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1743Re: [AtkinBoats] Re: Twinkle Twinkle little boat

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  • Kenneth Grome
    Jan 3, 2008
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      > Ken, do you really think Twinkle will go that
      > fast with 9hp?

      Maybe, but only if she can get "out of the hole" and onto plane with
      that 9 HP engine ... and I think this is the big question.

      I don't know what she will weigh when she's finished (do you?) ... but I
      do know she's a heavy boat compared with today's relatively lightweight
      boats, and it's going to take a lot of power to lift all that weight
      out of the hole and get the boat onto plane.

      Jim Michalak says it takes 1 HP per 50 pounds to get a boat to plane.
      This means the boat and contents should weigh less than 450 pounds if
      you're going to use a 9 HP engine to plane her. But won't this boat
      weigh more than 450 pounds with only one passenger? If so, I'm
      guessing it will not plane with 9 HP anyways, and if this is true you
      could probably get away with a much smaller engine ...

      > The article does read that with 3hp she should
      > make 6 mph.

      I suspect that this 6 mph figure is the boat's hull speed. A
      displacement hull wouldn't go much faster with additional or excess
      power, but this boat has a planing hull -- so additional power *might*
      move the boat faster in semi-displacement or semi-planing mode.

      Then again, maybe it would just run at 6 mph with a bow-high attitude
      and "remain in the hole" until enough more power were added to make it
      jump onto plane. I would hope that this is not the case, but if it is
      you may not be happy with the boat's performance if you try to push it
      faster than hull speed.

      You've selected a planing hull boat, are planing speeds important to
      you? How fast do you really want to go in this boat? Atkin has other
      inboard powered boats that may be better suited to displacement speeds.

      > I was planning a ply laptrake Ellon Jessup outboard
      > skiff but decided to go with an inboard design after
      > inspiration by the writings of the late Mr. White and
      > his Rescue Minor skiff.

      I think a lot of people are intrigued by Robb While's writings and
      boats. I've been designing a number of different tunnel-stern
      Seabright skiffs because of the inspiration I received from him as
      well ... and the Atkin's too of course.

      > This one has a simpler box tunnel than the RM. With
      > a less efficient tunnel and alot smaller engine she
      > should be a bit slower...but how much?

      Tunnel-stern Seabrights use a double-ended box keel which is basically a
      pirogue hull beneath an upper planing hull. The pirogue hull carries a
      substantial amount of the boat's weight and makes for a very easily
      driven hull with very low power requirements. I think this is the
      primary reason why the Atkin tunnel-stern Seabright skiffs move through
      the water more efficiently than most other hulls, especially in the
      range from displacement to low planing speeds.

      But Twinkle does not have the pirogue hull bottom to help with
      efficiency, so I don't think a direct comparison of these two hull
      types is appropriate here. It could very well be that Twinkle may not
      transition easily from displacement to planing speeds like the
      pirogue-bottom boats do.

      > Has anyone worked up the wheels for these
      > Atkin's tunnel boats?

      I don't think so.

      Ken Grome
      Bagacay Boatworks
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