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Old 14-07-2014, 22:21   #31
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Re: Reverse Gear while Sailing?

On my 3GM30 with a v-drive and three blade prop, locking in reverse creates a .5 knot drag over freewheeling while sailing at ~5 knots. Modern yanmars can be left in nrutral or reverse. On the older QM series with the Kanzaki/Hurth transmission, yanmar recommended reverse only.

The difference in resistance between an air plane prop and a 3 blade prop on a boat is angle /pitch, surface area, fluid density and of course diameter of the prop. Air plane props create more drag when freewheeling, Boat props (3 blade) create more drag when still.
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Old 14-07-2014, 23:05   #32
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Re: Reverse Gear while Sailing?

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Originally Posted by sailorchic34 View Post
On my 3GM30 with a v-drive and three blade prop, locking in reverse creates a .5 knot drag over freewheeling while sailing at ~5 knots. Modern yanmars can be left in nrutral or reverse. On the older QM series with the Kanzaki/Hurth transmission, yanmar recommended reverse only.

The difference in resistance between an air plane prop and a 3 blade prop on a boat is angle /pitch, surface area, fluid density and of course diameter of the prop. Air plane props create more drag when freewheeling, Boat props (3 blade) create more drag when still.
+1

Panope had a QM15 that we sailed for 18 years with a locked (in reverse) 2 blade prop aligned behind the dead wood (errr, dead metal).

Now have a JH3 series engine that I am not happy to use reverse while sailing. When sailing above 4 knots with the (3 blade) prop stopped, it is impossible to move the gear selector out of reverse. I just let that prop spin now. No worries.

Steve
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Old 15-07-2014, 06:31   #33
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Re: Reverse Gear while Sailing?

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Modern yanmars can be left in nrutral or reverse. On the older QM series with the Kanzaki/Hurth transmission, yanmar recommended reverse only.
Not according to Yanmar... See the official Yanmar MSA Here their words not mine..

"Yanmar requires that if sailing with the engine OFF (not running) the transmission shifter must be in the neutral position or internal damage to the gear or sail-drive will result."
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Old 15-07-2014, 08:22   #34
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Re: Reverse Gear while Sailing?

This question may never be resolved!!! But it's always interesting...

Interestingly, I just brought a 5 year-old Beneteau from BVI to FL, and on the shift lever was a a sticker with a clear statement from Yanmar that the prop should never be locked in forward, only in reverse. It didn't discuss the option of neutral.

Regarding which one creates less drag, and with total recognition of the fact that theory and reality are only distantly related in most cases, the reason a prop spins in theory when an external force (wind/water) is applied parallel to the axis of rotation is that it is trying to minimize resistance. Using deductive reasoning, one is forced to conclude that impairing the ability of the prop to spin must, therefore, increase resistance (drag). Of course, in the complex reality of an aerodynamic or fluid dynamic system, there may be other factors that modify this relationship, but in my professional opinion (I'm a scientist, not a physicist, but I've had plenty of physics), it's hard to see how you could completely invert these effects. Just my two cents. PS: I let my prop spin freely when under sail. Pete
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Old 15-07-2014, 09:00   #35
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Re: Reverse Gear while Sailing?

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I had a steel boat with a velvet drive transmission. The first passage (New England, Bermuda) almost drove me nuts with the howling of the prop. I didnt need a knot meter, I could tell by the howl. In Bermuda, I rigged a shaft break (a hook bolted to the coupling) that used a light line, so that it could break if I forgot to release it when putting the engine in gear. The speed didnt matter, compared to the noise. I had another larger boat, with a velvet drive, but the noise was only serious in the aft cabin, which I didnt use much at sea. My next boat will have a folder or featherer, for the silence and the slightly better speed on all points of sail. ____Just my opinion._____Grant.

Someone else on here said they made a shaft brake and used golf T's as a shear pin, I thought that was quite smart as I can see a golf T as being real cheap, readily available and an almost perfect shear pin that wouldn't damage anything.
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Old 15-07-2014, 09:02   #36
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Re: Reverse Gear while Sailing?

As far as a shaft alternator, I can see the gearing being interesting, maybe compound pulleys? Don't alternators need to be around 3,000 RPM to operate correctly?
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