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Old 04-06-2010, 03:32   #1
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Spinning Props

Leaving aside, please, the much debated question of whether it is more efficient to sail with a spinning or a fixed prop...... how much stress does it cause on a gearbox to stop a spinning prop by slipping it into gear with the engine not running?

Is it the case that, if you leave your prop spinning you are wearing the bearings and shaft, and if you park it with the gearbox you are doing that no good either.

And I'm not talking about a day sail, more of an ocean passage where this might be an issue over several thousand miles.
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Old 04-06-2010, 03:40   #2
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No more stress than putting in in gear with the engine running. It's designed to go into gear with one or the other spinning (engine or prop)
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Old 04-06-2010, 16:10   #3
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Over the short term, letting prop windmill (watermill?) or freewheel won't be a big issue, but on a long extended voyage, it does add wear to your cutlass bearing, stuffing box or shaft seals,and probably to the gear box seals as well. While this wear is not any worse, and probably less so, than cruising under power there is really no reason to accumulate extra wear when not actually using your engine. It would probably be a good idea to simply put a brake on your drive shaft or leave it in gear so the engine acts as a brake and prevents rotation. I have heard that letting the prop freewheel causes less drag than having the shaft locked, but my belief is that the amount of drag remains the same whether the prop is freewheeling or locked in place; the only difference is that left to freewheel, the drag is merely converting energy into prop rotation but not producing any thrust to cancel out the drag effect.
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Old 04-06-2010, 16:54   #4
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Freewheeling the prop will generate a minimal amount of thrust in the reverse direction since you are dragging it through the water and result in a minimal amount of heat being generated in the transmission.Next time you go for a lonigsh sail go below and feel the transmission to see how much heat is actually produced. The low load and heat generated means that the oil viscosity should remain high and ensure a good oil film remains between the moving parts keeping wear to a minimum. No metal to metal contact = no wear. The heat that is produced is mostly generated by the churning of the oil.
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Old 04-06-2010, 16:58   #5
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It's possible to let the prop spin and collect back that energy I think... might make what ever small wear there is worth it.
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Old 04-06-2010, 17:20   #6
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Good point. Maybe connect a small alternator to the shaft and generate some juice for the house.
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Old 04-06-2010, 17:30   #7
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One could, if you had a means of transferring the rotational power to a low speed generator (by belt or some such), use the freewheeling prop to produce a small amount of electricity. In this instance, the prop and shaft act as an impeller, but considering wear to stuffing glands and bearings, one would be better off with a purpose built impeller generator which would produce more current at less mechanical wear to your main propulsion. A shaft brake and a feathering prop, with a purpose built water gen to convert your boats motion produced energy to electricity makes much more sense to me.
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Old 04-06-2010, 17:43   #8
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No problem stopping the prop. There is far less power on the prop trying to crank over the engine than the engine cranking the prop.
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Old 04-06-2010, 18:09   #9
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Free wheeling or locked the controlling factor is your transmission manufacturer's manual. Some transmission have no problems with free-wheeling and other transmission manufacturers tell you to lock down the shaft so it does not spin.
- - Considering drag caused by cutlass bearings, shaft seals and internal bearings in the transmission - recovering any electrical energy from a free-wheeling propeller is minimal to none. A water generator with a free propeller dragged behind the boat is a better deal.
- - Some free-wheeling propellers make loud and squealing noises as they spin which can really "grate" on your nerves underway. And think of the creatures like whales, etc. that have to listen to your "screaming" free-wheeling propeller telling Henny Youngman jokes in Whale language. Folding or feathering propellers solve this problem nicely and completely.
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Old 05-06-2010, 12:16   #10
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It's my understanding that you're a little more efficient with it locked. That said, if you have a mechanical transmission, it's OK to lock it by putting it in gear (generally reverse). My new transmission is mechanical and so I can do that and sailing is quiet! I hated the noise of the spinning prop under sail. OTOH, if you have a hydraulic transmission, I understand that you're not supposed to put it in gear....have to use a brake or a pair of vice grips.
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Old 05-06-2010, 19:15   #11
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Next to zero damage. Imagine the box works loaded 99 percent of the time.

b.
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Old 05-06-2010, 20:58   #12
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Using a prop brake is the best way to stop a propeller and hold it stopped underway. Using the transmission works but the energy /torque imparted to the the shaft from the propeller is transmitted - through the transmission - to your engine. Fine so far, but on the fly wheel of the engine is a "dampner plate" with consists of two plates with springs holding them together. As the torque on the propeller varies with speed, lurches over waves, etc. those springs are being compressed and stretched. How long they last is unknown as nobody has ever done any studies on them - but - you may just shorten the life of the dampner plate by not using a shaft brake. And it is a bitch to change that plate.
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Old 08-09-2010, 15:13   #13
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There was a thread here a couple of years back that seemed pretty comprehensive. Net results:

-- Allowing the shaft to spin causes less drag than locking it. One poster did reasonably well controlled tests with a rig he created just for that experiment.
(found an active link to the test here )
-- There are some transmissions that require you to lock the shaft
-- There are some transmissions that require you to not lock the shaft, so you need to RTM for your own case.
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Old 08-09-2010, 16:30   #14
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Quote:
Originally Posted by gs41escapade View Post
There was a thread here a couple of years back that seemed pretty comprehensive. Net results:

-- Allowing the shaft to spin causes less drag than locking it. One poster did reasonably well controlled tests with a rig he created just for that experiment.
(found an active link to the test here )
-- There are some transmissions that require you to lock the shaft
-- There are some transmissions that require you to not lock the shaft, so you need to RTM for your own case.
I think the last statement might be better phrased as some transmissions do not need to be locked and can "free-wheel." But locking them with a shaft lock - not just by putting the engine in gear - is okay. After all when the boat is anchored or on the hard and not moving the propeller is not rotating.
- - The obvious best answer is to use a folding or feathering propeller and remove the drag and rotating shaft problems.
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Old 08-09-2010, 16:34   #15
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Of course, to feather properly, most designs I am aware of require the shaft to be locked (except for hydraulic, manually controlled props.
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