Prop rotation when sailing

[snooks]

However in the article I can't see any mention of measuring and offsetting the drag of the leg at any speed, only its "weight" - which I assume actually means the couple tending to hold it down. As far as I can tell from the article, only the total combined drag of prop and leg seems to have been measured. (Not sure what "figures without the leg" means?)
It seems to me the wording at the foot of p69 should be "which therefore equalled the combined drag of the prop and the leg." Have I misunderstood something?

Here's how the tests were done, first off the current/power consumption of the lecky outboard was measured on the hull without the leg down, to see the power needed to drive the hull, this was done at a range of speeds. Then the drag of the leg was measured with the spring balance and the power measured again as before, then the prop was added fixed, then spinning, then the folding prop etc etc

The weight/drag measurement was taken with the engine/arm in equilibrium, so if the leg it took 1kg to move 3 inches, so would the tiller arm where the spring balance was attached would move 3 inches and register 1kg

 

[Plevier]

It's not really a red herring just an example at the extreme end of a spectrum where the correct solution for minimum drag is reversed from the 'norm'. I grant you it's largely because the disengaged prop has low frictional loads but there is still induced drag present.

Induced drag is present as soon as there is lift and there is lift between the stall alpha and zero lift alpha (alpha = angle of attack) The frictional losses are still a big factor because they change the alpha and increase the lift coefficient of the prop - meaning more lift is neccesary to freewheel and therefore more drag is created.

Blade area makes a difference because it changes the value of the stopped drag. If this is high and induced drag is low because of low friction then there can be a set up where freewheeling is best, even in aircraft.

I think we are disagreeing on practice not theory. I agree with you about the frewheeling propeller (which if it has zero friction will I think have zero induced drag as alpha=0) on the rubber powered model but this discussion started in trying to explain why a conventional piston engined aircraft with a non-freewheeling propeller - with very high induced drag if it's turning the engine -cannot be compared with a boat and should not be quoted by stopped propeller exponents as validation of their views.

Yes it would, but the higher pitch prop will also spin at lower revs for the same speed. So it's really a question of advance ratio (pitch speed vs air/boatspeed). Fine pitch at high speed will freewheel at very high rpm and be much draggier than a course pitch prop at low speed.

If you really mean freewheeling i.e. little or no resistance therefore little induced drag yes the fine pitch will spin faster but I don't think it will be a lot draggier as there is only the rotational form drag. If you have significant resistance then the fine pitch prop will stall much sooner. Like trying to push a car in a low gear!

The comparison with aircraft props is completely valid (that's what Reynolds numbers are for ) Any propeller system that is operating in incompressible flow will obey the same principles.

Hang on - the aircraft prop is in highly compressible flow. The boat prop is in generally non compressible flow. However since we are not dealing here with either transonic tips or cavitation, what's the relevance? And I haven't noticed anyone doing anything analytical enough to bring Reynolds into it! Anyway what I said was not valid was to compare a prop that is freewheeling with relatively low resistance with one turning an engine over - nothing to do with the medium it's in.

The best drag condition will change depending on pitch, blade area, boatspeed and shaft load. That's my only point. There is no black and white answer like 'Freewheeling is always less drag' as some have concluded. It is not fair to scorn anecdotal evidence just because a test of one set of conditions leads to a certain conclusion. My point is simply that a greater understanding of the actual physics shows that there is no convenient one size fits all answer.

It may well be a 'one size fits nearly all' thing.

Each system should be examined for it's own best drag condition. It just so happens to be that for most set ups on boats freewheeling is lower drag. If you have a boat and prop like the one tested then that's great but don't take it for granted - an exception could be possible given the right conditions.

I do agree with that in principle, but in practice, unless you bring in special cases like the true freewheeling model aircraft propeller, or conversely a very high drag gearbox, it's not going to happen.

Goodnight!

 

[Plevier]

Here's how the tests were done, first off the current/power consumption of the lecky outboard was measured on the hull without the leg down, to see the power needed to drive the hull, this was done at a range of speeds. Then the drag of the leg was measured with the spring balance and the power measured again as before, then the prop was added fixed, then spinning, then the folding prop etc etc

The weight/drag measurement was taken with the engine/arm in equilibrium, so if the leg it took 1kg to move 3 inches, so would the tiller arm where the spring balance was attached would move 3 inches and register 1kg

I don't think one can deduce that from the article. So are you saying that the published prop drag v speed curves were produced by subtracting the previously measured leg drag v speed curve from the measured total drag v speed curve?

I hope the spring balance used had only a short range of movement otherwise the method in your 2nd paragraph would introduce errors. As the leg tilts back, it's righting moment will increase, whereas the opposing moment of the weight on your lever arm will reduce. This will make the spring balance under read. It would be more accurate to use a low deflection load cell or at least to move the spring balance mounting point each time to bring the leg back to its original position. The errors are maybe insignificant - did anyone work it out to check?

