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Old 22-09-2007, 13:06   #46
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Compleatly ignore and forget any advice by prop manufacturers. Head all advice by the box manufacturer.
The reason has nothign to do with efficiency. The reason to follow box manufacturers advice is to so with the longevity of the box itself. Each design of box has it's own set of issues. Some boxes like say the Velvet drive, has a final gear assembly that runs totaly free of the rest of the box, when the engine is not running. So the hydraulic side of the box is isolated and safe. The final gear side is happily bathed in oil and will free wheel happily with no issues. But it is possible to have a hydraulic box where that is not the case and damage can result. Some all gear driven boxes are the same. The final drive is happily turning on oil and not turning the rest of the gear train. But many don't work that way. Many require certain gears to pick up oil and distribute it around the rest of the box. Having the final gear free wheeling only, the oil may not be picked up and gears and bearings are spun with no oil on them. This creates heat and wear. A few boxes that are suggested that they are locked in reverse, may well be using the reverse gears to pick up oil and lubricate.
Some manufacturers suggest running the box for a few minutes every so many hrs so as oil is run around, Some suggest locking compleatly, so suggest no issues with free wheeling whatsover. So find out the recomendation and follow it. Forget about prop efficiencies etc, unless you can lock the box/shaft.
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Old 22-09-2007, 16:23   #47
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Originally Posted by rickm505 View Post
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.
Definitely referring to the same paper as you and have checked that the copy I have is identical to the paper you provided the link to by Beth Lurie and Todd Taylor December 1994.

As you have said this subject is like a religion but call me an agnostic. In fact there is no math on page 20 of the paper, as I posted earlier all they do on that page is 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 do make reference to inserting figures into the formulae for the non dimensional parameters they give earlier in the paper, but if you refer back to those you will see that they say of them Various nondimensional parameters are used in the study of propellers. These numbers are useful as they allow comparison of characteristics of different propellers without depending on the specific conditions of each test. That is they are useful for comparing propellers under the conditions of the conducted experimental test, they do not say much about the performance of the propellers in another not yet conducted and different experimental test exploring other performances.

There is no need to get concerned about my academic credentials, as I say I am an agnostic in this and not trying to discredit your opinion. However, personally, I tend to accept the wide work that has been done in aeronautics which is pretty much conclusive in that freewheeling propellers have more drag (eg see the link another poster has given on this) - but, again as has been said air is compressible and aircraft propellers are different (both in foil and aspect).

I have never found a conclusive answer - I do have a copy of one Navy's towing manual and it states that the propellers of a towed vessel should be left to freewheel, but I do not know if that is for drag or other reasons. Maybe it differs between prop types?

In the end I suspect the difference between fixed and freewheeling, whichever way it is, is not great - so perhaps something we can agree on is that if the drag of a fixed pitch prop is of concern enough to worry about that likely small difference, then one should consider changing to a non fixed pitch propeller (and of course, the Practical Sailor sponsored paper is very clear on the gain in making such a change )
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Old 22-09-2007, 16:46   #48
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Compleatly ignore and forget any advice by prop manufacturers. Head all advice by the box manufacturer.
Hi Alan, I have a feeling we might come across each other some day .

I hope I did not give the impression that a propeller builder's advice should be taken as to whether the gearbox can be safely locked or not. In fact I have never had any such advice from a prop builder and would not expect him to offer it.

For the sake of good order what I said (or meant to say? ) was that the propeller maker's advice regarding that prop was for it there was less drag if not allowed to freewheel and that the gearbox manufacturer's advice in the box manual was to freewheel the box when sailing (one assumes for the boxes well being). Two mutually exclusive things, neither considering the other.

But, as I said, on researching the specific box we found that in its particular case only it was actually ok to lock it in reverse and that is what we do (let's say that for the sake of keeping religion out of it we do so only to stop the rumble of the shaft turning ).

I suspect the advice in the manual was given for reasons other than lube, etc ones (eg the difficulty with some cone clutched boxes to disengage reverse if under or having been under an external load - unless one knows the knack of doing so).

