gears how to know

[icedogs66]

I have a 120 hp force that the leg failed. I took the boat in and the price was crazy to rebuild the mechanic said he could put a used one on, now that this was done my boat seems slower I can barely keep up with my dads 70 hp johnson on roughly the same size boat. does the leg from a 85hp force have a different gear ratio than a 120 and how would I verify this.

 

[pnwboat]

Re: gears how to know

Depending on the year of your 120HP motor, and the year of the 85HP leg, it may have a different ratio.

 

[jerryjerry05]

Re: gears how to know

Most definatly a different gear ratio.
You'll need to reprop.
Go for a ride and check the rpm's.
Post the size prop you have and rpm's.

 

[pnwboat]

Re: gears how to know

Depending on the year, the 120HP motors came with a 2.07:1 gear ratio. 85HP motors have a 2:1 gear ratio. Shouldn't be much change in performance. If the leg that was installed on your motor caused a decrease in performance, I suspect that it may actually be a 125HP leg that has a 1.73:1 ratio.

 

[icedogs66]

Re: gears how to know

I took it out and reached just shy of 7500 rpm, prop is a 15 pitch stainless. would the 1.73:1 ratio not give a higher top end? right now my boat stays even with a 70hp i get a better hole shot but the other boaat stays par with me going across the lake before I used to eat him up and get acrross and start fishing before he even got in sight.

 

[pnwboat]

Re: gears how to know

7500 RPMs seems a bit high. I turn my 125HP up to 6000, but that's about it. Maximum manufacturer recommended RPM is 5500 RPMs. I suspect that maybe you had a 1.73:1 leg and now you have a 2:1??? I don't know but 7500 RPMs is pretty high. Sounds like you are under propped. You might need to switch to a 17 pitch stainless prop to get your top end back.

It's been my experience with the setup that I've got that the 1.73:1 gear ratio gave me a poorer hole shot and less top end. That is with a stainless cupped 17 pitch prop. Each hull design and motor set up is a little different for everyone, so my situation may not necessarily apply to yours. I switched to the 2:1 gear case and haven't looked back. I used to use a 15 pitch stainless cupped prop with the 1.73:1 gear case, but since I switched to the 2:1 leg, the motor turns way above max recommended RPMs. Now I use a 17 pitch cupped stainless for most of my boating. Max RPMs with this prop is 6000. I also have a 19 pitch cupped stainless for long cruising. Max RPMs is 5000 with this prop. Hole shot is not as good though.

Bottom line, I think you are under propped if you're turning close to 7500 RPMs.

 

[jerryjerry05]

Re: gears how to know

7500 is about 2000 more than you need.
If it's a true reading,some tachs don't read right.
A 17 stainless is about $400new.
So get a used 19 and see how much it drops.
New it's about $115
A 15 pitch prop is usually used on heavier boats and pontoons.
With the right pitch you should(depending on the boat size) go a lot faster.
You need to figure out the year it came from?
Post some pics?

 

[ferdly1]

Re: gears how to know

wow 7500 rom. You definitely need to change pitch on your prop. You can do damage to motor by over reving engine.

 

[icedogs66]

Re: gears how to know

hit the wrong key guys rpm is 6500. got 2 props from shop (loaners) 19 and 21. 21 is to much and 19 seems to be the one to use. thanks for all the help but would still like to know how to find the gear ratio of the leg.

 

[pnwboat]

Re: gears how to know

To find the gear ratio of the leg, with the ignition key in the OFF position and the throttle pushed forward (to engage the gears), turn the fly wheel clockwise (as viewed looking down at it). Turn the flywheel a couple of revolutions to make sure the clutch dog has engaged. Then count the number of turns it takes the flywheel to turn the propeller one revolution. If it takes the flywheel about 1 3/4 turns to make the propeller turn 1 revolution, then you have a 1.73:1 gear case. If it takes two turns of the flywheel to turn the prop 1 revolution, then you have a 2:1 gear case.

Note: I would try the 17 pitch too, just to see how it runs.