I do realise space goes against publishing every detail!

 

[Rob_Webb]

What's a little bit of drag compared to the depreciation cost of a new gearbox!

Depends on your priorities. Could be the difference between winning and losing a race esp in light airs.

 

[snooks]

I don't think one can deduce that from the article. So are you saying that the published prop drag v speed curves were produced by subtracting the previously measured leg drag v speed curve from the measured total drag v speed curve?

No, as I understand it Emrhys measured the drag of the whole system, and plotted that......However, the drag of the leg on its own was very small, and was a constant for each test, but it had to be measured.

The spring balance had a very small movement, you can see it at the bottom of page 68, I was just plucking figures out of thin air for my example. Sorry for the confusion

 

[lw395]

It all comes down to the angle of attack that the prop needs to run at to drive the gearbox resistance.
If you look at a classic lift vs angle of attack plot, the lift peaks at somewhere in the range 15 to 20 degrees of angle of attack. Above this point the lift drops abruptly. Usually to about half its peak value or less.
So if the torque on the shaft is enough to promote a significant angle of attack, you may get less lift off the the prop blade with it stalled completely.

Which is why I view measurements made using unrepresentative or unknown shaft torque as basically worthless.

 

[Little Rascal]

I think we are disagreeing on practice not theory.

Troubadour, I wasn't taking issue with the practical application and I wasn't really disagreeing with you. I was just making the (side) point that there can be special cases. And that people (not meaning you) shouldn't assume it is always best to do something one way in a system that changes with such changeable factors. I think we are on the same page in terms of understanding.

Hang on - the aircraft prop is in highly compressible flow. The boat prop is in generally non compressible flow. However since we are not dealing here with either transonic tips or cavitation, what's the relevance? And I haven't noticed anyone doing anything analytical enough to bring Reynolds into it! Anyway what I said was not valid was to compare a prop that is freewheeling with relatively low resistance with one turning an engine over - nothing to do with the medium it's in.

Yes but in subsonic aerodynamics air can be considered incompressible for analysis anyway. But again: I wasn't actually aiming my remark about comparisons at you either.

 

[Maine Sail]

It all comes down to the angle of attack that the prop needs to run at to drive the gearbox resistance.
If you look at a classic lift vs angle of attack plot, the lift peaks at somewhere in the range 15 to 20 degrees of angle of attack. Above this point the lift drops abruptly. Usually to about half its peak value or less.
So if the torque on the shaft is enough to promote a significant angle of attack, you may get less lift off the the prop blade with it stalled completely.

Which is why I view measurements made using unrepresentative or unknown shaft torque as basically worthless.

Let's see:

*The "angle of attack" in my in-water tests was matched to that of my OWN BOAT, exact same angle, to within a couple degrees, even compensated for the attitude of the dinghy. The scale had a throw of less than 1/2" movement keeping the angle of the leg consistent. This test was first done for my own knowledge and secondly I decided to post it to the sailing community. That prop had been on our boat when we bought it and locking it in reverse was like dragging a large fish net behind, why I took it off. I wanted to see for myself just what the differences were.

*The "friction" was matched as closely to my own gear box as possible using the test jigs friction bearing set up. Hell I even took it up to still being stalled at 1.6 - 1.8 knots and the differences were still DRAMATIC in favor of letting it spin... But yeah these results that show MORE THAT DOUBLE the drag when locked are just "worthless" I guess, as are those by YM, MIT and University of Strathclyde Ocean Engineering Department....

Once again these results were NOT EVEN CLOSE.. "Worthless"?......


Once again I pose my challenge to the "doubters" pay my hourly rate and we will test traditional fixed props all day long. I already know the results. You can compress the friction bearing to your hearts content (I also know the results of that).

What's the catch? You must be allowed to be shown on video eating crow, and it will be posted on YouTube...... As of yet, no takers.. I put my money where my mouth is...

 

[lw395]

Let's see:

*The "angle of attack" in my in-water tests was matched to that of my OWN BOAT, exact same angle, to within a couple degrees, even compensated for the attitude of the dinghy. ....

It's the blade angle of attack as it sweeps through the water.
As used in general discussions of aerofoils.
It will be small for free spinning props and increase as the torque developed increases.

*The "friction" was matched as closely to my own gear box as possible using the test jigs friction bearing set up. Hell I even took it up to still being stalled at 1.6 - 1.8 knots ... [/B]

That is good attempt to match the friction to start the gearbox rotating.
Slightly flawed by friction bearings tending to have some 'stiction', i.e. a higher value of static friction followed by a lower lever of dynamic friction once movement is happening.
Many gearboxes will have a viscous element in their torque/speed characteristic, due to having oil in them.

