Skin On Frame Catamaran

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

  1. upchurchmr
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    upchurchmr Senior Member

    Skyak,

    Nylon does not heat shrink. Nylon is wet down, sewn in place and allowed to dry to shrink out wrinkles. But if it gets wet again it relaxes which allows whatever wrinkles were present when you sewed the skin on. The coating does stiffen it up some. So not as bad as it sounds.

    Polyester does.

    That would have been a real mistake for the OP.

    The sealant does matter.

    PVC impregnated polyester cannot be heat shrunk and it does not stretch much, so getting it to go around 3D curves involves cutting darts in the skin and bonding it.
    But you get a consistent color, strength.
     
  2. Skyak
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    Skyak Senior Member

    Real "heat shrink" plastic is stretched so that it will recover back to molded shape to some degree. None of these fabrics is real 'heat shrink' and I would not expect polyester fabric to shrink when heated.

    Stretch nylon wet and it will tension when dry -sounds good but there is no control. I have seen hair dryers and heat guns, even irons used on nylon on SOF kayaks. Watch enough youtube videos of SOF and you will see it. It's not the heat, it's the dryness. Practice on scraps first.
     
  3. upchurchmr
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    upchurchmr Senior Member

    Skyak,

    If it is not the heat that stretches Nylon then it is not heat shrink.

    I've built 6 polyester kayaks using either a heat gun or an iron and when it gets hot it shrinks and pulls out wrinkles.
    It is the same process used on fabric "skinned" antique aircraft (and some modern ones).
    If you heat it too much it will melt.
    Perhaps you could tell me how many times you have used polyester and "heat" shrunk it?

    Sounds like you have only looked at youtube.

    Especially since you repeated what I said about nylon. The only difference was you suggested drying the nylon with heat - a pointless distinction.

    Your definition of "real" heat shrink is a fantasy you created. It means nothing.
     
  4. Petros
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    Petros Senior Member

    Not that critical: I like oil based polyurethane, my preferred sealant. But I have also used hypalon paint (costly and toxic), oil based paint, water based polyurethane, and even Latex paint, the trimarans pictured in my earlier post use Latex. It is cheap, dries fast compared to the others, and works okay if you allow it throughly dry (or the water will wash it off), also not as abrasion resistant but regular touch up is not big deal. The paint will not stick to nylon or polyester fabric, you have to "wet it out" so the fibers are encapsulated by the sealant. So the type of paint or sealant is not realaveant as long as it penetrates the fibers (on thinker sealants, like the oil based poly, I like to thin the first several coats). It should get min 4 and up to 7 coats of paint to fill the weave.

    The term "dope" is a really obsolete term, it dates from when they covered aircraft with cotton fabric and the "dope" had to shrink to make it smooth. That has not been done (expect in the rare historic restorations) for perhaps 40 years. Light aircraft, both antiques and home builts, us polyester skin and many use latex based paint, and water based polyurethane. The fabric is stretched and glued onto the airframe, and than heat shrunk to correct tension. the sealant and paint do not participate in shrinking the skin, I have taken classes on covering aircraft structures with polyester fabric.

    This is not quite accurate, nylon elongates with moisture (it actually absorbs the H20 into the molecule making the fibers longer), nylon does not heat shrink at all, but it does stretch rather nicly. What I have done is mist spray it while I am covering the frame, stretching it tight as I go. Than when done I put heat lamps on it over night to drive out the moisture. It will be smooth and tight in the morning. You get the same results if you live in a very dry climate, it is just the moisture leaving the fabric. I have noticed when nylon skined kayaks are hanging in my garage, on damp foggy days the skin tends to relax and gets minor puckers, but it does not affect it enough to be a n issue. Everyone expects rag boats to have puckers anyway, even the factory folding ones have puckers here and there.

