1985 90Hp Johnson on waay to big deck boat

[69protec]

Hello all,

long time listener/first time caller

We have a 22', 13 passenger 1985 Hurricane deck boat; very heavy and loaded often.
The stock engine was a 185HP Johnson commercial (lower compression, high torque) motor.
Last week the VRO system malfunctioned (I changed oil line and did not bleed) causing extensive damage.
After looking at the rebuild cost on a 26 year old power head I found a 1985 90hp version of the exact same motor.
Spent a day pulling 185Hp motor (it is for sale ) and installed the 90HP.
Disconnected VRO and am premixing 50:1.
I am concerned about the prop selection as I know the barge is underpowered by about 150HP.
We will be using the boat as a "mother ship" for jet skis.
It will act like a pontoon boat and I plan to cruse at very low speed most of the day.
I know I need to order the correct prop for this set-up but have no idea what to get.
"help me Obi-Wan Kenobi"

 

[Texasmark]

Re: 1985 90Hp Johnson on waay to big deck boat

The 185 may have had a 2:1, 1.85:1, 1.75:1 gearbox. The 90 probably has a 2.3:1. What difference does this make? Where to start on looking for the right prop.

My SWAG would be to multiply the engine hp reduction to the pitch of the prop you have on the big engine. So you have .49 the hp you had so get .49 the pitch you had for starters buttttttt.....say you had a 17P initially, then you would consider a drop to a .49 x 17 = 8" pitch.

But you have to take the gear ratio into consideration. Say it had a 2:1 and your 90 has a 2.3:1 then that is a 15% reduction in prop revs vs engine revs which (could) translate into a 15% increase in the estimated pitch or instead of 8" pitch you could go with 9" of pitch. If the engine had a tighter gear ratio (like the others I mentioned), the pitch would be less by the ratios as referenced.

It is a long shot for you to get it right the first time and the next problem is finding a 13+ for the 90 and 14+ for the 185 in diameter prop in that shallow of a pitch.

My 2c,

Good Luck,

Mark

 

[Texasmark]

Re: 1985 90Hp Johnson on waay to big deck boat

I had a lot of spare time today and your problem intrigued me. So I spent considerable time surfing the www for a prop for you.....if my numbers are right and if not, you can bet someone will jump in and correct me....but that's ok. We're after the right answer here.

I found a site: USBoatSupply.com. Going to the J-E (OMC) section and looking at their "Hydrus". They had a listing for 3 cyl, or V4 engines in the 60-115 hp range which is just where your 90 is. There you will find 13 7/8 x 9" or 11", and another higher pitch I forgot (but wasn't interested in anyway). The item no for the 9" is 177201 and for the 11" is 177202, both priced at $201.70. I think this will solve your problem and I would guess the 9" would be your best bet.

Good Luck,

Mark

 

[steelespike]

Re: 1985 90Hp Johnson on waay to big deck boat

Mark very likely has it figured right and I'll second the 9" prop.
If it turns out to be a little light you can still use it and watch the rpm.
If the 11 turned out to be too much you just shouldn't use it.
Both props could be modified to slightly raise or lower rpm if necessary.