In my mind, you may well have the correct answer in the case of your boat, but I can see that it won't be the case for all boats and props, so the people who say they observe less drag with it locked may be right too.


It's all just a sad attempt to justify sailing with the brakes either on or half on, instead of buying a proper prop fit for a sailing boat.

 

[Little Rascal]

In my mind, you may well have the correct answer in the case of your boat, but I can see that it won't be the case for all boats and props, so the people who say they observe less drag with it locked may be right too.

Maine sail - I have no wish to undermine the validity of your test or the YM test. But it is important to see the empirical results in context with a theoretical understanding. There is nothing in the hydrodynamics that precludes the stopped prop having less drag than a spinning prop, if certain conditions are met. The problem with empirical testing is that there are at least four very important variables - testing for all of them in isolation is rather difficult.

The offer you make is less than practical for me to take up , but it would be interesting to see what the differences would be on a low pitch, small blade area prop fitted to a fast yacht with high shaft friction. This would be the best case scenario for stopping the prop. If there is still an empirically measurable gain by letting it freewheel then you might convince me - on the practical application side of things.

On my boat I find just pulling the OB leg up is enough

 

[Maine Sail]

On my boat I find just pulling the OB leg up is enough

When I tow my dinghy at low speeds and lock the prop the motor lifts out of the water as there is enough pressure on the stalled prop to do so. Towing at the same speed when allowed to spin in neutral the OB stays down... If I had an OB I would raise it too.. Kind of hard to raise an inboard engine...

We used to own a full keel Carl Arlberg designed Cape Dory. She had a two blade fixed prop. We marked vertical behind the dead wood and in neutral the prop still wanted to spin. My guess is that it was not as hidden behind the deadwood as we assumed. We had to lock it vertical, in reverse, to get it to stop spinning. Course on that old slow boat any difference between locked or spinning was not easily discernible, either way, and we left it locked to prevent it from spinning as we were certainly not racing her.... On our fin keelers the differences have been rather noticeable.. Heck on our boat now when the Flex-O-Fold is not locked in reverse it won't even fold and will continue to spin... This is a prop that will simply gravity close when sitting idle. The act of stalling the prop slams the blades shut instantly.. Clearly not enough "frictional resistance" in our boats drive train to stall the prop enough to even make it close... I find that fact very interesting in the context of the resistance of the drive train conversation in relation to drag of a free spinning prop.........

 

[snooks]

It would be interesting to see what the differences would be on a low pitch, small blade area prop fitted to a fast yacht with high shaft friction.

For the majority of cruising boats, with cruising propellers (as opposed to skinny racing ones) the test results are valid. However there will be exceptions, and fixing a two bladed prop which has a small blade area would probably be more beneficial, but this set up is rarely found on a YM readers boat, so there seemed little point in testing it.

When I contacted Jack Skrydstrup at Flexofold he said "There is a formula from which you can calculate whether a fixed propeller should spin or stay locked. The relation pitch to diameter is an important factor. Remember that the shaft rpm is directly related to the pitch. A low pitch will have far more drag than a high pitch."

So there is obviously a point when fixing the prop can be an advantage.

Also the slant of the shaft is a factor to take into consideration, on the YM test we suck with the saildrive scenario for ease and because we just wanted to measure the prop, rather than the stern gear. The steeper the shaft, the less difference, so the most difference would be found on our set up.

Take this to the extreme though and imagine a 90 degree slant on the shaft, ie it is pointing directly downwards, in that position you might have more drag from a folding propeller compared to a fixed or spinning or prop.

But YM wanted to find out the results for a typical cruising boat with a 3 bladed prop.

Here's a video of Emrhys Testing:



The figures he's getting on this run might be different from what you see in the magazine...Why? well on this run he had over 100kg of photographer and equipment in the front of the boat....but it lets you see a clearer picture of the test rig.

 

[Avocet]

Yes, I read that bit.
It matches the static friction.
Which isn't the same as the resistance to turning at speed, which I would expect to have a viscous component from the gearbox oil, which would increase at higher speeds.

Presumably though, whilst I absolutely take the point that the resistive forces in gearbox and cutlass bearings / stuffing box are likely to increase at higher speeds, I'm guessing that as the speed increases, you won't ever get to a speed where it stops turning again? To me, that suggests that at the low speeds (I think the post suggested about 1.2 knots, the stalled prop has the least resistance, and as the speed increases, the moving prop has least resistance. True, I can imagine that the speed of the turning prop won't necessarily increase linearly with boat speed, but the more I think about it, the more I think the prop (and whatever it's attached to) will always "take the path of least resistance" if not forced to do something else. So if it's left free to rotate, it will stay still until rotating becomes the easier (i.e. less resistive) thing to do.

