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Old 02-09-2007, 04:46   #16
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Freewheeling the prop is best

bruadair I find that simply to not be the case. My experience tallys almost exactly with the MIT research paper on this subject, which was written because of all the false information out there on this subject. I guess it's easier for me to see this since I have twice the prop resisitance of mono guys. I'd like to be clear that I'm not trying to convince anyone, and just letting you know what I've discovered. Like Alan said, we've all been there and done that already but it might be best if you read about this for yourself. I would recommend that you do a search for an online copy of:

"Comparison of Ten Sailboat Propellers" by Beth Lurie and Todd Taylor of the MIT Marine Hydrodynamics Laboratory. The charts and graphs are all there.

Freewheeling the prop produces the least amount of drag and results in higher boat speeds, of this there is no argument. The real issue is whether your engine/ tranny setup can do this without damage. In the end we have to comply with our engine manuals.
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Old 16-09-2007, 00:05   #17
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Quote:
Originally Posted by Alan Wheeler View Post
This is because a Propellor is a form of a "Rotary Wing". Each blade is a wing and once water flows over it's surface correctly, it creates "lift" just like any wing. If a prop is allowed to free wheel in the water, then it will act in the same way a Helicopter uses Auto rotate to stop it from going into the ground when the engine fails. The drag on the boat is created as the speed of propellor rotation increases. As it increases, more resistance to the rotation is applied by friction from the gearbox. So the propellor becomes drag to the boat, just like the helicopter blade. It is less noticable at slow speeds and greatly increases as speed increases.
If the propellor is locked, then water flows over the blade in the wrong way and the blade "stalls". This has a reverse affect. At low speed, the drag of the stalled propellor is large. At a higher speed, the drag is less noticable to the increased force from the wind.

I won't go into the issue of in/out of gear. As Rick505 said, there has been a lot discussed int he past and a search will reveal much information.
Hmm, thanks for that. This is the first time I have seen a plausible explanation of this phenomenon. I first read about it in one of the books of Nigel Calder, but he did not give any explanation.
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Old 16-09-2007, 09:06   #18
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Hmmm.

I have an OMC saildrive and I've been putting in neutral. In fact we usually put it in neutral and then pull out the wee black button that lets it start, whatever that's called.

Haven't tried it with the transmission engaged.

Anybody have any opinions on the OMC saildrive? Can't remember what the manual says, if it even mentions the issue.

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Old 18-09-2007, 08:34   #19
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All of the charterers recommend that the tranny be put in reverse, to lock the prop when sailing. Including Yanmars.

And yes the Yanmars are a bitch to get out of reverse to start the engine, more often you have to start the engine in reverse and then put it in neutral, then forward to pull in the sails.

The question of which has more drag a stopped prop or a turning prop, can not be answered in general, depending on the speed, the prop, and the setting, some may have more drag stopped then rotating or vice versa. It also depends on the amount of tranny drag there may be since this removes power from the prop system.

Indeed properly set-up a prop should be able to develop about some 60% of it forward power if rotated at the proper speed, into a generator arrangement. (freewheeling is of sourse different than this).

When using axial pumps has turbines the power curves for various set-ups often cross each other.
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Old 18-09-2007, 19:19   #20
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This is like religion...

It is transmission and prop specific. Do what works for you and your equipment.
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Old 18-09-2007, 19:49   #21
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the definitive research

Another expert opinion!!

Here's the definitive research on the subject.

The MIT paper

Please, only refute with your own research documentation
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Old 18-09-2007, 20:09   #22
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Another expert opinion!!

Here's the definitive research on the subject.

The MIT paper

Please, only refute with your own research documentation
Rick - This paper gets cited as the difinitive work everytime this subject comes up. It should be noted that they did not test under freewheeling conditions due to the limitations of their test rig. They extrapolated and calculated the data. I am not sying they are wrong. I am just saying it is not conclusive.

It's the first paragraph on page 19.
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Old 18-09-2007, 23:28   #23
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Dan

It's the only non biased research which has been published on the subject. Fortunately the research was done by a lab with impecable credentials, and in 13 years this report hasn't been refuted. I would think that ought to be conclusive enough in anyone's book. If MIT wrote it, I have no reason to question their methods, or conclusions and am a little surprised that anyone would.

I mean, come on, it's MIT..... not some fly by night outfit or rumors spread by marina rats.