One would hope others would similarly research the particular box before freewheeling or locking the box contrary to the manufacturer's manual. As you say, the recommendation for the good of the box should be what is followed as it may care whether it is freewheeled or locked - the prop is not caring of whether it is locked or not (but other drive components might be :-)).

John
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Old 22-09-2007, 18:08   #49
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What a thread! The RV forums are rather dim compared to this reading. Why are sailors so much more intelligent?

Anyway, to dummy it down a bit - I stand by my original post somewhere deep in this thread:

I lost close to 2 knots when I put the tranny in reverse to lock it on my boat. I also did it wile underway, after reading this thread (don't try this at home, kids!!) and made a clunk so loud I thought I had destroyed the tranny! Luckily, everything about my Perkins 4-108 is bullet proof, including the tranny. It works fine. I tried this experiment when the thread started way back when.

I guess what I'm adding is very non-quantitative, but I think I'd rather buy a new tranny than lose 2 knots boat speed. Seriously - and I'm not such a rich guy... lol

Also, from the data side of things (rather than theory), I have run every transmission I've had on all the different boats in neutral while sailing for more than 20 years. No problems yet.
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Old 22-09-2007, 19:04   #50
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I suspect that alot has to do with the particular prop design and drive chain characteristics (eg friction) and one can only go by what one finds with one's own boat.

In our own case we have a sailboat optimised 3 bladed prop (the blades have a very high aspect ratio ie long and narrow). I have not noticed any speed difference between freewheeling it or locking it (but I have never done any formalish tests with it, that because as I said earlier we lock it to avoid the noise ).

Also, boat speed is likely important in that drag probably varies as something like the square of the boat speed (haven't checked details of that at all, just wanting to make the point that it is not linear with speed ) and may also vary further as the nature of the flow (turbulant, laminar) over the prop's surfaces changes as speed changes (which will happen as even a "freewheeling" prop is not freewheeling due to friction in the drive chain).

My personal view would tend towards being that if one sees a big difference between locking and unlocking ones prop then one probably has a prop that is very draggy whether freewheeling or not and if concerned about drag the actual solution is to look to replacing it with a better one or a non fixed pitch one. Obviously, a prop on a light displacement efficient hull shaped sail boat (so includes cats) may appear more draggy than similar on a heavier less efficient hull shaped cruising one, say, but the foregoing still applies.

Another way of perhaps getting an idea on the matter is by watching what outboard motors left in the water while sailing with their prop in locked and freewheeled conditions (leg will kick back more for the case with most drag). In the two cases I've heard of the view was that there was less drag when the prop freewheeled, but, of course, that has to be considered in the light of a small outboard's prop being nothing at all like a keel boat's one (well hopefully not ) and the existance of the skeg in front of the prop.

John
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Old 22-09-2007, 19:36   #51
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I should add to my post a bit... for John's sake since he probably hasn't been around for my other 2,569 posts... ha ha

The boat I did this on is a 45' sloop with a PHRF of 120 and a 3 blade fixed prop. The boat is 13 tons. I was doing something like 6 or 7 knots and noticed a 2 knot reduction in speed.

I would have to take exception to the statement that a prop needs replacing if you see a great hull speed difference between locked and freewheeling. The non-linear (chaotic) effects of turbulence would certainly come into play when the prop is locked at more than a few knots. Also, ignoring those effects completely, it's simply a function of surface area presented when the prop is locked (assuming constant pitch and blade shape).

In the case where the prop is freewheeling, the velocity vector of the water (in relation to the normal vector of the curved prop blade surface) changes orientation, causing a sinusoidal (or cosinusoidal or tangential) reduciton of the drag force which slows the boat, allowing it to glide through the water with less resistance. (ie: the relative motion of the water is more ACROSS the blade rather than directly AT the blade)


I don't think this means I need to replace my prop. It just means I present a great resistance to the water when my prop is fixed in place vs when it is freewheeling. My hunch is that every prop works this same way. After all... why would they differ when they are all basically identical geometical shapes?

(excepting folding and variable pitch of course)
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Old 22-09-2007, 20:16   #52
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I should add to my post a bit... for John's sake since he probably hasn't been around for my other 2,569 posts... ha ha
Too true, where do I start ? On the other hand, perhaps don't tell me....

