Originally Posted by a64pilot
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
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
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.