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Old 21 May 2010, 12:16   #21
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Not according to Mr Newton!
Newton’s laws of motion, in particular the third law indicate that every action has an equal and opposite reaction, which in simple terms mean the amount of water thrown out the back of a boat is equal to the amount of forward thrust produced.
Yes, but the force only acts while the water is being accellerated; i.e. while it is in contact with the prop blade. Once free of the blade, it comes down to the "objects set in motion tend to stay in motion" thing, up until the drag of pushing through still water causes everything to stop.


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Stand on a skateboard and throw a medicine ball, you will move in the opposite direction to the ball even though you are no longer attached to it
Which illustrates the point: You will be set in motion until something creates opposite force to stop you; but you aren't going to keep accellerating in the opposite direction. The only accelleration you had was at the time of pushing the ball in the opposite direction. Once it left your hand, you were in a non-accelleration state. Doesn't matter if you pushed the ball hard enough to toss it 1 foot or half a mile, once it's free of your hands, it has no further effect on you.

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Old 21 May 2010, 15:40   #22
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Ahhh but the prop is continually throwing balls out the back....
The prop characteristics alter how the displaced water is handled.
And yes props are the work of witches!
It’s not a simple question because there are soooo many variables,
hull shape and size, weight distribution, amount of trim, rev’s......................and on'n'on!
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Old 21 May 2010, 18:26   #23
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Try to search out a vid of a small boat prop in action at speed, you will see that water is not being accellerated out the back as you are imagining. The water is almost undisturbed except for the blade tips which will always pose a problem. There was a good but short clip during the tv series, hosted by Richard Hammond, about things that happen too quickly to see. The clip was in the episode to do with cavitation and shock waves.
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Old 21 May 2010, 21:56   #24
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Fookinell what have I started


The answer is obviously not that simple
Yes it is! Just buy a fookin big Anchor and move it around
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Old 22 May 2010, 05:16   #25
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Try to search out a vid of a small boat prop in action at speed, you will see that water is not being accellerated out the back as you are imagining. The water is almost undisturbed
here's a small vid i took a couple of years ago of the the water being "accellerated out the back" and it is quite disturbed as you can see.
sorry about the quality it was taken on an old mobil phone.
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Old 22 May 2010, 05:32   #26
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Andy, sorry I don't have YouTube access at the mo but I presume your vid is taken at the rear of the boat where the hull has displaced the water and it's rushing back to fill the void, you have the waves created by the hull, the exhaust gasses being ejected at speed through the prop, the water disturbance by the lower unit and anything else protruding from the transom so there will be plenty of water activity at the rear of the boat. I maybe should have been clearer, look for a vid which is an under water view of a working propeller.
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Old 22 May 2010, 06:09   #27
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Something like that!
Im not trying to start an argument infact quite the opposite,
Just sharing an opinion.
Right im off to ocean village to give cookee some support
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Old 22 May 2010, 15:33   #28
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Right im off to ocean village to give cookee some support
If only he had been there
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Old 24 May 2010, 04:48   #29
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If only he had been there
I was all weekend!
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Old 24 May 2010, 14:33   #30
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I was all weekend!
Just Hiding
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