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What a lot of comment on prop walk -- and how strange that so many
erudite members of the list are unaware of the causes! No offence
intended but I thought that those causes were part of the basic
knowledge expected of anyone handling powerboats.

Perhaps the confusion arises because there is more than one cause
operating, with their relative contributions depending on the vessel and
its load condition at the time.

One cause: As George has pointed out, the greater pressure at the bottom
of the arc of a screw propeller does not make the water denser (at
least, not by enough for anyone to care). However, depending on the
shape of the stern and the depth of immersion of the screw, the blades
may suffer ventilation (rather than cavitation) as they pass through the
top of the arc, causing the prop to "walk" in the direction commonly
seen. Also, any tendency for the prop to discharge water radially at the
bottom of its arc will involve driving that water into other
(incompressible) water, whereas at the top of the arc, water thrown out
radially can move into air. Thus, there will be more lateral slip at the
top than the bottom, again causing "walking". I would imagine that these
factors come into play when cargo ships are in ballast but they can
rarely have much relevance in recreational craft.

Next cause: In the recreational craft (but much less in commercial
ships), prop shafts are usually angled downwards relative to the
waterline -- and hence relative to the water passing under the hull. As
a result, a prop blade encounters that water with a different angle of
attack when it ascends on one side of the shaft than when it descends on
the other. If you like, its effective pitch changes as it rotates. That
causes the discharge current to be stronger on one side than the other,
again producing an asymmetry in the forces acting on the boat, driving
the stern in the "walking" direction.

Third cause: Although the water in the lower part of the screw's arc is
no denser than that in the upper part, the lower water usually suffers
less drag from the proximity of the hull (whether that drag results from
the motion of the vessel or the suction and discharge currents generated
by the screw). Thus prop blades passing to port (as they go above or
below the prop shaft) are in a different water flow to when they pass to
starboard. Again there is an asymmetry of thrust which pushes the stern
sideways. This one probably applies to almost all vessels with screw
propellers, to a greater or lesser extent.

When going ahead, these various asymmetries are present, of course, but
they can usually be easily compensated for because the prop's discharge
current runs over the rudder, making the latter very effective. When
going astern, that advantage is lost but there is a far bigger problem:

Fourth cause: When going astern, the prop's discharge current is driven
against the hull. The asymmetries in the prop cause more water to be
driven along one side of the keel than along the other. (With a
right-hand prop mounted on the centreline and an angled prop shaft, more
water hits the starboard side than the port side.) Naturally enough,
much of the water encounters the hull and then turns outboard, while the
resulting equal and opposite force pushes the stern in the other
direction -- to port with a RH screw. (As George has noted, the tendency
of the prop's discharge current to be helical adds to these asymmetries.)

No personal experience of the effect but I understand that boats with a
single prop mounted under one quarter (usually conversions from pure
sailing craft) always move away from the prop when going astern,
regardless of whether the prop is LH or RH, the reason being that the
discharge current is entirely on the one side of the hull.


George's hypothetical submarine avoids most of this, of course. With no
chance for ventilation of the screw, with her prop shaft parallel to her
waterline, with the suction current drawing equally from all around her
hull (or, when going astern, the discharge current playing equally on
all parts), there should be no prop walk. I wonder whether that is so?


Note that none of this relates to prop torque. That is very real, if
very minor in most boats, but it doesn't have much to do with prop walk
so far as I can see. Aircraft, of course, are much more susceptible to
the effects of torque.


And finally, contrary to Jared's claim, screw propellers are not that
inefficient. If they were, they would not have been universally adopted
by the world's merchant fleets. It is true that, on the warships of
around 1850, they had the advantage that the machinery could all be
safely below the waterline. It is also true that high-speed merchant
ships of that era used paddles. However, once designs were developed for
high-power engines suited to the high revs needed by screws and once
arrangements were found for thrust bearings, shaft packing and so forth,
once ways were found for keeping hulls stiff enough that long prop
shafts did not bind in their bearings, paddles disappeared very quickly
(aside from in a very few specialized vessels) -- largely because
screws were more efficient.


Trevor Kenchington


--
Trevor J. Kenchington PhD                         Gadus@iStar.ca
Gadus Associates,                                 Office(902) 889-9250
R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555

                     Science Serving the Fisheries
                      http://home.istar.ca/~gadus

   

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