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post #1 of 10 Old 10-06-2006 Thread Starter
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Transmission setting while sailing

I always thought that the inboard engine transmission setting while sailing was supposed to be neutral to allow the propellor to feather. I just saw, on one of my pictures of boats, a sticker next to the throttle that reads
"To lock the Prop shaft
Warning
When sailing select ASTERN only to lock the propellor
SELECT AHEAD WILL DAMAGE THE GEARBOX"

Does this mean they wish the prop to be locked in reverse, or if a lock is to be done (I have no idea why this would need to be done, perhaps to reduce the noise of rotation) then only in reverse.

Does anyone have more detailed information on this?
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post #2 of 10 Old 10-06-2006
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My previous boat was a Catalina 34. This was discussed extensively on the C34 website. Most our boats had fixed three bladed props, so feathering wasn't the issue. While sailing with the transmission in forward or neutral, some of the gear train was spinning while the gear box was locked when the gear shift was put in reverse. I don't remember the reason "why" the manufacture suggested locking the transmission but I suspect it was probably a lubrication issue.

Our current boat has a Maxi-prop feathering propellor on it. When making the transition from motoring to sailing it can sometimes take a couple of attempts to feather the propellor so the shaft doesn't spin while sailing. As long as the shaft isn't spinning with a feathering propellor, I don't think it matters what shift position the selector is in, as long as you continue to monitor whether the shaft is spinning or not (we can hear it on our boat)

Ray
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post #3 of 10 Old 10-06-2006
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Locked Prop

I just read an article the other day which showed empirical proof that a windmilling prop will produce approx twice the drag of a locked one. This is also true in air where it is vital for control to feather the prop on a dead engine if it windmills. If the engine siezes and you therefore cannot feather the prop it is much less of a problem. Yes, the prop blades create turbulent flow in the water in one of the worst places for such turbulence (just aft of the keel and just ahead of the rudder) but nowhwere near as much as a wind milling prop. Just look at true efficiency numbers on props and reverse the principal. Props are designed to pull/push water not to be rotated by it. Water turbines (axial) use an entirely different plade profile/shape/pitch. Take a look at the spinner on a shaft log.
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post #4 of 10 Old 10-06-2006
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oops

I just read my last post (which I should have done before I posted) and it is enormously confusing unless you understand that when I was referring to air I meant aircraft which work on the same exact phisics as sailboats.

sorry bout that!
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post #5 of 10 Old 10-06-2006
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This topic was discussed in a recent thread

There is some excellent information including Navy research on this topic in the following thread.


http://www.sailnet.com/forums/seaman...=prop+spinning
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post #6 of 10 Old 10-06-2006
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Some transmissions are designed to be cooled by the engine raw water while the engine is running, and they will overheat and be damaged if the transmission is spinning without the coolant flow. So, if the prop is auto-rotating while you are sailing with the engine off---it turns the transmission and damages it. (I think the actual problem is that internal parts are "glazed" from the heat damage, and the fluid itself overheats and is damaged.)

For these transmissions, it is important to put the shift to reverse, so you lock the prop and don't turn the transmission while sailing.

With a folding prop, locking the shaft this way also ensures the prop will stay folded and not flop down to create extra drag, regardless of whether the transmission requires it. (You'd normally mark the prop shaft so you can visually align it to "blades closed, facing sideways" before locking the shaft or putting it into reverse to lock it.)
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post #7 of 10 Old 10-06-2006 Thread Starter
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Quote:
Originally Posted by feetup
I...when I was referring to air I meant aircraft which work on the same exact phisics as sailboats...sorry bout that!
Actually it is important to understand that the physics are quite different as airflow is compressible while hydraulic is not. I haven't seen the 2x drag numbers on a windmilling prop but can understand what underlies the turbulent flow behind an unused boat propellor. I guess I have trouble understanding how a speed of 8Kn on a very small prop could do damage to an automatic transmission (unlike not feathering a failed engine at 190Kn on a Baron).

The odd thing is that I sailed a sistership to the one with the sign with the gear in neutral and you could hear the prop spinning while underdeck. Perhaps that one had a different transmission.
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post #8 of 10 Old 10-06-2006
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Practical Sailor had an article in May 7, 1987 issue. one survey by a reader gave emperical data. The conclusion was that at lower speeds lock the prop & let it freewheel at higher speeds. but freewheeling does cause wear in the cutless bearing & trans thrust bearings even if lubrication in the trans is not a factor.
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post #9 of 10 Old 10-08-2006
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With engine off, the Volvo MD17c (1977 motor) will stop the prop if shifted to reverse. It won't stop it if "ahead" is selected, and it will windmill like crazy. When it windmills, the noise will drive you insane, and it will wear the stern bearing.

Perhaps some gearboxes need the motor running to feed the lubricant. The MD17c has a splash lubricated gearbox and does not need the motor running to lube it.

I could imagine some serious and rapid wear if a 'box is allowed to windmill and it needed the engine to be running to feed the lubricant.

I think it best to stop the shaft spinning. For me, it would be too noisy if it did.

On a long haul, I try to line the two blade prop with the keel to minimise drag.
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post #10 of 10 Old 10-13-2006
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It really depends on the transmission if you can freewheel the prop. Some transmissions are fine with it, some other transmissions with lubrication pumps driven off the engine should not be freewheeled due to lack of lubrication. Which gear the the transmission should when you lock the shaft depends on the transmission. Reading the manual for your transmission is the only way to know for sure what is safe for your transmission.
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