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Boat Forum / Building / October 2005Cold molding with thin strips
Hi, A number of weeks ago, I posted a question on this group about building a composite hot tub by "borrowing" some boatbuilding methods. I've been looking at cold-molding vs sheathed strip (is this synonymous with wood cored glass?). If I used sheathed strip (with 1/4"-1/2" or greater core), I might not be able to obtain the desired curvature. I could probably obtain the desired shape with cold-molding using veneers, but I think I that I would have to make a decent mold to support the veneers during construction, and it would still be awkward. I read some posts about people using multiple layers of thin wood strips for cold molding, instead of using veneers. I'm going to have some exterior framing surrounding the tub, and I think that I could simply attach the first layer of wood strips directly to this frame (probably via staples). Then add a second layer of strips, diagonally oriented from the first, working from the inside. Then a third diagonal layer, with the opposite orientation. The strips would probably be 1/8" each. I'm not sure exactly how many layers I would use, I would gauge the stiffness of the structure after 3 layers and go from there. My question is this: If you are planning on using this type of "cold molding", and plan on encasing everything in glass in the end, what type of glue/epoxy can you get away with for the layers of wood in between? I've read that a lot of the sheathed strip kayak builders get away with using yellow wood glue (some even say they've used hot glue), knowing that the fiberglass is going to provide a moisture barrier. Keep in mind that I would be edge gluing the strips together, and also gluing each layer to the previous one. Thanks for the help. Shamus If you can take the weight it would be a good application for ferrocement. I've seen government programs for making water and grain storage tanks in "developing" countries that way. -- ------------------------------------------------------------------------------ William R Watt National Capital FreeNet Ottawa's free community network homepage: www.ncf.ca/~ag384/top.htm warning: non-FreeNet email must have "notspam" in subject or it's returned > Hi, > > A number of weeks ago, I posted a question on this group about building > a composite hot tub by "borrowing" some boatbuilding methods. I've been > looking at cold-molding vs sheathed strip (is this synonymous with wood > cored glass?). <snip> > My question is this: If you are planning on using this type of "cold > molding", and plan on encasing everything in glass in the end, what > type of glue/epoxy can you get away with for the layers of wood in > between? Epoxy. > I've read that a lot of the sheathed strip kayak builders get > away with using yellow wood glue (some even say they've used hot glue), > knowing that the fiberglass is going to provide a moisture barrier. With sheathed strip, there is only one single layer of wood, which is kept together by the glass on both sides. Glueing the strips together is only necessary to keep them together until the glass goes on. Nails could be used as well. However, with could moulded, you have more layers of wood which will only keep their shape when glued together very rigid. So in this case you MUST bond the layers together very well.The glass is not providing any or much help to retain the shape. Could moulded can even be used without glass sheathing, as Paul Gartside does. And since it is impossible to privide the necessary clamping force needed by ordinary glues to get a decent bond, you'll simply have to use epoxy for this which does not need any clamping force at all. And as said earlier, don't go blind on the cost of epoxy, it will only be a fraction of the total cost of the boat. Meindert > And since it is impossible to privide the necessary clamping force needed by > ordinary glues to get a decent bond, you'll simply have to use epoxy for > this which does not need any clamping force at all. Make that "thickened epoxy", as it will have to be applied thickly enough to fill any gaps and irregularities. Sounds like a huge pain the butt to me. Brian Nystrom, Yes, I'm starting to think that myself. However, due to the need for a custom design (the shape I need isn't available off the shelf), my options are somewhat limited. I could learn to build a "real" fiberglass foam mold, but from what I hear, it would be a long learning process, and I'd probably mess it up the first time. The "fallback" option is to use plywood sheathed in fiberglass, with straight vertical walls, and add seperate benches. Meindert Sprang, Thanks for the advice. I've found some relatively cheap sources of 5-gallon thick epoxy kits on eBay. They're not suitable for wetting fiberglass, but probably fine for this task. William Watt, I haven't actually calculated how much cement I would need to use, but I have a feeling that it would weight too much. Maybe I should look into it, because the water will weight around 4000 pounds after all (though there may be less water if the tub has a curved bottom and seats). Let's say the tub is 5' x 6' x 2.5', and for now, it is just rectangular. The surface area (including bottom and 4 sides) would be 85 square feet. If the cement was 3" thick (I'm not sure if this is reasonable), the