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Re: [bolger] Presto Cruiser

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  • John and Kathy Trussell
    Before PCB, the expert on sharpies was Howard I Chapelle. The original working sharpies were cheap, shallow draft, were used for tonging,and had to carry huge
    Message 1 of 7 , Feb 1, 2007
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      Before PCB, the expert on sharpies was Howard I Chapelle.

      The original working sharpies were cheap, shallow draft, were used for tonging,and had to carry huge loads. Chapelle says they generally had between 3 1/2" and 4" of flare for each foot of depth. Since many sharpies had nearly vertical stems, there was often less flare forward and more aft. Chapelle states that less flare was used for speed and more for seaworthiness.

      A basic problem with sharpies is that water flows at one speed under the bottom and at a different speed along the side. The difference in speed creates a pressure differential and water tries to move over the chine to equalize the pressure. This movement takes the form of edies and these create drag, slowing the boat. One effort to reduce edies is to match the curve of the side to the curve of the bottom, thus reducing the difference in speedof thewater on the bottom and side, the pressure differential, and the edies. Another approach is the 'presto' sharpie with rounded chines. These provide a smoother course for the equalization of pressure and, in theory, reduce drag. Jim Michalak has several designs in which he cuts the corner off the chine and replaces it with a plywood panel. I have one and it seems to sail fine, but without a similiar sized sharpie for comparison, I can't state that it works better (or worse) than a more normal sharpie.

      If I were building PCB's Presto, I'd build the bottom and sides of plywood and strip plank the chines, fiberglassing the whole thing. I'm to old to tackle a boat building project this size, but the 'Presto Sharpie' sure is a pretty boat.

      John T
      ----- Original Message -----
      From: Will Parkes
      To: bolger@yahoogroups.com
      Sent: Wednesday, January 31, 2007 7:10 PM
      Subject: [bolger] Presto Cruiser


      Did PCB complete plans for the Presto Cruiser design in BWAOM? If so,
      does anyone know if it is all strip planked or is any ply used in the
      construction?

      Over the weekend, in the presence of a naval architect, I was
      discussing sharpies with some fellow Bolger fans. We compared a number
      of hard chine sharpie designd, like Skillygallee, with more round edged
      designs like Karl Stambaugh's Trailer Sailer 24. Stambaugh uses ply on
      the sides and bottoms and strip planking on the corners. Some opined
      that the additional work involved in the strip planking wasn't worth
      what was thought to be a small gain in performance.

      We also compared the square sided sharpies (AS-29 etc) with those with
      more flair from Howard Chappelle as well as PCB. The flair averaged
      about 23 degrees from the vertical amidships. Does anyone have a view
      about the value of sharpie flair?







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    • mkriley48
      lets just see what the extra weight of making the bottom and hull sides of steel. Bottom has plate of 1/2 10 x 24 = 2000lbs will make 1/4 bottom for 2000 lbs
      Message 2 of 7 , Feb 2, 2007
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        lets just see what the extra weight of making the bottom and hull
        sides of steel. Bottom has plate of 1/2 10'x 24'= 2000lbs will make
        1/4" bottom
        for 2000 lbs minus the weight of the original bottom so a net gain in
        reducing weight. hull sides of 1/8" steel{2000lbs?} minus the weight
        of the original sides and the weight saved on the bottom all else done
        to the original plans in wood and fastened to the hull with bolts,
        onto tabs welded to the hull. I think weight gain would be minimal.
        can someone run numbers on the weight of the plywood hull and the
        steel hull skin only. No need for steel internals, if the wood
        structure was good enough for a weaker ply hull it will be fine for a
        stronger steel one.
        This kind of wood deck and house with steel hull has been used for
        over 100 years with success <starof india>
        can someone run the numbers?
        thanks
        mike
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