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Old 12-02-2012, 03:06   #1
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Gear Ratio- How Do You Know?

We are re-powering our 1976 Ford which has no markings on the gearbox with a new Yanmar 4JH5E and note that the new Yanmar is offered for sale with different gearbox options/ratios.

How do we know which would be the correct gearbox to choose....do we tell the supplier our boat is 39 ft and displaces 11 tonnes and they figure it out or do we hand turn the crankshaft to see how many times it turns before the prop goes around once....???

Please help..
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Old 12-02-2012, 03:52   #2
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Re: Gear Ratio- How Do You Know?

Hi Greg and Debi,
surely the Yanmar dealer will be able to suggest you the right gear ratio for your boat considering lenght, displacement, draft, beam and so on. The Yanmar dealer I think will also consider your actual prop and pitch and will eventually a new one. Anyway to check the Ford ratio moving manually the crankshaft as you have written is correct. Hope this will help.
Max
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Old 12-02-2012, 06:05   #3
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Re: Gear Ratio- How Do You Know?

Were you happy with the performance with the old engine? Are you planning to keep the same propeller? If so, the prop rpm probably needs to stay the same. Otherwise the gear ratio and the prop should be chosen together. Dave Gerr's book (http://www.amazon.com/Propeller-Hand...9051586&sr=1-1) has all the formulas and instructions for doing this, but a good prop shop or engine dealer should be able to help. I would also contact Michigan Wheel (Home // Michigan Wheel Marine). They were quite helpful when I was repowering my boat a few years ago.
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Old 12-02-2012, 06:07   #4
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Re: Gear Ratio- How Do You Know?

Agree with MAX59. You should check the ratio of the old gearbox, just as you suggested. Work out as much of this as you can on your own, and you'll be in a better position to evaluate what the dealer is telling you.

Every boat is different, and dealers' experience levels vary, so it's important to take the time to understand as much as you can about the trade-offs you are making with your boat and your money.

I posted comments on the trans. in my response to your other post, where you asked about the generator.
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Old 12-02-2012, 14:49   #5
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Re: Gear Ratio- How Do You Know?

Go to a prop seller and have them do a calculation for prop size for each ratio available for your new engine. One of the may be the same prop that you have already. Even if it is, the chances are that you will need to have the pitch altered a little to fine tune it in use.
If it was me, I would go for the lowest ratio available if the prop applicable to that ratio would fit in the aperture, as the slower the prop , the more efficient it is and the less wear on shaft bushes etc.
Regards,
Richard.
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Old 21-02-2012, 14:09   #6
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Re: Gear Ratio- How Do You Know?

Hi Please could someone advise if there is such a thing as a shaft extension used in lieu of a new longer shaft? Are they recommended and are there in fact pro's and con's?
Would you need to, or is it advised to then also use two flexible couplings, one on either side of the extension?
Really appreciate some advice as our new engine is way smaller than the huge old cast iron Ford we had and will sit too far aft, in fact into our aft cabin cupboard...
Thanks Greg
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Old 21-02-2012, 14:19   #7
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Re: Gear Ratio- How Do You Know?

I didn't use a CV joint when we repowered because I didn't have room for it in the configuration on our boat... But if I had YOUR problem, that my shaft wasn't quite long enough, I would have put one in.

Check these guys out
Welcome to Aquadrive

It would fill in the gap in the length of your shaft as well as providing for one of the best if not the best solutions to the engine alignment problem. It also is supposed to quiet things down.
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Old 21-02-2012, 14:20   #8
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Re: Gear Ratio- How Do You Know?

best to get a new,correct lenth shaft made up

or use an aquadrive,but they are very expensive
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Old 26-02-2012, 11:15   #9
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Re: Gear Ratio- How Do You Know?

The data plate fell off a Velvet drive transmission long before I bought my current boat. I wanted to know the transmission ratio so I bought one of those optical RPM meters for something like $15 us.

Then with the transmission in gear measure the engine rpm and then the shaft rpm. Do the division and there you have the transmission ratio.

Good for checking the engine RPM gauge too.
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Old 26-02-2012, 11:31   #10
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Re: Gear Ratio- How Do You Know?

Re. shaft extensions / CV axles:
If you're a cheapskate and know your way around a machine shop, you should be able to make something similar to an Aquadrive out of a thrust bearing and a front-drive car's half axle for a few hundred dollars plus some lathe/mill time.

Re. gear ratio:
This calculator ( Vicprop - Prop calculator for Displacement and semi-displacement hulls ) will give you a decent first guess at prop size for a given boat, engine and gear. If you want to keep your current prop, try tweaking the gear ratio until it spits out something close to what you already have.
The gear ratio of your old engine doesn't matter; the choice for the new engine should be made based on the new engine's ratings, the available propeller options, and the performance characteristics of your boat.
I'll second Ziggy's recommendation of Gerr's Propeller Handbook if you want to work through all the math properly.
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Old 26-02-2012, 11:54   #11
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Re: Gear Ratio- How Do You Know?

When out and about cruising and maybe not having Internet access, it would be good to have the simple calculations tucked away someplace. A typical marine small diesel will produce peak hp at around 2900 rpm and peak torque at around 1600 rpm.

If you have a 2 to 1 reduction in your gear box, then your top hp rpm at the prop is 1450 rpm X pitch (say 10"). So 1450 X 10 X 60 / 12 / 6072 = 11.9 kts with no slip. For best (least) fuel burn, cruise at 1600 rpm, so again 800 X 10 X 60 / 12 / 6072 = 6.5 kt of speed no slip. All speeds of course slower after you apply slip.
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Old 26-02-2012, 14:51   #12
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Re: Gear Ratio- How Do You Know?

Quote:
Originally Posted by deckofficer View Post
[...] A typical marine small diesel will produce peak hp at around 2900 rpm and peak torque at around 1600 rpm[...]
Peak horsepower is almost always at peak rpm. Peak torque varies from engine to engine.
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