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??

"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.
Gordon

On Thu, 3 Jun 2004 12:06:33 -0700, "Gordon"
wrote:
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.

==========================================

This sounds like it ends up closing the loop on the "prop walk"
discussion and starting around the mullberry bush again. We pretty
well established that boats with horizontal shafts had just as much
prop walk as those with angled shafts.

My guess is that it all comes down to a resolution of force vectors in
the vertical and horizontal plane, using shaft angle with the bottom
of the boat as the vector direction.

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.

Steve

(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.

Steve

Why?

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

(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.

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

Steve

Why?

Because while the propeller shaft is not parallel to the surface, the flow of
water past the propeller is. The physics of this relationship make one side of
the prop
more effective in reverse than the other, depending upon which way the prop is
rotating.

Prop walk is the "sum" of a number of factors - prop rotation (left or
right), prop pitch, shaft angle, vessel trim, wind, current, hull form.
A right hand prop, SHOULD, back to port .... a left hand prop (G fixed
pitch), SHOULD, back to stbd .....
BUT, results will vary for any and/or all of the reasons above, and in
addition, whether you are all ready turning and/or have headway or sternway.
When you start out on a boat, figure the "SHOULD" first, then standby to
learn the "oops" and vagaries, that WILL apply due to any of the
factors, listed above.
Try not to think of it as your enemy and learn to use it.

otn