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Old 04-07-2008, 19:00   #46
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I think you answered it perfectly and better than all of us put together. The answer is....It depends!
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Old 04-07-2008, 22:36   #47
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Midland, with the purpose of contining this conversation rather than debating any particular points I add the following comments.
BTW, I have really enjoyed all the posts to this thread and thank the OP and everyone else for their comments.

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.... But as has been said, as far as aviation is concerned, is well proven in air that freewheeling props of the aviation type have more drag than those not freewheeling. As said by others it possibly depends on the boat, prop and drive - especially when you see so many sail boats with big elliptical bladed "power boat" type props.
Of course in the case of a fixed pitch airplane prop, free wheeling is a bit of a mismomer considering the prop is bolted to the engine and has to turn the engine over as well. Akin to the boat tranny in ahead and the engine being spun over while sailing although I accept there is a reasonable difference in the compression ratio's between the respective engines. The pilot has only one option to stop the a fixed pitch prop windmilling, slow the airplane down until the prop stops turning and keep it stopped. Refer Ex's link for greater detail.

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... The worst I can imaging is that the drag, when locked, is something of the order of being similar to as if 3 men on board took to dragging one hand each through the water as we sailed along. Not something I would panic about on a cruising boat from a performance point of view - we have no rule on board about not dragging hands in the water .
Good point
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...While I have on a few short occasions tried watching the log comparing with the prop freewheeling and when it is locked, any advantage one way or another was lost in the "noise" of speed fluctuations from waves, wind velocity changes, etc so I quickly lost interest.
This has been my experience, Sully must have a faster, better boat or can sail better than me - actually it's probably all three , lucky bugger

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.. Might be different if just ghosting along in little wind and flat sea and tried the same test, but that is when you use the engine anyway; Is it not? .
I can't do that test as my prop doesn't rotate even if in netural when ghosting along, just not enough force on it from the water (a bit like the pilot who slows the plane down isn't it). This usually the time I start thinking about if only I had...[insert light weather sailing gear of choice] or more diesel on board .

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...Which leads to we lock it with the gearbox just because of the noise if it freewheels ...
I just hate that noise.....
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Old 05-07-2008, 04:04   #48
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Of course in the case of a fixed pitch airplane prop, free wheeling is a bit of a mismomer considering the prop is bolted to the engine and has to turn the engine over as well.
Yes, but that isn't the example I would have in mind as it is so obviously different (unless trying to jump start a sail boat engine ) -

Examples - the freewheeling unit in a helicopter which disengages engine (but not transmission) for auto-rotation, or the rotor (no engine or gearbox) on an autogyro.

In my previous post I was just noting the matter for a field where alot of experimentation has been done, not making any claim that any aviation case proved any particular sail boat propeller case. Really just had in mind putting to one side the difficulty of accepting that it is possible for a freewheeling prop to have more drag than a locked one. Obviously there are many differences between aircraft propellers/rotors and sail boat propellers, such as aspect ratio and the fluid they operate in, and mechanical ones as well.

Similarly, the examples above are not meant to prove one thing or another with respect to sail boat props, just examples of freewheeling props which have more drag than locked ones by whatever means they go about achieving that.

John
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Old 05-07-2008, 19:40   #49
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I think for "your" boat it will depend. I read a cruising book recently and the skipper was very detailed about keeping a log. After something like 5,000 miles he locked the prop due to a mechanical proble and noticed the boat speed went up. Over the nest X thousand miles he averaged more than a know faster while sailing. This was a big 60+ foot heavy ketch.

If I were about to do 18,000 miles I'd probably want to know. Especially the trade off between wear on the running gear and speed through the water. If the wear is higher, the speed is slower and all then locking it would be a good thing. Also even on 2,000 mile passages it could ad up to a day or 2 more on the water and faster passages are better, if only to avoid weather changes.

We have a max prop feathering prop. It is the way to go IMO.

And once again - Do what the gear manufacturer recommends first...
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Old 12-07-2008, 03:49   #50
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Quote:
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I think you answered it perfectly and better than all of us put together. The answer is....It depends!
Yes it depends...... The rule of thumb is if the transmission has it's own independent lubricating oil sump then freewheel. If not, lock the transmission. My Yanmar manuals say to free wheel.

As far as performance is concerned, there is no comparison as free wheeling is much better on my boat. I have two engines and two (2 blade) props, and when they are locked in reverse it's a 1 knot penalty versus free wheeling.
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Old 29-01-2009, 20:57   #51
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The manual to my Volvo saildrives say to let freewheel too. Hmmm.., Never noticed much difference except it's noisier when freewheeling.
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Old 30-01-2009, 05:22   #52
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OK, outside of the fluid dynamics the transmission type and manufacturers recommendation is important. This can probably be generalized as follows.

Mechanical Transmission: Splash lubricated - does not matter whether locked of free wheeled unless specified by the manufacturer.

Hydraulic Transmission: Pressure lubricated by a pump on the input shaft. The engine must be running to maintain lubrication - Transmission should be locked.
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Old 30-01-2009, 08:57   #53
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Kevin, do you have fixed props?
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