    Polyester will shrink with heat, either heat gun or iron. It is a result of the way it is made, the aircraft type polyester (brand name Ceconite) is specifically stretched during the manufacturing to get maximum heat shrink, about 15-20 percent. But all raw polyester will shrink about 3 to 5 percent with heat, which is usually enough to get the puckers out of it on a kayak hull. You have to bring the fiber temp up to just below melting it, so there is some hazard to using this method (on aircraft you are required to use a calibated thermometer on the iron to make sure you do not exceed the max safe temperature). Polyester does not stretch, so it is tougher to get a smooth skin on the frame, but once on and shrunk, it will not relax and pucker up with moister like nylon.

    You generally do not have compound curves on a a skin on frame hull, so you would not have hollows. I have taken stitch and glue kayak designs that use plywood, and converted it to skin on frame, all the individual panels are simple flat wrapped. It also makes it fairly easy to get the skin on fair and smooth even without stretching fabric.
     
  5. WestVanHan
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    WestVanHan Not a Senior Member

    I've wondered about SOF,and also the heat shrink wrap for boats. A couple thousand square feet for a couple hundred dollars.

    Put it on,shrink it-then add a layer of some sort of FG or kevlar for puncture resistance etc,than another layer of FG then another outer layer or two. Ought to be easy,cheap and fast.

    Good idea..or not??
     
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  6. upchurchmr
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    upchurchmr Senior Member

    There is a guy named Bill Hamm over on the Guillemot Kayak forum who does something similar.
    He uses a 3oz polyester skin, then puts a single layer of glass epoxy on a wood frame.
    He use to teach a class on the method.

    Your method sounds very similar, without the extra layers. that would depend upon the use you are expecting, I guess. I've never seen the heat shrink wrap actually used.
     
  7. ElGringo
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    ElGringo Senior Member

    I do not remember seeing any of the videos on youtube where they said.

    is it OK to use epoxy resin to coat the fabric? Would it be any better than using polyurethane?

    When the frame is ready for fabric, is it sealed with epoxy prior to the fabric?

    If I were to apply the fiberglass "Shoe" to the bottom, should I use the kind that PAR has mentioned a few times, I think it is called Xylenole or "Poor Mans Kevlar"

    I'm going to go to the flea market and a few garage sales today and try to find an old pressure cooker to make a steam box with.
     
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  8. upchurchmr
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    upchurchmr Senior Member

    I have applied epoxy on polyester fabric, for local attachment. Only around the cockpit opening to bond the fabric to the wood frame. For that it worked, but it is tough to get the thick epoxy to penetrate thru the fabric.

    Epoxy will be a lot more expensive - about $200/gallon (mixed) verses $40 for polyurethane.
    I don't know if the brittleness of the epoxy would be OK, but I had some brittle sailboat paint I used and it cracked everywhere - another expensive mistake.

    To my knowledge, very few people coat a kayak frame with epoxy. Some use an oil, I used polyurethane.

    What do you want a steam box for? At least for kayaks, there is no steaming required, you just bend the frame members into place. You do need to make sure there are no knots or wild grain, but that would apply to steamed material also.
    If you were to make a SOF hull similar in shape to the Tornado I mentioned none of the frame members would need to be steamed.

    I don't know anything about Zylenole, but putting a stiff shoe on a flexible skin is a sure path to failure of the joint between them. To really guess you'd have to show exactly how you meant to do it.
     
  9. Tiny Turnip
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    Tiny Turnip Senior Member

  10. Rurudyne
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    Rurudyne Senior Member

    Has anyone done any work using SOF techniques to build light weight superstructures for cruising boats?

    I've wondered about using a light frame, outer and inner skin, lined with a batten material for insulation and sound reduction, as a way to get a larger cabin volume in a light displacement craft without weighing it down (say a modified Bolger Tennessee) or on a boat not made to carry a substantial superstructure (like many a pontoon boat).

    My interest are, I should hope it is obvious, intercostal or riverine use.
     
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  11. Petros
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    Petros Senior Member

    Do not use epoxy, it is brittle and has no UV resistance and will crack on fabric that will flex and stretch. It will get weakened by sun light, and flake off. Polyurethane is a far better product, particularly the kind intended for outdoor use (has UV protection), costs less, and is very durable for this use. Most brands of oil based paints work well also, but takes days for each layer to cure/dry.


