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Old 16-09-2015, 09:35   #1
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Changing gear ratios

A question for those more knowledgable then me.

I have a hurth transmission in need of a rebuild, came with a Volvo D2-40, gear ratio is 2.14. Bought a rebuild kit and its the wrong size, I am in a place where getting parts is time consuming and difficult.

It just so happens someone here has a new hurth of the same size, will bolt right on, but the gear ratio is 1.85.

Both the gear boxes are "15" size hurths, both turn the same direction, and everything lines up. So how drastic would a change from 2.14 to 1.85 be if I decided to use this transmission rather then get the right rebuild kit (which would cost $1000 and take a month or more to get). Would it be so drastic that the prop would be in absolute need of repitching? Or something I get away with for a few years at least until I was someplace that actually has a prop shop?
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Old 16-09-2015, 09:54   #2
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Re: Changing gear ratios

That is a pretty big change. You might not be able to pull max rpm and it would load up the engine at full power. If you just want to poke around at reduced power you could probably live with it. Kind of like being stuck in overdrive. Not sure if changing prop pitch will work as it will change the power band the engine will operate in.
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Old 16-09-2015, 10:02   #3
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Re: Changing gear ratios

Another possibility, that I would rather not do if i dont have to, I could take this new transmission, pull the plates out of it and put them in the old transmission, best i can tell looking at sites that sell the rebuild kits, the different gear ratios of the same model transmission still use the same plate sizes.
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Old 16-09-2015, 10:19   #4
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Re: Changing gear ratios

That sounds like a better plan. Plus you get to know your transmission.
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Old 16-09-2015, 10:25   #5
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Re: Changing gear ratios

You will need to change your prop pitch... at least at some point I imagine. But it may depend on how you were propped before. It seems you will experience about a 15% lower rpm than before right?
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Old 16-09-2015, 10:54   #6
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Re: Changing gear ratios

at engine 2000 rpm at
2.14=934.6 prop rpm
1.85=1081.0 prop rpm 146.4 faster
or at engine 1500 :
2.14=700.9 prop rpm and at
1.85=811.0 prop rpm 110.1 faster
depending on HP and prop pitch can your engine take the difference it will spin faster at normal engine cruising revs. As you push the engine you may not have the HP to reach a efficient working load on the engine. You may have to re-pitch the prop if that is with in the range of the prop?? Pitching the prop to run faster may cause cavitations.. I wrestled with a similar problem I phoned Michigan Wheel the propeller manufactures and got a ear full of great information and worked out my problem.. too much information missing to predict a healthy solution. Prop size, aperture, engine hp , boat stats, they have the questions to ask you to get you started If you do the re-build make sure you use ATF fluid not oil or the plates wont work. luck
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Old 16-09-2015, 11:04   #7
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Re: Changing gear ratios

I do have a smaller prop from when I had the previous 4-108 in it, I can possibly put that on as well as it should be a better match to the higher RPM of the prop. There is no prop shops anywhere around here, so repitching is not an option right now (but can be done in the future).
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Old 16-09-2015, 15:59   #8
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Re: Changing gear ratios

You may have to shorten the blades and/or change the prop pitch. Blade tip speed will be higher and may cause problems. Can you get a prop worked on where you are at??

If you are over pitched with the new prop it's not the end of the world. You may actually get a more efficient cruise speed after the change. You will lose high rpm power so bucking into a headsea/wind could be a challenge though motor sailing can ameliorate the effect. Cruised for a year with too large a prop. Would motor at a fuel sipping 5 knots in our W32 but engine would begin to lug and smoke if it was pushed to higher rpm, wouldn't go much faster and wouldn't run up to max rated rpm. Didn't hurt the motor as it lasted more than 20 years and 3 cruises to SoPac.
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Old 16-09-2015, 16:54   #9
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Re: Changing gear ratios

Yeah, that's why I asked what engine and current max rpm is. If it was pitched to reach max rpm, some prop people recommend 10% less than max anyway. And the new ratio he's looking at is 15%.....
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