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Old 19-09-2007, 09:12   #31
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Originally Posted by drh1965 View Post
Do you mean that the "case" is actually nearly filled with oil?

Does fluid ever come out of the breather on the OMC Saildrive?
The "case" is filled only to the approx. center of the gears - in other words only about 3" up in the bottom of the gear case.

The OMC sail drive is not like a 4X$ transfer case (where you can change the oil after going thru a river). There is no "breather" per se on the sail drive. If there were, moisture could enter which is what you don't want since you can't change the gear oil without pulling the boat out of the water.
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Old 19-09-2007, 09:26   #32
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Originally Posted by Wannafish View Post
The "case" is filled only to the approx. center of the gears - in other words only about 3" up in the bottom of the gear case.

The OMC sail drive is not like a 4X$ transfer case (where you can change the oil after going thru a river). There is no "breather" per se on the sail drive. If there were, moisture could enter which is what you don't want since you can't change the gear oil without pulling the boat out of the water.
Interesting... I noticed on my Hurth that there isn't a breather either. I figured that was why the case had what seemed like a tiny amount of fluid. If it had much, the pressure would build up radically in the case without a breather... I bet if you look closely, the OMC saildrive has a mechanism to disperse the fluid when the transmission is free-wheeled.

PS. We had snorkels on the breathers that went up to the roll-bars on the transfer case, transmission and axles to avoid having to change the darn fluids all the time. The cost adds up replacing that stuff...
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Old 19-09-2007, 11:02   #33
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I agree and already mentioned this.

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Originally Posted by Pblais View Post
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, 11:37   #34
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The report also mentions the impact of prop settings and transmissions etc on the conclusions.

There are certainly cases where "freewheeling" will induce more drag, I would think that these would be less likely to occur.

The worst "drag" will occur when the prop is turning and producing power into the drive system, perhaps intentionally.

I would like install a electric motor drive that can produce power as well as consume it. Then drive this motor with a diesel generator, through a VFD (Variable Frequency Drive models are available to produce pwoer back as well). This would allow the boat to produce power when underway, lock-up or drive the boat off batteries, and solar power or just motor as normal. The driven efficiency will almost certainly be higher since the prop and motor can be operated a different optimal RPM, much like a feathing prop (The electrical transmission efficiency can be as high as 70%)
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Old 19-09-2007, 13:18   #35
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It's more about the issues with the transmission. If the transmission vendor prefers the shaft to be locked then it should be.
The above statement by Paul has already been echoed, but it is important enough to state it once again. No one make of box will determin the rule for all others. Each box is different and the advice of the manufacturer needs to heeded for each.
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Old 19-09-2007, 22:24   #36
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Although I am loathe to extrapolate aviation data to the nautical world the physics involved are very similar. The main difference is the incompressibility of the fluid that a boat propeller works in.

In powered operation the propeller meets the water just on the concave side of the leading edge. The concave side of the prop "paddles" water aft and some water rushes around the leading edge. In air this creates lift thanks to bernoulli. In water it must create some small drag. A big difference is that most boat propellers are not airfoil cross sections because the medium is incompressible. But ultimately there is a thrust vector, most nearly at right angles to the blade.

Lot's of interesting things must be happening on a props that are not mounted along the longitudinal axis of the boat - i.e. the shaft angles down. In this case the blades going up and the blades going down see a different angle of attack to the oncoming water and therefore must produce different thrust - mathematical ice cream headache...

When the prop freewheels water strikes the blade on the convex side. The blade rotates and the unbelievable thing is the thrust vector goes negative. Negative thrust is drag. Added to this is the hp required to turn any machinery such as a gearbox and engine.

Another difference in boats is that in most cases you can disconnect the machinery, at least the engine and only turn the relatively easy to turn gearbox.

So bottom line is that you have to follow your manufacturers recommendations or do your own testing and weigh the factors.

One final thought regarding the autogyro. It works right? But the forward thrust engine is used to overcome the drag of the rotor. If the engine fails energy stored in the rotating rotor is converted by angle of attack to create a flair for a safe landing.

How well do you think that would work if the gyroplane's rotor was locked in place?

For an interesting cartoon about windmilling and the forces have a look at this.

Drag From Windmilling Propeller
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Old 20-09-2007, 05:46   #37
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This topic is like discussing religion
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Old 20-09-2007, 05:59   #38
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It is very difficult to debate a (religious) point of view that is concerned with the very core of one’s existence. A person of faith in God has that faith planted firmly in their emotional and physical core.
I hope that none of us on the CF adhere’s that emphatically to their technical opinions on free-wheeling prop’s.
Most religions (and secular cultures) follow some version of the Golden Rule, "do unto others as you would have them do unto you". Why then is it so hard to be kind, patient, understanding, etc, with people who cherish beliefs different from our own?
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Old 20-09-2007, 06:32   #39
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The good news that this is not religion. It's physics.

The better news is I have a feathering prop so I'm just stirring the crap from the bottom of the pot - LOL!
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Old 20-09-2007, 13:09   #40
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The better news is I have a feathering prop
Arrrr, feathering props. Don't get me started on those ;-) :-)
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Old 20-09-2007, 13:47   #41
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Arrrr, feathering props. Don't get me started on those ;-) :-)
I gotta hear it, Alan. I'll have to search to find out what you've said previously. Or would you give a brief statement about pillow (feathering) props
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Old 21-09-2007, 01:23   #42
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Nah no no no, I was joking. See the Tongue in cheek I put up. I was just yanking the chain. Sorry, I guess it didn't come over right.
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Old 21-09-2007, 06:40   #43
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The OMC saildrive has a cooling water intake/outlet system, if that's what you mean. Because it's a two-stroke it doesn't have the usual oil system lube the cylinders, but there is definitely oil in the lower end of the engine, because you have to change it every now and then. (Well, at haulout, usually.)


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Old 22-09-2007, 05:38   #44
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It's the only non biased research which has been published on the subject.
In fact they did no research whatsoever on freewheeling propellers - not even a teeny weeny bit, just zilch .

All they did was as an aside to their experimental work with locked props was to make a couple of extremely shallow assumptions drawn out of free air and then concluded (sic) that freewheeling props would create less drag than locked ones. They may be right or they may be wrong but there is nothing in the paper that gives any credibility whatsoever to their conclusion being correct.

With respect to following the gearbox manufacturer's recommendation it may be that the prop manufacturer has a view as well. In our case the gearbox manufacturer states in the manual that the box should be in neutral when sailing (ie freewheeling unless a shaft brake is fitted) except for a feathering propeller in which case it should be in reverse.

But the prop manufacturer, who is a well respected designer and builder of props, is adamant that least drag for our sailboat optimised fixed pitch prop is with it locked.

We ignore (after some research) the gearbox manufacturer's recommendation in favour of the prop builder's recommendation for least drag and sail with the box in reverse to lock it.
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Old 22-09-2007, 12:00   #45
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I would want to know what paper you were reading. In the document I referenced earlier, their conclusions are stated clearly along with the math on page 20.

Of course, to demonstrate that both MIT and Practical Sailor Magizine (they sponsored the research) are wrong.... you do have GPS printouts to post here for sailing with your propeller both locked and freewheeling?? And will tell us exactly where you have done your post graduate Hydodynamics work? And where we can read your Thesis?
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