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Re: [AtkinBoats] Unsanctioned motor question

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  • John Kohnen
    Unsanctioned is a full-on planing boat, so she ll go faster if you put a bigger engine on her. Playing around with George Crouch s formula* for guessing
    Message 1 of 2 , Feb 1, 2009
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      Unsanctioned is a full-on planing boat, so she'll go faster if you put a
      bigger engine on her. Playing around with George Crouch's formula* for
      guessing planing boat speeds we can work back to find that Unsanctioned
      probably weighs around 2,300 lb. when she goes 15.65 knots with 25 hp. Big
      Twin. If we could find a 50 hp. motor that weighed 85 lb. she'd go about
      22.1 knots, but a 50 hp. Honda weighs 205 lb., so with that engine we'd
      only get about 21.6 knots -- 24.8 mph. A 90 hp. Honda weighs 365 lb. and
      would push Unsanctioned 28 knots, but at that power you'd have to consider
      carrying more fuel and beefing up the construction to handle the power.
      Lets say 150 lbs. for that -- 27.25 knots, 31.3 mph. Weight matters, so if
      you take a couple of corpulent friends along you'll go slower. At an
      all-up weight of 3,230 lb. top speed with a 90 hp. Honda would be around
      25 knots, with that same couple of big friends and the old 1951 Big Twin
      speed would be around 14 knots. So, it'd be advantageous to have more than
      25 hp. (the Big Twin was one of the most powerful outboards available in
      1951), but going bigger than about 50 hp. would be pushing things, IMHO. A
      50 hp. Honda will carry you, the boat and those two fat friends around
      19.6 knots, or around 22.5 mph. Plenty fast enough for a cruiser. Of
      course these are only guesses... Even with the 50 hp. motor you should pay
      extra attention to making the transom area strong. You've got to consider
      that the heavier motor is going to be straining the boat more while you're
      trailering down the road, as well as when you're on the water.

      Playing with Crouch's formula quickly drives home the point that _weight_
      is the enemy of planing boat performance! Build strong, but don't add a
      little extra something here and there to make it "hell for stout." It'll
      add up and spoil the boat's performance. Something to think about when you
      load the boat, too, or thinking about an extra helping of dessert. ;o)

      * Kts = 150/(Weight/Horsepower)^0.5

      On Fri, 16 Jan 2009 14:14:05 -0800, Troy wrote:

      > Hello all,
      > I am looking at the boat unsantioned and was wondering what would the
      > upper speed limit and hp max be for this style of hull. I love the
      > looks of it and the "ease" of building with plywood. It mentions a 25
      > hp would get it to 18 mph.
      > Thanks for any help

      --
      John <jkohnen@...>
      Distrust any enterprise that requires new clothes. <Henry David
      Thoreau>
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