volume of cement needed would be 85 ft^2 * 0.25 ft = 21.25 ft^3. Let's see, 1 foot is about 0.3 meters, so 1 cubic foot is about 0.027 cubic meters. So 0.57 cubic meters. At 2300kg/m^3, that would weight about 1367kg, or a little over 3000 pounds. I think that would be too heavy for my application, though it all rests on my guessing that the walls would be about 3" thick. Thanks for everyone for the help. Here's another thought. Picture the profile of the tub you want. Cut this profile into a ply rib, and repeat every 6 inches or so. Then cut a profile of the tub at rt angles to the first every 6 inches, with slots to fit in the other ribs. 2 inches depth would be stupendously strong, with an upper and lower glass skin. Brian Whatcott >Hi, > [quoted text clipped - 31 lines] > >Shamus Hi Brian, I wish there was a standard way to draw diagrams on the net. I'm not quite sure if I've understood correctly. Do you mean that the first set of ribs would be mainly vertical (for a "conventional bathtub", they would look like U's), and the second set would be longitudinal? ie, a see-through grid of ribs. If so, would you recommend attaching a sheathing of thin ply before applying the glass skin? Also, by 2 inches, do you mean the width of the ply strips? Sorry if I'm missing the obvious, I haven't yet learned much about boat building, and probably don't see some of the things that are clear to others. > Hi Brian, > [quoted text clipped - 4 lines] > see-through grid of ribs. If so, would you recommend attaching a > sheathing of thin ply before applying the glass skin? See http://www.gartsideboats.com/jessie.php This is a could moulded design, where temporary stringers are laid fore and aft, to hold steam bent frames. This is covered with two layers of wood, one 1/4" thick, the second 1/8". When you add a 3rd layer, you can probably leave the frames out. BTW, this boat is not covered with glass. If you build the same boat with strips, you need glass inside and out but no ribs are needed. Meindert > See http://www.gartsideboats.com/jessie.php > This is a could moulded design, where temporary stringers are laid fore and > aft, to hold steam bent frames. This is covered with two layers of wood, one > 1/4" thick, the second 1/8". > When you add a 3rd layer, you can probably > leave the frames out. Leaving the frames out would be good - otherwise I'd have to put a layer of wood on the inside too (for comfort reasons - it is a hot tub). > BTW, this boat is not covered with glass. > If you build the same boat with strips, you need glass inside and out but no > ribs are needed. I think sheathed strip would be the easiest construction method, if I could get it to work for the desired curvature. Could I use very thin wood strips (1/8" or so), and rely almost entirely on the fiberglass for strength? Basically the only purpose of the wood would be to avoid making a mold (as would be needed for 100% fiberglass construction). > Meindert Thanks again > Leaving the frames out would be good - otherwise I'd have to put a > layer of wood on the inside too (for comfort reasons - it is a hot > tub). Don't know if anyone has mention this before, but as I understand epoxies are highly sensitive to temperature. This was googled out on "epoxy temperature" search: http://www.epoxyproducts.com/novolac.html I anticipate the tub being about 104 degrees F. So notwithstanding any problems with my heat source, I should be below the 140-160 degree maximum (for wet conditions) of regular epoxy. Although my heat source is wood-fired, I have a thermostat of sorts worked out. I can probably control the temperature to within a few degrees. >Hi Brian, > [quoted text clipped - 10 lines] >building, and probably don't see some of the things that are clear to >others. This is a speculative idea, rather than the fruit of long experience. Yes I am thinking of ply ribs in two directions at right angles, Whose faces are all vertical, so that in plan view, they would look a noughts n crosses game of lines say 1/4 in thick, and in side view they would be 2 inches deep and wide (at the sides.) An internal and external skin would then provide a strong stiff light structure. The crucial detail is making the inside skin fair and smooth. This is left as an exercise for.... :-) Brian Whatcott > This is a speculative idea, rather than the fruit of long experience. > Yes I am thinking of ply ribs in two directions at right angles, [quoted text clipped - 6 lines] > > Brian Whatcott Interesting idea. For the inner/outer skins, did you have in mind multiple layer of wood like the boat Meindert Sprang pointed out, or fiberglass, or both? I wonder also if it would be wise to put a coat of epoxy on the inner ribs, to prevent rot (just in case steam managed to work through the outer skin somehow). Perhaps regular wood sealer could even be used instead of epoxy, since its adhesive properties aren't needed, only a moisture barrier. >> This is a speculative idea, rather than the fruit of long experience. >> Yes I am thinking of ply ribs in two directions at right angles, [quoted text clipped - 16 lines] >instead of epoxy, since its adhesive properties aren't needed, only a >moisture barrier. I thought, fix thin laths to the inner rib contours. The laths need not be gapless. Then a layer of glass on top. Brian W |
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