BoatBanter.com - View Single Post - Angle of prop shaft

(Steven Shelikoff) wrote in message ...
On 4 Jun 2004 04:17:35 -0700, (basskisser) wrote:

(Steven Shelikoff) wrote in message ...
On Thu, 3 Jun 2004 12:06:33 -0700, "Gordon" wrote:

"Gary Warner" wrote in message
...

We have an inboard and, of course, the prop shaft exits the hull at an
angle. I just drove by a similar (but different brand) of boat and it
appears it's prop shaft comes out at less of an angle. My first though it
that a angle that gives a more push forward and less push "up" would be
more
efficient. Is that about right??

Yes but the real gain in effiency comes from the fact that the pitch on
the blade on one side now more closely equals the pitch on the opposite
side. This also means it takes less rudder to maintain a straight line and
therefore less drag.

You'll also get less propwalk when reversing the boat with the lesser
shaft angle from horizontal.

Why?

My theory is that , all else being equal, a boat with a greater shaft
angle will have less thrust in the horizontal direction to counteract
the sideways force from the prop paddle-wheeling and at the same time
will increase the sideways "spinning" component of the rotational force
from the prop.

Think of it this way: If you have the shaft completely vertical and
spin the prop, the main motive force will be straight up and not push
the boat anywhere. You've minimized the "paddle-wheel" sideways effect
but that total force is very small. However, you've also maximized the
torque rotational force that will try and spin the boat in the same way
as putting a single mixer blade in a bowl of batter will try and spin
the bowl and that force is huge compared to the paddle-wheel force.

As you lessen the angle of the shaft, you start to increase the sideways
paddle-wheel force but again, that's a very small force. The direction
of the "mixing bowl" rotational force changes from trying to spin the
boat to trying to list the boat. Also, you get more motive force in the
proper direction to allow you to counteract both the "paddle-wheel"
force and the "mixing bowl" force.

When the shaft is completely horizontal, the rotational force is all in
the direction that creates list as you apply power and none is trying to
rotate the boat. I believe this has the greatest effect on reducing prop
walk since the rotational force can be very large. While a small change
in shaft angle has a small effect on the ratio of the vector components
of that force, the total force is so great that a small change in the
ratio of the vectors can have a large effect on the handling of the
boat.

Also, you have the maximum amount of thrust in the proper direction to
help handle the remaining sideways paddle-wheel force, which is the
least of all of them.

That's my theory and I'm sticking with it. lol

Steve

I don't agree. If the direction of the prop and shaft, relative to the
centerline of the boat never changes, and the cause of the "prop walk"
is from the rotational force, it would make no difference what the
angle of the prop is relative to the horizon. It could be anywhere
from horizontal, through 90 degrees to horizontal, and the rotational
force would remain the same.