Thought experiment

[cindersailor]

Popular wisdom seems to have it that stopping the prop from rotating while sailing reduces drag. I have never understood this and while thinking about shaft driven alternators developed an argument that appears to demonstrate that it cannot be true.

Driving an alternator from the rotating prop shaft while sailing will result in a slowing of rotation and the generation of power by the alternator. On the basis that a stationary prop results in less drag than a rotating one, slowing the rotation will presumably reduce drag until, in extremis, the prop stops turning and the drag is at a minimum. Thus, extracting power from the rotating shaft reduces drag and makes the boat go faster. Either I have just broken one of the fundamental laws of physics, or the drag argument is wrong.

Looking at this another way, if the slower a prop being pulled through water turned the less drag it created, why would it turn at all, since by turning it is creating a force which opposes its rotation!

Any comments?!

 

[tome]

The argument for stopping the prop does defy apparent logic. Best explanation I heard is of a sycamore seed rotating as it comes down to slow its descent to earth, thus demonstrating that a rotating prop has more drag.

 

[Das_Boot]

I dont believe the outcome of stopping the prop is reducing drag the desired outcome is reducing noise and wear in the gear box and bearings.

 

[BrendanS]

The answer is it depends on the pitch and size of the prop. A student did a thesis on the subject, though the data is not as conclusive as possible, and it would be nice to see someone expand on it with marine props

http://www.goshen.edu/physics/PropellerDrag/thesis.doc

 

[cindersailor]

In my view the seed rotates because by doing so it creates less drag !

 

[forestay]

If resistance was an issue, it wouldn't have blades or wings and would drop straight down. It has wings or a blade so that it will rotate and therefore stay airborne longer. With a little help from the wind, it will land further from the tree thus reducing competition for water and light and enabling it to spread further afield.

Can't think how to explain this because I am having a blond moment, but Newtons law (For every action or reaction there is an equal and opposite reaction) would prove that a spinning prop will have less drag.

 

[Lakesailor]

being simplistic (which I excell at) If you had a prop on a shaft, but not attached to a gearbox it would rotate as the boat moved through the water. If you grasped the shaft in your hands to stop it, the amount of energy you used to stop the shaft turning must be the amount of extra energy the boat would need to overcome the stationary prop and maintain it's boat speed( I am being simplistic here remember).
This is the way a brake dynanometer works to demonstrate the power an engine produces.
So the stationary prop requires the boat to use more energy than the revolving prop to overcome the drag.

Does Newton spin in his grave?

 

[moodycruiser]

[ QUOTE ]
Does Newton spin in his grave?

[/ QUOTE ]

Not if you stick him in astern /forums/images/graemlins/smile.gif

 

[Evadne]

Things always seek the lowest energy level. Thus if a prop is free to rotate when being towed, the lowest energy level is the one that it adopts, i.e. rotating.
This would imply that to stop the prop rotating, or to slow it, would require more energy than to leave it be, and indeed, if you put a shaft alternator onto it to extract energy (from the movement of the boat rather than the rotation of the prop) then the prop will slow down.
Unfortunately this is only true if the rotating prop doesn't actually slow your boat down. The only real measure would be from boat speed measurements, and as Brendan implies, I suspect that it is not that simple in theory. After all, the source of energy is the wind, translated onto forward movement through the water. The source of drag we are talking about here is more subtle than a flat plate or even an aerofoil. I can see at least one hole in my simple argument above when you startt to take all these things into account, namely that it assumes that all the effects are linear.

Hope this helps

 

[RickMoss]

As any helicopter pilot will assure you, the increased drag (or lift) of a spinning propeller (or rotor) is all that keeps them alive if the engine should happen to fail. A setting of -1° pitch gives a nice steady 45° descent into a theoretically controlled landing. If the rotors were stopped, the descent would be vertical and the landing final.

The movement of the blades of an autorotating helicopter though the air provides lift in exactly the same way that the movement of the freewheeling prop blades though the water provides lift. Backwards. Stop the prop and lose this effect if at all possible.

 

[tcm]

newton

No, Newton does not spin in his grave, and neither does Coriolis - unless ofcourse an external force is applied. Kepplers cleaning staff obey the same rule but spin at variable angular velocities.

 

[Evadne]

I have always believed that my propellor, fixed, slows me down a mite less than when rotating. I also believe that this pales into insignificance compared with the cost of the wear on the bearings of an incessantly spinning prop., which is the main reason for locking it.
The theory is not simple, I suspect, and that means that you will only get it right if you are able to take into account all the possible influences. My point was that an empirical approach is more accurate, i.e. measurement not speculation.
I wouldn't guarantee that a helicopter prop in air works in exactly the same way as a boat prop in water, except in the broadest sense. I had always thought that one of the main reasons for spinning the prop on a stalled helicopter was to build up momentum in the blades then convert that to lift at a lower altitude by changing the pitch, thus preventing fatal contact with the ground. Increased drag on the way down is just an added bonus. But then I'm not a helicopter pilot and never likely to find out!