As I said earlier, this is the definitive work on the subject. If you'd like to tell us that their scientists, methods, and conclusions are all wrong, all I can say is that I'll stick with MIT's report.
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Old 19-09-2007, 01:31   #24
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I have no problem with any credible results. The problem lies in extrapolating those results beyond the assumptions and facts of the test environment.

The MIT study is conclusive for the props and conditions they tested. It is not conclusive for a windmilling prop of any kind.

They specifically state that they did not test windmilling props.

Once you attach a transmission and the associated drag the numbers in the study go out the window. The study was one of prop efficiencies not installation effects.

In regards to it being unrefuted for 13 years. How long was the world flat? how long did man believe the sound "barrier" was impenetrable?
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Old 19-09-2007, 04:39   #25
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Point taken but this isn't rocket science. Besides, I own the perfect test platform as I have two of everything. So results are amplified so to speak.

Freewheeling is much better.
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Old 19-09-2007, 07:00   #26
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Quote:
Freewheeling is much better.
It's more about the issues with the transmission. If the transmission vendor prefers the shaft to be locked then it should be. The two boats I've owned both require the transmission shaft not to freewheel. The more common situation is there is a problem with the transmission being able to freewheel without damage and more or less drag is moot.
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Old 19-09-2007, 07:39   #27
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This topic is very interesting and ironic (at least to me). One of my other hobbies is 4-wheeling (Jeeps). A problem 4-wheelers often have is getting their heavily modified trucks to the trails. Often, these trucks are unpleasant to drive on-road (huge/load tires, axle lockers, under-powered brakes, etc) to say the least. So, often we flat-tow our Jeeps to the trails. That is, a tow bar is attached to the front bumper that is then attached to a tow-vehicles hitch. For the most part, this sounds like an obvious solution. But noooo.. few transmissions/transfer cases can handle being free-wheeled. The problem is that the drive shafts continue to spin the gears in the front and back of the transfer case that attach to the axles. But, because the transfer case is left in neutral (to prevent spinning the manual tranny gears which wear rapidly when left in neutral), the bulk of the transfer case does not get lubricated. This causes wear and failure over time. Well, clever 4-wheeler guys have come up with a solution. They weld little paddles on the shaft inside the transfer case so that when the front and rear of the transfer case is free-wheeling, the little paddles toss the lubricant all over the inside of the case. No more problems

Does this not seem like the exact same problem that sailboat transmissions have when free-wheeling? I wonder if some manufacturers have installed paddles inside the transmissions while others have not. I know that Hurth claims that the tranny in my boat can be left in neutral or in reverse. Hmmmm??? I wish I had a parts diagram for my transmission. I'm very curious how Hurth allows for free-wheeling the tranny...
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Old 19-09-2007, 08:13   #28
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Quote:
Originally Posted by rickm505 View Post
Dan

It's the only non biased research which has been published on the subject. Fortunately the research was done by a lab with impecable credentials, and in 13 years this report hasn't been refuted. I would think that ought to be conclusive enough in anyone's book. If MIT wrote it, I have no reason to question their methods, or conclusions and am a little surprised that anyone would.

I mean, come on, it's MIT..... not some fly by night outfit or rumors spread by marina rats.

As I said earlier, this is the definitive work on the subject. If you'd like to tell us that their scientists, methods, and conclusions are all wrong, all I can say is that I'll stick with MIT's report.
Didn't I read somewhere that MIT's research was funded by the 'Organization of Cutlass and Transmission Bearing Replacers Local 110'??
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Old 19-09-2007, 08:39   #29
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Originally Posted by Connemara View Post
Hmmm.

I have an OMC saildrive and I've been putting in neutral. In fact we usually put it in neutral and then pull out the wee black button that lets it start, whatever that's called.

Haven't tried it with the transmission engaged.

Anybody have any opinions on the OMC saildrive? Can't remember what the manual says, if it even mentions the issue.

Connemara
OMC Saildrive - the "transmission" is basically identical to the bottom end of a regular outboard motor, hence the gears are in an oil bath. Freewheeling should cause no ill effects to the gear train.
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Old 19-09-2007, 09:05   #30
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OMC Saildrive - the "transmission" is basically identical to the bottom end of a regular outboard motor, hence the gears are in an oil bath. Freewheeling should cause no ill effects to the gear train.
Do you mean that the "case" is actually nearly filled with oil? Another solution I've heard of in the 4-wheeler world is to fill the entire transfer case with fluid prior to flat-towing. The problem with that solution was that there was localized super-heating of the oil because of the lack of circulation.

Does fluid ever come out of the breather on the OMC Saildrive?
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