 

[Little Rascal]

So if it's left free to rotate, it will stay still until rotating becomes the easier (i.e. less resistive) thing to do.

Not quite. The prop turns because of lift not drag - in fact the induced drag is trying to stop the prop rotation.

The idea that resistance of the shaft is what causes induced drag is not quite accurate. There is a connection because the shaft load increases the lift neccessary to turn the prop and therefore the drag increases too. But the idea that the boat speed at which the prop starts turning is directly related to the shaft frictional load is an oversimplification. A high pitch prop will begin to turn at a different speed than an low pitch prop with the same shaft load. It is all to do with the blade angle of attack, and the lift/drag ratio and lift coefficient of the prop blade. Using the speed at which the prop starts to spin would not be an accurate representation of shaft load if you were testing more than one prop.


What is "the path of least resistance" for the prop system is not neccessarily the least drag condition for the boat as a whole. It's sorting out all these different factors and different types of drag in different conditions that makes the whole subject rather complex and counter-intuitive.

 

[Avocet]

Whilst I'm happy to accept that the individual components of the total resistive force are indeed many, complex, and variable, I'm still struggling, looking at the system as a whole, to get my head round the rest of it. Can we agree that the prop, the shaft, and the gearbox on any given boat, left free, will do whatever is "easiest" for them? (i.e. rotate or not rotate) in any given set of conditions?

If so, we're left with whether what they end up doing is the "least drag" option for the boat as a whole. I must admit, having seen a couple of practical tests that I can usnerstand, that I'm still leaning towards the freewheeling prop (and whatever it's attached to) being the lowest drag option for the boat as a whole.

 

[lw395]

..... Can we agree that the prop, the shaft, and the gearbox on any given boat, left free, will do whatever is "easiest" for them? (i.e. rotate or not rotate) in any given set of conditions?

....

No.

The prop always wants to turn.

 

[lw395]

...... I must admit, having seen a couple of practical tests that I can usnerstand, that I'm still leaning towards the freewheeling prop (and whatever it's attached to) being the lowest drag option for the boat as a whole.

I think you are probably right.
But I would also strongly suspect that increasing resistance from the gearbox at higher rpm is probably quite significant, and probably bring the drag up towards the locked case, to a greater or lesser degree depending on the boats, speed etc.
In some cases it may exceed the locked drag.

I'm saying that these tests don't really prove anything for most real-world cases.
To do that, you'd have to be sure the torque on the prop was realistic, by either measuring it directly or ensuring that the prop's rate of turning was the same as when driving the gearbox.

It might be easier to measure the drag on a real yacht, or perhaps a matched pair of real yachts, using methods like Frank Bethwaite describes in his books.

I think these measurements probably under-estimate the typical drag with a real gearbox, which might lead people to wrongly under-estimate the benefits of a feathering prop.
It's another missed opportunity to get some good information.

 

[Little Rascal]

....I'm still leaning towards the freewheeling prop (and whatever it's attached to) being the lowest drag option for the boat as a whole.

It normally is. I've never questioned that. But the norm comes about because the typical set up has low friction from the gearbox, and large blade area props. Change those variables (and/or P/D ratio and boatspeeds) enough and it becomes quite possible that the least drag condition could be prop stopped, in theory anyway.

The 'fact' that freewheeling is lower drag is the norm but by no means a rule.

The prop always wants to turn, given enough boatspeed to overcome the initial friction. This is because even when highly stalled the blades have a positive angle of attack and some lift (ie turning force) even if the lift/drag ratio is very poor. (In flying terms, it's quite common to think of there being no lift in the stall but in fact it's the massive increase in drag that is the issue.) As soon as the prop begins to turn the angle of attack improves to the point at which the blades are no longer stalled.


The Strathclyde study does make interesting reading...

 

[Plevier]

The Strathclyde study does make interesting reading...

Very surprised they gave constant shaft torque regardless of rotational speed as a major assumption without any justifying evidence.

I see the object was to quantify the drag and validate a prediction method, not to evaluate the fixed vs free argument. It's clear from page 3 that they started from the premise that the locked prop is always higher drag quote "the myth of common currency among many yachtsmen that the practice of locking a fixed blade propeller to prevent rotation results in less drag than would allowing it to freewheel"

 

[lw395]

Very surprised they gave constant shaft torque regardless of rotational speed as a major assumption without any justifying evidence.

.....l"

It looks like a typical theorist's paper, lots of impressive equations, bit lacking in terms of what numbers to put into those equations. It would be useful to put some of those drag figures in context, such as comparing with the overall drag of a typical yacht.

Both the yachts I've owned have been 'not quite right' under motor, with folding props that were probably not ideal diameter or pitch. I don't fully understand props but this discussion has reinforced my conviction that a fixed prop is a compromise too far on a sailing yacht.
Each to his own though.