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Old 10-05-2018, 12:31   #61
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Re: Stuffing box... Sailing.. prop left to spin

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Originally Posted by Cheechako View Post
I've often wondered about the supposed "truism" that a stopped prop will drag less than a spinning one. It appears from this discussion/test results shown that a spinning prop drags less. My gut always said the spinning is relieving the pressure on the blade allowing it to "slip off". OTOH, the blades are in water and no matter how much they move or not move, they still have the same amount of surface area confronted with the water....? So how does spinning help?
So I still cant logic it out.
I can tell you this though. If you are casting a spinning type of lure fishing in water, you can tell if it's not spinning/tangled as it often feels like less drag retrieving it than when it's spinning.
That is a great observation about the spinning lure, which I can attest to. Again I think this has to do with the speed at which the spinning object is pulled through the water (or air). At slower speeds the energy hitting the blades causes the prop to rotate, thus minimum energy in the form of drag. At some point the blades can rotate only so fast and so any additional energy added due to greater speed through the fluid, just becomes drag.
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Old 10-05-2018, 21:21   #62
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Re: Stuffing box... Sailing.. prop left to spin

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Originally Posted by a64pilot View Post
I say there is no energy required in holding a prop stopped, cause there is no movement, therefore no work done.
Think of a parking brake on a car, it can hold one on a hill forever, no energy input required to keep it there. Stopped prop is the same as a parked car.
Takes energy to stop a car or prop, but not to hold one stopped.
Hold the prop stopped by hand. If it takes no energy to keep it stopped and you let it go, surely it would stay stopped but we know it doesn't.

Park the car on a hill and without using the park brake or chocks, hold it there by hand. If it takes no energy to hold it there, it will stay there when you remove your hand - but we know it rolls down the hill.
So yes it does take energy to keep the car stopped on the hill or the prop locked on a moving sailboat.

Allow to me explain a bit more

Energy, work and movement are related but are not interchangeable. Rather than explain in perhaps my not so good ways, have a look at this link that explains it fairly simply Work, Energy and Power

Back to energy, movement is not required for a system to contain energy. There are two common forms of energy - kinetic energy (movement) and potential energy (stored). Some common forms of stored (potential) energy are a compressed spring, compressed gas in a cylinder, a weight suspended above the ground, chemical elements contained in a battery etc.

Back to the locked prop; consider the prop shaft to be spinning and then you attached a big pair of vice grips to it. If the handle of the grips has enough room to rotate, it of course it rotates. However if there is not enough room, the handle will smack up against the hull and stop the shaft from rotating. Where has the energy gone or better said, what has the kinetic energy of the shaft been transferred into?

The handle smacking up against the hull will have altered the heel of the boat. In which direction is immaterial, it will simply change the existing heel as soon as it contacts the hull and stops the shaft rotation. We will assume the hull is strong enough not to break at this point

Now the angle of heel is just a another stored form of energy. Prior to locking the shaft and in a steady state system with a steady wind, the boat had an angle of heel of say 10 degrees, the wind on the sails and the weight of the keel and the underwater shape and the steady boat speed kept it at 10 degrees. Now lock the prop with the vice grips and the handle will apply a force to inside of the hull and cause a change in the heel. Thus the stored energy in the system has increased - some of the kinetic energy of the moving prop has been transferred into stored energy and some of it into other kinetic energy (namely increased turbulence of the water behind the stalled prop).

Finally to your parked car analogy; once parked on the hill, the car still has at least two forms of stored energy. A minor one is the compression of the suspension springs which has altered once you have applied the park brake and bigger one which is gravitational potential energy. This is the same (or very similar to) say the stored energy a weight has that is suspended above the ground.
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