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20835Re: [bolger] now hull speed, was Re: mast options for Ruben's Nym ph

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  • wmrpage@aol.com
    Jun 4, 2002
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      In a message dated 6/4/02 7:46:17 AM Central Daylight Time,
      nickerb@... writes:


      > These AIAs
      > (acronyms, initialisms and abbreviations) are not only confusing to me, but
      > I sometimes cannot understand if they are technical terms from the topic
      > under discussion, or cyber-speak (like BTW).

      Oh Dear Me! I see I've inspired our humorists to new levels of cyber-speak,
      and I can't get the jokes because I don't understand the "AIA's" ("AIA" being
      an new "initialism" for me.) (Thanks for including the definition in
      parantheses, BTW. :-)) (The AIA "BTW" and "emoticon" are my attempt at
      cyber-humor.)

      I'll try to provide a de-coder for my AIA's, in the off-chance that you are
      actually interested and not just setting me up as a figure of fun:

      S/L = "speed to length ratio"; mathematically this is a boat's speed in
      nautical miles per hour ("knots" or "Kts.") divided by the square root of its
      waterline length (LWL) in feet. The significance of this ratio is that the
      energy expended in creating the surface waves generated by boats that
      progress through the water at "displacement" speeds is what limits their
      speeds. A 16' boat at 4 Kts. will produce the same wavelength relative to its
      length as a 36' boat at 6 Kts. Both boats would be moving at an S/L of 1.0.
      At that S/L ratio the wavelength of the wave and the length of the LWL
      coincide. Above that S/l the boat has to start climbing up its own bow wave
      to go faster. Above an S/L ratio of 1.0, the amount of power required to
      increase speed increases at an accelerating rate as speed increases.

      D/L = "displacement to length ratio"; in the form used by Gerr, this is the
      displacement of the boat in "long tons" (2240 pounds per "long ton") divided
      by the cube of one percent of the LWL in feet. The dividend is a
      dimensionless number, which is perfectly arbitrary, as far as I can tell, but
      commonly used to compare boat designs. The higher the ratio, the heavier the
      boat is for its length and the more power it takes to drive it to a given
      speed. Unless the boat is designed to "plane", there is an S/L ratio where
      adding additional power results in rapidly diminishing returns.

      O/B = outboard motor

      Kts. = "nautical miles per hour"; a nautical mile is about 1.15 statute
      (U.S.) miles

      C ("Crouch Formula") = this is just a constant; George Crouch was an eminent
      designer of high performance planing power boats in the '30's and this is
      derived from his work. Gerr gives values of "C" ranging from 150 ("average
      runabout") to 230 ("racing power catamarans").

      Kts. = "nautical miles per hour" a.k.a. "knots"; a nautical mile is
      approximately 1.15 statute miles; an hour is an hour, I think.

      SHP = "shaft horsepower"; this is the power available to turn the propeller.
      With inboard engines deductions from crankshaft horsepower have to be made
      for power losses to accessories, transmission, bearings and etc. O/B motors
      are now rated for SHP, but I have recently purchased a 1983 vintage Mercury
      O/B "40" that was subsequently marketed as a 35 SHP model when Mercury
      changed its ratings from "crankshaft" HP to SHP.

      LWL = I believe this is an "intialism" for "Load Water Line"; for purposes
      like calculating S/L and D/L, it is the distance between the stern of the
      boat and that portion that contacts the water at its most forward point.
      Overhangs, bowsprits, pulpits, etc. do not interact with the air/water
      interface and so "LWL" is used as a reference datum, rather that "LOA"
      (Length Over All), a different dimension, with its own valuable uses. (e.g.
      docking fees, trailer length, storage facilities, etc.)

      Ciao for Niao,
      Bill in MN


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