*JimH* wrote:
Why 65 replies on a simple question. Amazing. ;-)


Hear is number 66

Every body is getting drag mixed up with lift. The reason you should
not allow the propeller to spin freely is not because of drag it is
because of all the lift it generates opposite the direction of travel
of the boat.

Drag is always in the direction of the relative fluid flow that causes
it. The relative fluid flow that causes the drag on the spinning prop
is not at all the same (direction and speed) as the relative fluid flow
that is influencing the rest of the boat hull but every one is
determining drag as though it was the same. The more the prop rotates
the more the relative fluid flow influencing it becomes circular, and
the more the direction of the lift it generates becomes parallel to the
prop shaft. As the prop starts to spin the angle of attack becomes less
generating more lift and less drag because the prop is operating well
below the stall angle.

The best aeronautic analogy of this would be an autogyro. The rotor is
tilted slightly rearward to facilitate its rotation from on coming
relative airflow. The rotating blade not only generates enough lift to
support the weight of the aircraft but unfortunately some of the lift
has a rearward component. Igor Benson wrote one of the disadvantages of
the auto gyro is all the drag it generates that opposes thrust. He got
drag mixed up with lift also. Lift and drag are very very similar.
Aircraft have flown non stop around the world in spite of drag while
others have done it as a result of drag.

Also be aware that the water is not flowing around the prop the prop is
moving while in a fluid (rotation) as it moves linear thru a fluid. If
the fluid were moving around the prop drag would cause its motion not
oppose it. Thanks to aeronautic texts we all know that to move thru a
fluid you have to overcome drag. This represents a fraction of what
drag is all about for it is also true that to remain still while in
moving air you must also over come drag. I have about reached the
limits of my third grade education that took me four years to get.