Skin On Frame Catamaran

Discussion in 'Boat Design' started by ElGringo, Sep 14, 2016.

  1. ElGringo
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    ElGringo Senior Member

    Yep, flat bottom would be easier. I got off on that chine thing when the idea first came to me. Manfred.pech says the flat bottom works fine, and I have a lot of respect for what he thinks.
     
  2. upchurchmr
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    upchurchmr Senior Member

    Richard Woods has done a lot of flat bottom designs that seem to work well also.
     
  3. ElGringo
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    ElGringo Senior Member

    Yes Richard Woods has some nice boats and I love the looks of the Skoota 28 but I don't want to deal with the 5,000 pounds. I like the idea of two people being able to move one hull much better.
     
  4. Petros
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    Petros Senior Member

    to build a 16 ft skin on frame kayak it takes one 16 ft 2x10 carefully selected from home depo lumber stack (about $20). it is all the wood you need, it is ripped down on a table saw into stringers and ribs. The only other thing you need is some dowel stock (chop sticks work, but it only cost about $2 to buy one), and lashing cord (about $6), heavy nylon or polyester fabric, typically about $30 from on line suppliers, and than wood finish and oil based paint to seal the skin, about $30. Less on sale, or I like to buy the "mis-tinted" paint for $1 per gallon. Also, both lumber and paint is easy to salvage.

    You can buy a lot costly deck hardware, rudders, seats, and other crap you do not need, but I either make it from scratch or use webbing and cord for gear slings, carry handles, etc that costs next to nothing. It uses no fasteners other than to lash the frame with polyester lacing chord (stronger, faster, cheaper and lighter than metal fasteners), and it uses a small amount of glue in some of the joints, either Tight bond III or Polyurethane glue, epoxy can also be used but it is more troublesome and costly.

    where would the costs be higher? I have built perhaps 30 boats this way, some were as cheap as $35 out of pocket, using salvaged materials, the most I spent was about $300, an all plywood dingy (stitch and glue).

    Below is my wife's skin on frame kayak that cost about $80 to build. And two different skin on frame trimarans we built for a contest that cost less than $200, buying all new materials, including tarp material for the sail.

    Building durable boats does not have to be costly, you just have to get creative on finding suitable materials, and designs that are less costly to build.
     

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  5. Skyak
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    Skyak Senior Member

    I think your plan goes off the rails as soon as you hang two outboards on it. How big and how fast do you think this boat will be? Power boat scantlings are based on impact ie proportional to displacement * velocity. You would be much better off making the bottom out of plywood at least. Better still, everything below the waterline, eh just build a stitch and glue boat, eh there are tons of power boats for sale cheaper than you can buy two motors....
     
  6. ElGringo
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    ElGringo Senior Member

    Skyak, less than 20 feet, less than 10 horsepower, and probably 10 to 12 mph. Just the usual stuff like, Light enough to float on damp pavement and stop a 50 caliber bullet.

    I really don't know, I just wanted to find out if it was possible. I think now that it is possible, and maybe even add less than 600 pounds with motors to that list. I saw a lot of Kayaks in the 16 foot range that weighed less than 25 pounds.
     
  7. Skyak
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    Skyak Senior Member

    10 to 12mph puts you back in skin territory -and good news! you only need one motor 2-5hp. I would still have a solid ply flat bottom and just have skin over frame above for light weight.

    If this is mostly for easy deployment, have you seen what the russians do with inflatable hulls and aluminum frames?
     
  8. Petros
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    Petros Senior Member

    On skin on frame construction, all of the strength and structure of the hull comes from the frame. the skin just keeps the water out. for an outboard you would want a stout frame, particularly on the rear where the motor mounts. If you use heavy 20 oz ballistic cloth as the skin, no reason to use plywood, it is stronger than plywood.

    It would be a fast and easy build, and reasonably light if you can optimize the frame member sizes to your speed and power needs. An inland lake sailboat can be fairly light frame since the only power comes from the wind.
     
  9. ElGringo
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    ElGringo Senior Member

    Skyak, you are right, plywood below the waterline would be better and I doubt that the thing would have more than a few inches of draft. The flat bottom would be easier to build. I'm just gathering information at this point, And I think this is something that I could build even with my back problems.
     
  10. ElGringo
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    ElGringo Senior Member

    Petros, when you decide what design you are going to use, I would sure like to know.

    I saw the video on youtube of the guy beating on a SOF kayak with a claw hammer and he was not messing around. He was hitting it. That was the moment that I started thinking of a lightweight catamaran. The wind here where I live is nothing or way too much and can change from one to the other in minutes. AND, I don't know how to sail a boat. I think mine will have at least one 10 horse motor.
     
  11. upchurchmr
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    upchurchmr Senior Member

    You might think about an alternative.

    A 20' fiberglass catamaran - the Tornado - has been used to build 2 different powerboats.

    The Water Bug was designed and built by Russel Brown. It was recently seen being used a photography boat for the "Race to Alaska" in the Puget Sound area (Washington state).
    I've never seen what the power used is but there is a little description here: http://proafile.com/multihull-boats/article/water-bug-an-suv-catamaran-powerboat

    Tornado hulls are about 80# each, so the entire boat might not weigh more than 400#.
    The original tornado is 10' wide, but you could make the cab or deck any way you want.
    [​IMG]
    The other boat is by Gary Dierking a multihull designer.
    http://outriggersailingcanoes.blogsp...max-results=35

    Tornados are not real common, but there is a guy here in Ft. Worth who has two sets. Never seen them but probably the same as a boat I had.

    [​IMG]

    Tornado hulls have very low resistance in the water, so long as you don't sink the transoms deep.
     
  12. ElGringo
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    ElGringo Senior Member

    Where would a person find these Tornado hulls? Are they sold as just a set of hulls?
     
  13. upchurchmr
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    upchurchmr Senior Member

    You can buy them as new for a huge amount of money.
    But the only sensible way is to buy them used - you just have to look around.
    They were not common, it was a high dollar Olympic sailboat.

    You can also find plans to build the original plywood set online.

    An alternative could be a Hobie 18, Prindle 18-3, but the Tornado had more size - able to carry more weight.

    You might check the Tornado International web site and see if anyone is selling an old boat. I suppose you could advertise for wanting some hulls on the same site.
     
  14. Petros
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    Petros Senior Member

    I will probably develop my own design, borrowing lines from something that sails well. The skin on frame structure does not take point loads (like beams or mast or shroud lines) the same way a hollow shell hull does, so I usually develop my own structural details.

    For your purposes I would make it 8 ft wide and 16 ft long, that way you do not have to take it apart to get it home. use something like 1x4 gunwales and something similar laid flat as a "keel". 3/4" solid pine plank can be used to make frames that you just screw the stringers to it. You might also use a pair of 1x2 chine stringers. That would make a very stury hull.

    You can use a pair of 2x6 for beams, and than either frame a light weight plywood deck, or use a fabric deck, between the beams. mount a single outboard on the rear beam (5 to 8 hp max). And use as heavy of fabric as you can find for the skin, 12 oz/sq yard min, 20 oz would be better. Nylon or Polyester. could be built for under $300 I would expect.
     

  15. Skyak
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    Skyak Senior Member

    You did not mention the sealant or dope -important for cost and compatibility.

    Major differences in nylon and polyester. Nylon is hygroscopic -you can heat shrink it which is important for non conic surfaces. You need to seal it well or it will lose tension in water.

    Polyester doesn't heat shrink -do you think that will not be a problem on his long thin hull? If you don't need to heat shrink you can just buy PVC impregnated polyester.
     
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