 

[fireball]

If you read the thesis from the link posted above you'll see that both arguments are correct ... as it depends on the speed, pitch and length ...

As it is a fairly easy experiment to do - sail along in flatish water and measure your speed, then stop the prop and see what happens to the speed. I doubt there is much change in boat speed unless you have a massive prop and no wind ...

Personally, I'll continue to stop the prop (conventional shaft on Yanmar 2GM20 with 2 blade fixed) cos I don't like the sound of it turning... if that looses me 0.2kn then so be it...

 

[tcm]

reasons why the prop/rotor shd not spin

unless powered, the screw blade simply acts as a brake and as its descends (in a helicopter) or moves along (in a boat) it pushes some air or water outta the way, slowing down whatever the prop is attached to. But whhy does its spinning make a difference?

I wonder...and think that this is because the spinining rotor/prop repeatedly gets hold of lumps of air beneath it that are not moving and has to accerate each lump of air - whereas if the prop or rotor doesn't move it will soon have accelerated the local air/water to its own velocity. As the stationary prop is exposed to less "new" stationary air/water - which it would need accelerating - the drag is less.

The very fastest olympic swimmers use the same principle in the past few years when using front crawl (freestyle) - instead of pulling each arm back through the water in a straight line, they slightly move their arms from side to side in front them as they pull thru the stroke - thus repeatedly exposing the hand and arm to stationary water instead of staying with the same line of water - part of which they have already accelerated and which therefore gives less impetus to their own forward movement.

 

[Evadne]

Entrained water

I hadn't thought of that, but entrained water is one of the sources of drag when towing something (or hauling it ) through the water. When you have a big cage-like structure, for instance, you have to allow for the weight of water within it when calculating how much load it will exert. The swimmer's waggling arm "catches" more water than a straight sweep. On a prop, the flatter and bigger the blade(s), the more water it would drag with it. A narrower blade, on the other hand would probably entrain more water by spinning. (I've not had time to read the article but will try and do so later). Air has different friction and (obviously) density, so that is one reason why aerodynamic and hydrodynamic arguments may differ.

It would also follow that a big 3-bladed prop on a P-bracket would have far more effect than a 2-blader on a long-keeler.

 

[IanR]

I think of it like the skid pan analogy

Skidding does not slow you down as much as allowing the wheels just to turn hence ABS. If you stop the prop it cavitates in moving water creating turbulence. If you allow it to turn slowly then the turbulence is less and because there is almost laminar flow then it has higher drag, let it spin at speed and the drag decreases until it turns at the same speed (with no bearing friction - like a windmill).

On my boat I have experimented and with no clutch drag the boat sails faster with the prop spinning (Its a three blade prop) and so it would exert a LOT of drag. I also don't sail that fast say 4-5K and so I don't think the cavitation does not build up enough to release or Skid in the water /forums/images/graemlins/confused.gif

Thats my theory and I am stickin to it.....

 

[tross]

The blades of a helicopter have the same profile as an aircraft wing. If you can keep them rotating then they create lift. Think of an airplane going forward - it stays up because of the lift generated by the air flowing over the wings,, stop moving forward an forward at it falls out of the sky. Stop the rotor blade on a helicopter the same thing happens.

 

[Clive_Rigden]

Hmm - all very interesting if somewhat theoretical:-

Without going into too much physics, sailing with a fixed prop locked is broadly equivalent to towing a bucket. Letting it free wheel, I believe, reduces the drag, but wears out the stern gland and the p-bracket bearing.

Your choice.

 

[Lakesailor]

The Answer

Course the obvious thing to do is to tow a sailboat with a MOBO (aaaagh) at a steady speed (below it's hull speed) in still water on a windless day with a spring gauge (as used on mobile cranes to measure the lift load) in the tow-line. The yacht then stops and releases the prop on a signal from someone observing the spring gauge.
If the load increases with a locked prop and decreases with a free prop we have our answer.

 

[bobfrost]

Re: The Answer

It all depends on the propeller and position relative to the hull.

From what I can remember of Bernoulli and company the ideal is to have streamlined flow and avoid vortices, the production energy of which is taken from the kinetic energy of the boat.

I would suggest that in general with three bladed prop letting spin will allow boat to go very slightly faster.

Where it will make a difference is with a long keeled boat like mine and twin bladed prop it's better to stop the prop as long as prop is in vertical position. This way prop is in lee of back of keel and should not cause too much drag.

David Acheson is one of the main guys on fluid mechanics (as well as being a good jazz guitarist)

His home page of

http://home.jesus.ox.ac.uk/~dacheson/

has some interesting vortex simulations.

Well I find them interesting anyway.........