On 08 Jun 2004 03:14:52 GMT, (Shen44) wrote:


That was a good explanation and I got what you're trying to say. The
only problem with it is the overhang. My prop is about 3 ft down. The
clearance to the hull above it is around 3 to 4 inches. There's still
about 5 or 6 feet of waterline behind the prop. So on the upward
stroke, the blade is pushing the water column against the hull.

First, what is the shape of your hull? Secondly, look at the pitch of your prop
- the main push is back, the "UP" angle is only slight and not directly "up"
against you hull, even WITH (I'll bet) 5-6' of overhang.
Even in reverse, with any degree of deadrise, the push to the side and up will
not be mainly impacting on your hull for the full revolution, if at all, and

The area directly above the prop is fairly flat then as you go sideways
it flares out after it gets a few feet from the prop on either side.
The prop is only a few inches below the fairly flat part. As you go
back a few feet it starts to form a pronounced V that gets pretty sharp
where it meets the rudder. But since we're talking about reverse, it's
more important to go forward from the prop. That way, there's plenty of
boat hull on either side of any prop wash.

I'm looking at pictures of it right now and I'd say there's only maybe
15 degrees of rotation in the upward direction off to the right (it's a
LH prop so it comes up on the right side) where it only has air behind
the column of water, and maybe only 10 degrees of the 90 up and to the
left part of the rotation where it's pressing against air.

These are all just estimates from looking at a bunch of pictures of the
boat on the haulout crane. It's a LH prop and follow a blade around the
forward rotation from 0 degrees (straight up) down to 270 degrees (off
to the left) down to 180 degrees (straight down) and then it starts up.
It'll go for another 10-15 degrees with the column of water backed by
air off to the right, but then from around 190-195 all the way back to
355 degrees the column of water is backed by the hull. Then from around
350 to 360 it's backed by air off to the left side of the boat.

So basically, for around 15 degrees of the 360 rotation you have a
column of water backed by air in one direction and about 10 degrees the
column is backed by air in the other direction. The entire rest of the
rotation you have either the botom of the sea or hull.

I just don't think a tiny minute loss of efficiency during that small
amount of rotation is enough to be the main contributing factor of the
prop walk. It certainly is a factor though, among many others. And
because there is an overhang, it's a factor *more* than if there was no
overhang... see below.

even if you have no deadrise, the water will shortly leave the confines of your
hull and be allowed to push up into the air.

Not
only is there no noticable bulge of water behind the boat from the prop
but there isn't even a sign on the surface that there's a prop turning
at all. I just don't get that stream of bubbles behind me or a bulge in
the water that powerboats get or anything other than the same sort of
wake I get when sailing. And yet there's pronounced prop walk when
reversing.

My feeling on this is that the visual effects of prop thrust are not always
readily apparent, but this does not in and of itself, alter what is occurring,
i.e., you don't need a big white water wash, astern or ahead of you to be, in
reality, experiencing a lessor degree of efficiency from the prop during half

It's true that the effect can be very subtle, barely noticable. For
instance, one of the ways to find a moving submerged submarine is that
there is a pressure wave it creates as it moves through the water which
creates a wake-like bulge on the sea surface. It's tiny, but it's there
and theoretically could be picked up with a very sensitive radar looking
at the sea surface. You're not gonna see it with your eyes though. It
shows up as a minute wakelike bulge in the average sea level.

(maybe a little less, maybe a little more) of it's rotation, which is causing
the unequal pull to stbd or port, depending on it's rotation.

But in my case, it's nowhere near half the rotation, it's only around 15
degrees one way and 10 the other. It can never be more than half the
rotation. But the main problem is with your theory that you have an
almost equal amount of inefficency in either direction. I.e., think of
a completely unshrouded prop with no overhang. As it's coming up, for
90 degrees of the 180 upward travel it is less efficient in one
direction and the other 90 degrees it's less efficient in the other
direction.

So here's why I don't buy your theory for an unshrouded prop. When I
say efficiency below, I mean strictly the efficiency from the column of
water being backed by something solid vs. backed by air.

Going through the rotation of an unshrouded RH prop in forward, from 0
to 180 it's pushing against solid water so it has good efficiency. But
from 0 to 90, it's pushing the stern to the left with good efficiency
and from 90 to 180 it pushing the stern to the right with good
efficiency, the same equal and opposite efficiency as from 0 to 90. Now
on the whole way up from 180 to 360 the prop is pushing against water
that is backed by air, so you get the bulge and some loss of efficiency.
From 180 to 270 degrees it's pushing the stern to the left with poorer
efficiency and from 270 to 360 it's pushing the stern to the right with
poorer efficiency, the same equal but opposite efficiency it was pushing
to the left with. As you can see, that all cancels out.

Now, add an overhang and visualize the angles of rotation that the prop
will go through where it has neither the bottom of the ocean or the hull
of the boat to back up the column of water. You'll see that because of
the overhang and the fact that the blade of the prop is off to the side
of the centerline during most of it's upward travel, the amount of
rotation with only air behind the column is different for the 180 to 270
degree push to the right than it is for the 270 to 360 degree push to
the left.

This is where I think your theory can contribute to prop walk, but only
by a small amount. i.e., only by the amount that an overhang causes the
amount of rotation with air behind it to be different between the 180 to
270 degree part of the rotation and the 270 to 360 degree part of the
rotation.

If you followed all that, I'll be amazed. Hopefully I can somehow
make it clearer if you didn't.

Steve

I have often wondered if the phenomenon is partly caused by the same
forces that act on a helicopter rotor (or any other rotating mass) ... a
control input force to the rotor is applied 90 degrees ahead of the
desired output force. Applying that to the marine propeller means that
the right hand prop applying a force (better "bite") at 6 o'clock would
generate a resultant force at 9 o'clock in ahead rotation and 3 o'clock
astern. The force at 3 o'clock would result in the stern moving to port
when backing.

Rick