Noisy Sailing!

[john_morris_uk]

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One final thought. If you were a helicopter with a failed engine, you wouldn't lock the blades in the hope of producing more drag as you descend!

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For gads sake not that one again. You can't even begin to compare the situation of a stalled helicopter rotor to a stalled prop - the Reynolds numbers are of orders of magnitude different.

The drag on a free-turning prop is about one tenth of the drag of a locked and stalled prop. If you don't want to believe it, you are free to bask in ignorance for the rest of your lifef, but I would suggest you get a book on fluid dynamics and propeller design and study it.

[/ QUOTE ]Not sure if I should rise to someone who claims "sailing, winding up rightards on the internet" as their occupation/interests on their profile"

Reynolds numbers are apposite to what I believe (and have found experimentally) to be true regarding propellors and drag.

On a power boat or big ship propellor I believe that allowing the prop to rotate is less drag.

However I have found that on a sailing yacht with a small blade area (Typically a compromise be drive efficiency and drag under sail) then locking the blade produces a smaller drag. Its what many people find in practice, as most yachts don't have big bladed propellors. When you lock such a prop there is obviously no laminar flow and the turbulent flow of over the stalled prop produces less drag.

I don't claim to have any great insights into fluid dynamics. Its over twenty years since I was reading physics and I wasn't very good at it then! I do know what I have found in practice. Being rude and insulting people doesn't win arguments.

 

[Richard10002]

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Reynolds numbers are apposite to what I believe (and have found experimentally) to be true regarding propellors and drag.

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I cant believe this hasnt been sorted out. How difficult would it be to fix a prop and shaft to a block of something, fit a load measuring thing to the block, and tow it through the water, (in a tank?), with the propellor free and fixed, maybe at a variety of speeds, with a variety of propellors, and a variety of blocks.

Would probably make a good research project for Southampton University if they still do that kind of thing.

Not sure whether it would be BSc, MSc, or Phd level, but it should be done.... unless it has been??

 

[VicS]

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Not sure whether it would be BSc, MSc, or Phd

[/ QUOTE ] "A" level I would think /forums/images/graemlins/wink.gif

 

[BrendanS]

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Not sure whether it would be BSc, MSc, or Phd


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"A" level I would think

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25 years ago. Full Professorship now.

 

[Marsupial]

Richard, It has been sorted out many times but I cannot find any of the old papers I had on tank tests - it was 15 years ago. . .. But I do remember that the prof explained it like this "free spinning props act as brakes because the boat is powering the prop and the only energy available to turn the prop is the kinetic energy in the boat - so the boat slows down"

I love these threads! /forums/images/graemlins/smile.gif

 

[Glyka]

Three-blade prop, relatively large blades, boat slows down when reverse engaged. That's my case. I'm positive.

Maybe two-blade props actually create a larger (than when stationary) area when rotating thus creating more resistence.

On the other hand, excess noise may come from a worn-out reverse. It does so in my case and I'm going to replace the ball bearing this winter... Normally the noise of just the propeller and shaft rotating is not that ennoying.

 

[Glyka]

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But I do remember that the prof explained it like this "free spinning props act as brakes because the boat is powering the prop and the only energy available to turn the prop is the kinetic energy in the boat - so the boat slows down"

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Your prof does not understand physics at least not it's meaning. If we fit in a boat a wheel like the ones used in mills (I don't know the word) does he claim that if kept stationary it will create less drag because it doesn't produce any energy?

Hint: Is the heat produced on the prop energy?

 

[Inselaffe]

I couldn't agree with the need for empirical testing (I do mostly experimental research), but that doesn't mean anything theoretical is heretical /forums/images/graemlins/smile.gif

http://news.bbc.co.uk/2/hi/uk_news/england/somerset/6315987.stm

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[Lakesailor]

In one of last years posts on this subject I suggested a real-world test in which a yacht is towed on a still day (no wind, no current, Windermere has these) by a vessel on a longish warp with a spring balance in the tow.
Take an averaged reading over a reasonable distance with a turning prop, then lock the prop and take and average reading over another similar distance. Repeat in the opposite direction. Keep throttle opening constant on the tow vessel.
Try again keeping speed constant by altering the therottle opening if needed and log those readings as an alternative.

I will bet that the locked prop will show a greater reading on the tow line in both circumstances. Also that the throttle will need opening to maintain speed with the locked prop.

It may not suit the professors amongst you, but sailing happens in the real world, not tanks.

 

[Vara]

Put this one back in the box,it's not due out again until March.Current old chestnut is the tinning of wires in a marine environment,currently running on a forum near you.

(experimental researcher?.............beats working for a living! /forums/images/graemlins/smile.gif)