engine in gear when sailing or.......?

Thanks for that - I was beginning to think no-one read it. I have to admit that I have probably oversimplified it a bit where inboard installations are concerned. Even in neutral there is significant resistance to turning a prop with its seal, shaft and sundry bits inside the gearbox so energy is being taken out of the system by having the prop rotate (an outboard in neutral has practically no resistance).

The habit of stopping the prop I think dates back to the days of long keels and a skinny 2-bladed prop that you could stop lined up vertically hidden behind the deadwood. I have seen this on an old wooden boat and there was a mark on the shaft to show when the prop was lined up.

It's probably a lot different with a 3-bladed prop on a P-bracket.

And the helicopter analogy is the wrong way up! Think about it!

And my bike goes down hill faster if I let the back wheel go round.

Geoff

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OK.

The least possible resistance created by a fixed prop is when it is rotated at a rate that is just less than the rate at which it produces thrust. This will be when the angle of incidence on the foil section is zero. Agreed?

On a sailing boat this will be when engine revs are reduced from those that were creating thrust to revs at which 'just' no thrust is produced. Any more revs and the angle of incidence will go positive and create thrust, while less will produce negative incidence and 'drag'.

It's important to think of the drag in two ways. First is the 'true' drag. Consider a symmetrical foil section with the the zero incidence angle being parallel with the line of symmetry through the cross section. The drag produced is directly downstream of the moving foil when the angle of incidence is zero.

The aft facing component of this drag is the force that concerns us, that which is slowing the boat.

So, none of the foregoing is contentious. Increase the revs and we get thrust, decrease them and we get drag. We're only interested in the decreased revs here.

The question is how much does the drag increase with each succesive reduction in revs (or increase in the negative incidence angle).

Let's turn it upside down and just call it angle of incidence, because all we're interested in is the increase in drag with change of angle.

Now, imagine the stationary blade on a yacht at rest. Accelerate the boat and it's obvious the the flow is creating an angle of incidence at a right angle to the rotation direction of the blade, and an angle of, say, eighty degrees to the blade itself (because of the angle the blade is mounted on the shaft). Despite the fact that the blade is stalled, some flow will be forced around the blade, which, although massively turbulent, will eventually produce a thrust in the blade which is sufficient to get it to overcome shaft friction and spin.

Back to incidence. The maths of lift/drag are hugley complex and the result of work by some great minds over the last two centuries. It is an area where empirical evidence was used to create and apply mathematical theories. Almost all of the work done has been with a view to finding the optimum lift with the minimum drag. The object being the pursuit of perfect foil sections for various speeds and densities of fluids. A secondary field has been the behaviour of foils at and around the stalling point, looking at how the centre of lift moves forwards at stall so that safe stall can be designed into aircraft.

Unfortunately, Reynolds spotted (and this is a simplification) that the lift/drag of an object moving in a fluid bears a direct relationship to it's size.

In the study of foils, the apparent change in size of a foil (if you imagine your view of the foil if you were a particle of water about to hit the foil) that results with the change of angle of incidence from zero to ninety degrees (from a very thin thing to the whole planform of the blade) is enormous. The maths just can't deal with this effectively at large angles.

Consequently, if you look at a typical graph plotting lift against incidence, or drag against incidence, they give up at about twenty degrees in air or ten degrees in water.

So, to find the lift from a prop operating between zero and ten degrees incidence is relatively easy. To find the drag from a stopped prop requres a different method.

Next though, we have to wonder what angle our freewheeling prop might be operating at. We know it started at eighty degrees in the case where we started from rest. We know that it is now rotating, so the angle must be less than eighty. We can assume that it has not managed to get anywhere near,say, three degrees, because at that speed it would be producing maximum lift and that would have stopped the boat.

From that we can see that our freewheeling prop cannot be near to the zero incidence case outlined at the beginning, to get there it would have had to get through the three degree point, which isn't possible.

Has it even managed to get into the ten degree zone where calculation is possible?

Apart from boatspeed, what are the restrictions on it's rate of rotation? Friction is one, but principally it is it's own dynamic drag.

I've spent the better part of my morning on that, so while I'm doing some maths later, anyone have a problem so far?

<hr width=100% size=1><P ID="edit"><FONT SIZE=-1>Edited by boatless on 24/09/2004 13:04 (server time).</FONT></P>
 
Pardon me while I go a bit quiet for a while.

My aerodynamics qualification (PPL) expired on 10/4/59 and I've only used Mr Reynolds briefly in 1970 to enquire something about making liquids go round corners in a pipe.

So far though: All the boat props I can lay hands on have camber on the front surface and hollow in the back. So any windmilling is going to be pretty inefficient (vampires (symmetrical aerofoil) fly upside down quite well, tiger moths (asymmetric)) don't.

I need to make up a device to dangle over the stern with a prop on a shaft with a brake/torque gauge towed by a drag gauge. If I figure out how to do that I'll take it for a spin round the bay.

In the meantime Mr boatless, perhaps we should avoid cluttering up this thread and go PM? Or even email - pics easier!

Geoff

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Mr Reynolds is also handy in pipes! If it wasn't for him you could pump oil from the Iraq to my house at a billion miles an hour through a hosepipe... he has a lot to answer for.

Wasn't going to mention that the foil section is upsidedown yet, although a prop blade isn't actually hugely cambered. Didn't want to confuse.

Experimentation being very much a key to all the theory, it's a good idea. I used to work at a towing tank, and over the last few days have wished I was still there, so easy to do in a tank. Definitive testing would be enhanced if you could put the apparatus over the bow - in clear water, where propellors really ought to be....

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You are a brave man Boatless, taking on the conspiracy theorists /forums/images/icons/smile.gif.

I do not need convincing but will look forward to and value every episode, as I am sure many others will. So don't give up there and please don't get side tracked into Disneyland when the going gets tough as I am sure it will (I know, have been down that road many times /forums/images/icons/crazy.gif).

John

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And I'm beginning to think I must be a brave man to take on Boatless - someone who clearly knows what he is talking about! And we used to have a saying at work that the entertainment value of any argument/discussion is inversely proportional to the amount of relevant knowledge available.

That's why I suggested that we take the heavy stuff (and as soon as Mr Reynolds rears his (ugly) head you know it's getting heavy) off the public domain for a bit until we can get back to conjecture and prejudice.

There's a danger that we might even get to the bottom of it all and spoil what must have been one of the most long-running subjects ever. I remember it coming up years ago on the usenet in uk.rec.sailing and it was just as much fun then. So if we do we should keep it secret!

Geoff


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Test data

I have also asked for hydrodynamic test data every time I see this subject but no-one has ever shown any. Lots of theoretical waffle presumably assuming bu****it baffles brains. This link has test data on model aircraft props but I am not sure how relevant this would be to boat props. It does show some interesting points though. http://www.goshen.edu/physics/PropellerDrag/thesis.doc. Hopefully someone can come up with a similar test on yacht size props in water. I have just found some summary data mainly aimed at fodling/feathering props showing a 15 x 13 fixed pitch 3 blade prop at 8 knots has a locked drag of 420 N and a turning drag of 180 N. Interesting to see an Autoprop self adjusting pitch has drag of 75N locked and 670N turning. May be a mistake in the table, how can I find out? These tests were done in a circulation tank of the Technical University of Berlin and are the only "evidence" I have found to support my belief that a fixed prop has more drag than a free rotating one. Link is (I don't know how to put in clever linky things) http://www.actionsailing.co.uk/FoldingProp.htm. I am not claiming this is cast iron proof and I would be interested to see some more data if anybody can find any with different props at different speeds. At least it is a start.

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