    Seal the wood with oil or water based polyurethane or spar varnish. some people like boiled linseed oil, but that is traditional wood finish. It is really obsolete, common commercial exterior varnish is superior in all ways, fast drying, excellent handling and application, good attractive surface finish. Do not use epoxy as a wood sealant, it is brittle and will flake off, unlike the finish designed for wood surfaces. Epoxy is a "glue" that bonds fiberglass, kevlar, or carbon fibers to a surface, applied to add strength (all of these fabrics are very low stretch). On a boat you still have to give the epoxy several coats of exterior finish to protect it from sunlight. No point in adding epoxy since the strength of the hull comes from the structural properties of the wood, not a coat of glue.

    I would not mix materials such as fiberglass and polyester fabric. they stretch at different rates and they would likely just separate, it would add a lot of weight and cost too. To make the bottom more durable (but I have never had an issue with this in 30 some skin on frame boats I have built) you can either add a wood "rub strip" screwed down the keel line (that keeps the fabric off the sand or rocky bottom), or you can also cut 4" to 6" wide strips of the same fabric (cut on a bias, so weave is diagonal) and lay it over the keel stringer after you have the hull skinned over, glue it down with some kind of water proof fabric glue (I have used polyurethane glue applied with a squeegee, than lay on the fabric layer) and than paint six or seven coats of paint over it. Do not use the foaming type of polyurethane glue, but the construction adhesive type that comes in a tube.

    your fear appears to be that you think the fabric is vulnerable to getting damaged, that is just not the case. A heavy fabric hull (12 to 20 oz/SY) is actually more durable than plywood or fiberglass that would weigh and cost twice as much to use. you can bounce rocks off a skin on frame hull that would leave "bull's eye" damage to a fiberglass hull. The fabric will give, stretch and yield slightly to impacts, bumps and scraps, a "hard shell" of fiberglass will not so it has to be much heavier.

    I suggest just building one and using it without the extra layers. Inspect the bottom after one season of use, if it appears to have significant abrasion damage that another coat of paint will not fix, than add hard wood strips (screw into place) to the bottom keel stringer.


    Good place to buy wood working tools too! But I have found that boiling the wood works better anyway. I use a big turkey type pan with a hot plate under it, and put each half of each rib I intend to bend into it. Bend one side at a time and rotate them out of the heated water (it never quite boils, but works well enough). I have found a simple bending jig (made from a 1/4" piece of plywood or particle board) helps a lot to prevent splits.

    If the ribs are thin enough you only need to soak them, but heating makes it bend easier. I have also used thin strips (like 1/8" thick) and just laminated them together to the thikness I want, typically 1/4" for kayak size boat, more layers for something larger. It is more work to cut the strips, but faster to create the ribs, for a catamaran you can "free form" them so they make half circle shape (this would not be a good shape for a kayak, but works perfect for a cat hull).

    Good luck.
     
  12. ElGringo
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    ElGringo Senior Member

    There is a series of videos on youtube of a pontoon boat that has Coroplast walls and a lot of other alternative building methods, I think the videos are listed under Bret Becky, I don't know how well it would do in a hard wind.
     
    Last edited: Sep 18, 2016
  13. Manfred.pech
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    Manfred.pech Senior Member

  14. ElGringo
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    ElGringo Senior Member

    Hi Manfred, I have been looking around to see if there is anything I need to consider in my design for a half plywood and half fabric type catamaran with a flat bottom. It looks like a long box with a pointed front is about all you can say about doing it that way. I do think I will make the front of the hulls like Bernd did on his small power cat that we have been following. I am thinking 16 feet long and 18" wide and 24" tall for the hulls. I am going to try to make it as light as possible. I think 1/8" plywood and 1 layer of 6oz. fiberglass on the hulls. I'm not going to be out in bad weather, and I don't want to go very fast, so I think maybe 10 horsepower.
     

  15. Manfred.pech
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    Manfred.pech Senior Member

    It might be helpful to think about the displacement/length ratio first.
     
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