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I would have to take exception to the statement that a prop needs replacing if you see a great hull speed difference between locked and freewheeling.
Didn't mean that "a prop needs replacing" but that should "look to replacing it" ie consider doing so if fussing about drag ("fussing" is a new flavour added here in order to add clarification to my meaning ).

I've considered replacing ours, even tho seems to be a good prop as far as fixed pitch ones go, but decided against it - would rather have money and drag rather than no money and no drag .

John
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Old 23-09-2007, 07:25   #53
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For those who are interested, I checked out the users manual for the OMC 'Zephyr' saildrive yesterday. It makes no mention of the transmission during sailing.

OTOH, we always sail with the lock-out switch on (Dunno if that's clear. There's a wee black button and when it's pulled out, the engine is disconnected from the transmission so you can rev it without driving forward into the dock. We sail with the button out.)

And yesterday, in 10-17 knots, with the genny halfway out, we consistently hit 6.3 SOG and were above 7 a few times (probably surfing a wave, but fun). This on a beam reach.

So for those who have an OMC saildrive (a dwindling few, I'd guess) I don't think having the engine out of gear hurts performance.

And I like to think that if sailing with it out of gear was bad for the equipment, the manual would have said so. But I'm open to persuasion on that point.


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Old 23-09-2007, 10:47   #54
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Sorry but although "all props look the same" that does not imply that the are geometrically similar. Thats what the advance coefficient and the other dimensionless numbers are for. Props with similar coefficients will behave similarly, not props that look the same.

Your test result losing 2 knots is very a very significant. I can gain 2 knots of speed, 4.5 to 6.5 by motorsailing in light air at about 1700 RPM, That would mean the loss is about oh 10 HP at least.

I have hit 10.9 knots in 20 kn of wind surfing a Hunter 45 with the prop locked (broad reach). Pretty sure I would not have made 12.9 knots with it freewheeling (maybe?), so clearly the conditions for the test matter quite a bit.
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Old 23-09-2007, 12:33   #55
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I call for more real world testing. Rick and Ssullivan have given their in and out speeds. If it is a question of tranny cooling, letting it freewheel for the minute it will take to determine speed isn't going to boil the oil. Besides you've probably done it by accident more than once. Though perhaps luffing up and slowing down before cramming it in gear will produce less trauma.

So far we have two real world data points. A heavy cruiser and a lightweight cat that say freewheeling is faster.

I would add a data point, but when I put mine in gear when it is freewheeling the max prop feathers.

John
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Old 23-09-2007, 13:00   #56
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Hi Alan, I have a feeling we might come across each other some day .
Yikes, I hope I don't come across as a Grumpy ole fart or something. I wasn't aiming at you if that was how it read. I was just wanting a point to be made clear.
To re state in different words,
Firstly follow the box manufactures recomendations.
Then if you can, try the prop locked and unlocked and see which best suits your situation.
I think if there was not such and issue with drag in either case, manufacturers of feathering props would not have a business. No one is going to spend three times the money on a prop if it made no difference.
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Old 23-09-2007, 16:56   #57
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Hi Alan, I have a feeling we might come across each other some day .

Yikes, I hope I don't come across as a Grumpy ole fart or something. I wasn't aiming at you if that was how it read.
Whoops, no my I have a feeling we might come across each other some day comment was an allusion to the fact that we periodically cruise the outer Marlborough Sounds so would look out for you (but if you insist that you are a Grumpy ole fart I might not ).

John
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Old 23-09-2007, 17:53   #58
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The best thing about this thread is that on one hand you have people saying a locked prop is better and on the other hand you have people saying a freewheeling prop is better.

We all agree that the transmission manufacturer has a great deal to say about it and I hope we can all agree that the different transmisions (if engaged) will add different levels of drag.

We have a paper done at MIT by Lurie and Taylor. Several of us read clearly that no testing was done on freewheeling props. None, nada, zilch. Taylor did go on to a very successful career as a hydrodynamiscist(?) and has other papers published. Lurie pretty much disappeared as far as internet available references.

The dialog points out that it is not cut and dried. I would submit again that you need to test with your particular setup, consider the mechanicals of your boat and make your choice.
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