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Hi George.

You are certainly right about the density of the water.  What do they call 
that thing I was talking about? 'the weight of the water column'?


Dave W

-------Original Message-------
From: George Huxtable 
Sent: 04/22/03 08:16 PM
To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM
Subject: Re: Prop-walk.

>
> Some comments on prop-walk from George.

I'm not convinced by any of the explanations so far.

Dave Weilacher was certainly wrong when he said-

>The bottom end of the prop is more efficient than the top half.  The
water
>is more dense >by 3% of an atmosphere at the bottom than at the top.

That simply just ain't so. Water is virtually incompressible. Any
difference in density is infinitesimal.

Perhaps list members may like to ponder on the following hypothetical
situation.

Take an immersed cylindrical submarine, ballasted so as to be neutrally
buoyant (and so as not to be rotated by the prop-shaft torque). Will that
show prop-walk? To me, simple symmetry implies that there will be none.
Turn the sub through 90 degrees, so that its stern is vertically down.
Now,
will there be a force, at right angles to the prop-shaft? No, there's no
way of choosing one direction over another: it's symmetrical. It's like
using a propeller-type paint-stirrer, driven from an electric drill: the
type which always ends up flinging paint around the garage. But before
that
happens, is there any sideways force on it, or on the paint-tin? Symmetry
says no. Does anyone disagree so far?

Go back to the horizontal cylindrical submarine. Attach a vertical fin to
its tail, behind the prop (just where a rudder would be), extending
exactly
as much below the prop as above it. Again, this is symmetrical, so there's
no overall sideways force. Now, remove the bottom half of this fin,
destroying the symmetry. The water-flow from the prop is a backward-facing
jet of water, but it also picks up a spiralling motion from the rotating
propellor. If the propellor is turning clockwise (seen from aft) then the
upper fin will be pushing the stern to starboard, because of the
water-flow
impinging on it from the propellor. That would have been exactly balanced
by a corresponding pressure on the lower fin, pushing the stern to port,
if
the lower fin was in place. But now we have removed the lower fin, so that
balance has been lost, and the result is prop-walk.

That thought-experiment was designed to remove any influence of the nearby
water-surface, or any ship-shape of the hull. Prop-walk can arise from
surfaces of the vessel which intercept some of the spiralling outflow of
water from the propellor.

Now, back to surface vessels. For a vessel with a thin vertical sternpost,
and a vertical rudder which extends well above and below the outflow from
the propeller, I suggest that propwalk would be minimal. For a boat such
as
mine, with a sternpost and rudder-pivot at 45 degrees, I would expect to
see a lot, because there's a much more hull-and-rudder-area behind and
above the prop than there is behind and below it. If any vessel had its
sternpost angled the other direction (going further aft as you descend), I
suggest propwalk would be in the opposite direction: but I know of no such
vessel.

In reverse, a corresponding effect must occur. The spiralling outflow goes
forward from the prop, embracing the hull, divided into two parts by the
keel. With the propellor turning anticlockwise, water leaving the prop on
the port side of the keel is free (to a large extent) to pass downwards
underneath the hull without being intercepted. However, water leaving the
prop on the starboard side of the keel, with a partly upward motion,
becomes trapped between the hull and the surface, and has to be deflected
by the hull before it can flow away.

Try fixing your rudder in the straight ahead position, and with the boat
lashed to its berth, put the engine into reverse. Give a few seconds for
the resulting current flow around the boat to stabilise. If your boat
behaves like mine does, and your prop is a clockwise-forward
(right-handed)
one like mine is, you will see a strong surface current leaving the
starboard quarter, in a direction about 45 degrees clockwise from the bow:
but on the port side, little or nothing. That strong flow direction must
imply a corresponding reaction on the stern of the boat in the opposite
direction, pushing the stern to port.

I'm sorry not to be able to put numbers or experimental results into the
arguments above. I just hope to have persuaded some of you that the
detailed shape of boat and hull may have a large part to play in the
effect
of prop-walk.

George.

================================================================
contact George Huxtable by email at george@huxtable.u-net.com, by phone at
01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
================================================================
>

Dave Weilacher
.US Coast Guard licensed captain
.    #889968
.ASA instructor evaluator and celestial
.    navigation instructor #990800
.IBM AS400 RPG contract programmer

   

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