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    Re: Slip
    From: George Huxtable
    Date: 2003 Apr 24, 23:23 +0100

    Perhaps Doug Royer will explain why a day's "advance" of a vessel,
    calculated from shaft rotations, will be calculated and recorded in statute
    miles rather than in the nautical miles a simple-minded navigator might
    He said-
    >Slip is determined by speed.So each time a vessel changes speed slip must be
    Then that will devalue, to some extent, the usefulness of shaft turns as a
    measure of distance travelled.
    I wonder if Doug can comment on my earlier question about the effects of
    weather. Does a ship's navigator expect to allow for the effect of heavy
    weather on the slip factor, when using shaft-turns? Or, may I add,
    different states of lading or fouling of the hull?
    In my early teens I spent a bit of time on an ageing 2000-ton coaster on a
    voyage between Liverpool and Belgium (coal-burner, triple-expansion, single
    screw: it was wonderful). There was a Walker log trailed from the taffrail
    and read, from time to time, sometimes in quite rough conditions. Off North
    Cornwall there was a lot of pitching, and on each big wave the screw would
    emerge and race away. It gave the engineers a lot of work to do in quickly
    closing and reopening the steam regulator. Under those conditions, there
    must have been much slip, and counting the shaft turns would have given
    little information about distance travelled. But the Walker log would have
    done the trick.
    Doug, is a taffrail log (or modern equivalent) ever used at sea these days,
    in the merchant service? My small boat usually trails a Walker log, but it
    gets brought in whenever I'm trailing a mackerel line, as otherwise the two
    get tangled. Food comes first!
    I can understand the use of "apparent slip", as Doug describes it, but
    "true slip" would need a knowledge of the speed of water under the ship's
    counter (where that body of water is carried along with the ship, to some
    extent). I don't see how a master would be expected to know that "wake
    factor" at the location of the propellor. This is the water that the
    propellor has to draw from, to then accelerate it and eject it astern. It
    would obviously be more efficient if the propellor could be working in
    "clean" water, water that was stationary when it met the blades. If it's
    moving forward, to some extent, with the ship, the propellor has to first
    bring that water to a stop, and then give it a backward velocity, which
    must take more power from the engine.
    To show how important this matter could be, in a vessel with a stern that's
    designed badly for power, I will quote from a paper by William Froude, the
    guru of tank-testing, in 1865, "Remarks on the mechanical principles of the
    action of propellers". The naval vessels he refers to are the
    "motor-sailers" of the day, three-masted square-riggers, with an auxiliary
    steam engine driving a propellor . The propellor could be hoisted out of
    the water when sailing, as it ran on bearings in a frame located between
    the ship's sternpost and the rudder-post. In use the screw was driven by a
    dog with could be pushed out to engage it from within the shaft-tunnel. You
    can see this clever arrangement if you ever visit HMS Warrior, which lies
    afloat in Portsmouth, not far from Victory, beautifully restored. I do
    recommend a look over Warrior, for anyone visiting England. But I
    Here is what Froude had to say-
    "Behind the sternpost of any ship when in motion, and behind her square
    tuck if she has one, there is collected a mass of what is termed dead
    water; water which accompanies the ship almost without change. When the
    sternpost is thick, and the tuck broad, as in our timber line-of-battle
    ships, the volume of water thus carried is very large, insomuch that it was
    mentioned to me by a wardroom officer of such a ship that when the screw
    was hoisted out of the water, and the ship was under canvas and going seven
    knots, he had descended into the well and bathed there without
    This would be a terrible environment for a rudder to live in, and a great
    deal of "slip" would result.
    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.

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