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    Re: Prop-walk.
    From: Trevor Kenchington
    Date: 2003 Apr 23, 23:28 -0300

    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

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