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    Re: Slip
    From: Trevor Kenchington
    Date: 2003 Apr 24, 22:22 -0300

    Doug has answered with respect to the modern ships of his experience.
    Books and old movies give a guide to the practice in the days when towed
    patent logs were normal on merchant ships. (I can just remember them on
    cross-channel ferries in the mid-'60s.)
    In that pre-electronic era, checking the taffrail log was a job for the
    deck department, which dealt in knots and miles run. The engineers had
    no way of knowing how far or how fast the ship was going but they did
    have gauges showing the revolutions. So when the captain rang the
    telegraph for half speed ahead, the engineers had a table showing what
    revolutions corresponded to half speed and they adjusted the steam valve
    and cut off until the engine steadied at the appropriate revolutions. I
    imagine that the deck department then used the same revolutions/speed
    relationship when estimating the speed of advance along their intended
    track but relied on the patent log for the actual distance run when
    plotting their DR positions.
    That being roughly the practice of half a century ago, I am not in the
    least surprised that Doug, during his exams, was expected to be fluent
    in the calculations linking revolutions to speed and distance -- even if
    they had been obsolescent for decades.
    Naval practice was probably a bit different -- not least because I have
    never heard of warships of the first half of the 20th century towing
    patent logs. (Did they use hull-mounted instruments even then?) They
    also needed to adjust speed for station keeping and hence you will
    sometimes come across references to the captain or other watch keeper on
    the bridge calling the engine room (by voice tube) and ordering some
    number of revolutions, based on whatever is needed to keep station on
    the flagship or flotilla leader.
    You wrote:
    > 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.
    I expect that "true slip" is the sort of abstract concept beloved by
    authors of textbooks, though it may also be used by naval architects
    when they design screws to suit particular hull/engine combinations. It
    presumably has some relevance when models of new screw designs are being
    developed and tested.
    The empirical slip, estimated by a run on a measured mile, will of
    course be the "apparent slip". I doubt that any other kind is of
    practical relevance once at sea.
    > 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
    > digress...
    > 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
    > inconvenience."
    > This would be a terrible environment for a rudder to live in, and a great
    > deal of "slip" would result.
    A word of explanation for those unfamiliar with the terminology of
    sailing warships:
    The after face of sternpost was only a foot or so wide. Sure it dragged
    dead water behind it but not that high a volume. (In a pure sailor, of
    course, the rudder was immediately behind the sternpost and the dead
    water followed the trailing edge of the rudder.)
    A "square tuck" is what we would now call a transom stern. Most
    textbooks will tell you that they went out of use in the English navy in
    the 1600s, English warships then being distinguished (for the better
    part of a couple of centuries) from those built in other European
    nations by the "round tucks" on the English ships. Inspection of
    original drafts shows, however, that some big English ships were built
    with square tucks well into the 18th century. Still, by Froude's time,
    immersed square tucks must have been rather unusual on anything but the
    lowest classes of cargo carriers. The Severn trows seem to have dragged
    such sterns around but even the spritsail barges of the Thames lifted
    their transoms out of the water. (Did hydrodynamic efficiency not matter
      to the trows because they worked the tides more than actually sailing
    through the water?)
    So Froude was over-stating his case more than a little. There was
    nothing so clumsy as a square tuck in the steam line-of-battleships or
    the armoured "Black Battlefleet" which appeared from 1861 onwards --
    though there is no denying that their buttock lines were very bluff.
    And as a digression on the digression: The problem of dead water dragged
    astern was understood long before the need arose for a clean flow to the
    screw. 18th century textbooks of naval architecture confirm that the
    reason that contemporary ships were given fine runs was that otherwise
    the rudder would lie in dead water and would be ineffective. Putting
    enough buoyancy into the run to resist hogging (under the added weight
    of quarterdeck and poop) without eliminating effective steering was
    quite a challenge. [A better solution might have been the use of quarter
    rudders but those had long since been lost to history in European cultures.]
    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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