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Subject: length vs beam

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  • Rob Rohde-Szudy
    Hey Mike It s been said that nearly any boat can benefit from more length. Not so much because of streamlining , but because of a higher hull speed. This is
    Message 1 of 1 , Jun 5 10:35 AM
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      Hey Mike

      It's been said that nearly any boat can benefit from more length. Not so much because of "streamlining", but because of a higher hull speed. This is certainly true as regards speed, but not necessarily for handling or comfort.

      But be VERY careful with any stretch beyond 10% or so, especially when the designer was a real structural engineer like Atkin. The parts are sized to hold up to the sea AS DESIGNED. Make it longer and you might break a keel or gunwales for example. It might be worthwhile to have an engineer look at it to be sure you're not removing a needed safety factor.

      You're talking about a MAJOR stretch of of close to 40%. This has serious structural implications and is not to be taken lightly. It makes me think you're really after a completely different design, actually. George Buehler has designs that might fit the bill better, and you'd also get a living designer who can answer questions. In any case you'd really have to pay someone like Buehler or Michalak to go over the stretch and make sure it won't break up in the waves. You can rest assured that if it does break, it will be at the worst possible time. (I'd go with Michalak for sheltered water, Buehler for open water.)

      Or at least consider other Atkin designs. Mr. Simms might be better if you really need all that space. Don't be afraid of the round shapes, since Martha Green cannot be built in plywood anyhow.

      This, of course, gets to the harder questions of what you want to DO with the boat. Why do you need 33 feet? Will that many people REALLY come along? Often enough to justify it? Be very wary of building too large a boat. A small one is plenty enough of a money pit and you almost never get back more than the cost of materials when you sell. It's much harder to sell a large boat, too. Not many people can afford to keep one. It's a shame how many of them sit around in marinas and get sailed maybe once a year, then sold for a song when the divorce comes.

      Finally, if you simply must have a boat along the lines of a longer Martha Green, I would seriously consider buying an unloved sailboat and refitting it as a motor-sailor. You may well get more for the money, and of course you wouldn't have to build a hull.

      Sorry if I rambled a little, but please think very carefully about what you will ACTUALLY use OFTEN. And absolutely consult an engineer for any stretch over 10%. It's not worth your life to guess.

      --Rob


      as i've been thinking of building a stretched martha green, i've found
      many thoughts on the subject from various builder and designers web
      sights. one said that in an ideal situation beam should be close to
      1/4 of the boats length. as mg has a beam of 8'4" this would equate to
      a length of a little over 33'. i don't think i would want to go that
      far but i think that a loa of 31'would be okay. she might roll a
      little more at displacement speeds but the added loa might give me
      less resistance, and more speed with the same hp due to better
      streamlining.



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