop should do it.

**speedfreak** · 12-31-2011

Agree, and for what I use it for it is perfect (the beefy lower unit). I'll keep you posted as I experiment with the prop to try and reach that 35mph number!

**chris3298** · 12-31-2011

I think what you're doing is pretty dam good. my 15x52 V alweld with a 50 tohatsu and 11x14 prop I can do 39.1. If you can do 32 what you have now is well pretty dang fast

**captbone** · 01-02-2012

I would get a good 15pitch SST OMC wheel. Have it labbed and it should run pretty good. I would also raise the engine up on the transom 1/2-3/4 inch.

25:1 oil ratio seems crazy to me. Not even insane race engines run that. 50:1 should be fine in my opinion.

**speedfreak** · 01-03-2012

Going try a 14P OMC first see where that puts me and go from there. I already have it raised off the transom 1 1/2" and have went back to 50:1 ratio. I saw no difference (performance wise) from 25:1 to 50:1, other than it didn't smoke as bad.

**JohnsonM50** · 01-03-2012

Originally Posted by speedfreak

Going try a 14P OMC first see where that puts me and go from there. I already have it raised off the transom 1 1/2" and have went back to 50:1 ratio. I saw no difference (performance wise) from 25:1 to 50:1, other than it didn't smoke as bad.

Oil, If you have to endure 'go-slow' zones like around here

excess oil could foul up the works I guess but if your mostly WOT then probably not, Whats right? [the 64 Dollar question

]. If your within the stock motors rated RPM's then 'book' mix is fine. When you run 1000-1500 R's above, then more for sure. 32:1 seems to be a happy medium or even more on a hi value vintage motor.

**Mark Poole** · 01-05-2012

Keep in mine that all OMC portable motors (35 and under) were made to run on crap fuel. They told us at school one year that small OMC's can run down to 70 octane. Lot of room for carb and timing mods knowing that. Also, no need to run extra oil in one of these little motors especially if you are not turning it carzy rpm's. Use a thinner, 75W 90 synthetic gearoil instead of 80w90 for longer gearlife and a few more rpm's.