scantlings, strip composite?

Discussion in 'Fiberglass and Composite Boat Building' started by kbowen, Nov 28, 2010.

  1. kbowen
    Joined: Nov 2010
    Posts: 22
    Likes: 1, Points: 0, Legacy Rep: 25
    Location: Chicago & Maine

    kbowen Junior Member

    I am looking for advice on scantlings for strip-composite construction. The "Western Lakes Mackinaw" (w/ centerboard), as drawn by Chapelle in "American Small Sailing Craft" is 26'7" x 7'8", displaces about 6000lbs to the waterline as drawn. No construction specs are given in the 3/4" scale drawings from the Smithsonian, but assumed to be traditional carvel, and the keel rabbet appears to be 1" or more. What size strips and weight sheathing should one consider to build this in composite?

    Other traditional Mackinaw types from the Smithsonian drawings are built in 1x3 (5/4) Douglas fir, & 3/4 x 9 White Oak lap-strake. Additional historical sources reference 1 1/4" square pine edge-nailed strip construction.
     
  2. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 487, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Traditional strip plank would use very similar planking thickness as carvel. These thicknesses would get progressively thinner as the planking became more of a core in a sandwich laminate, compared to a lightly sheathed planking.

    Making a full up set of scantlings for this design aren't especially difficult for a designer or NA, though there can be some pit falls for the novice. Are you sure you want this type of boat as it's a fairly heavy thing to move around a puddle. I have a 28' x 8' 8" powerboat, with V8, transmission, 50 gallon steel fuel tank and heavy bronze gear hanging under, it that's only 4,500 pounds.

    To directly answer your question, traditional strip with a light abrasion sheathing would be 1 to 1.25" thick strips, maybe 1.5" tall, with 10 ounces of cloth on the outside only. The cloth really isn't necessary except for abrasion resistance. 3/4" thick by 1.5" tall strips with two layers of 12 ounce biax on each side will work, though you may need some ballast and you might want a finishing cloth over the biax to make fairing easier, say a 6 ounce. It all depends on what you want to do, but given the hull design is fixed, you're stuck with the displacement and any weight savings will have to be put back in the form of ballast, which could make for a jerky ride.
     
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