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Boat Forum / Building / August 2005pontoon construction
Dear Group, I am interested in some feedback on pontoon construction. I have looked at several designs for wood pontoons with fiberglass over them. I would not be building something huge, perhaps about 18 feet max. Has anyone had experience building a pontoon? I have built wood/fiberglass construction before, but never a pontoon. Ideas on floatation foam and bulkheads? Proper support for deck attachment? Etc. Thanks a ton for any feedback. M Russon If you plan on using pressure treated plywood or lumber, there is some concern that neither polyester or epoxy resins will stick for very long to the new preservatives they now use. Also the new formulations are very corrosive to metal so all fastenings and fasteners would have to be compatible. If epoxy won't stick to the stuff, I would also wonder about any glues and adhesives. Sam >If you plan on using pressure treated plywood or lumber, there is some >concern that neither polyester or epoxy resins will stick for very long >to the new preservatives they now use. Also the new formulations are >very corrosive to metal so all fastenings and fasteners would have to >be compatible. If epoxy won't stick to the stuff, I would also wonder >about any glues and adhesives. Sam Sam, Thanks for the reply. I was going to use a non pressure treated plywood, most likely a marine grade ply on the deck and an exterior grade on the actual pontoons prior to fiberglass. I am a bit gun shy of the pressure treated wood since i believe they still use arsenic as a chemical in that stuff. Any other suggestions for the actual pontoons? MRusson > I was going to use a non pressure treated > plywood, most likely a marine grade ply on the deck and an exterior [quoted text clipped - 4 lines] > > MRusson I would divide the pontoons into watertight sections for emergency flotation and skip the foam which would only add redundacy, price, weight and a good starting point for rot. I would also make each watertight section inspectable with a hatch or seal big enough to make the inside accesable for repairs, etc. The hatches wouldn't have to be store bought but could be made of plywood that was secured in place with screws and sealed with a non-permanent caulking, say silicone or White Lightning house caulk as opposed to 5200, as you wouldn't have to open them but once a year, more or less. The bottom corners (chines?) of the pontoons will have to be slightly rounded for the fiberglass to wrap around without creating air pockets and they should also be reinforced with extra layers of glass or something like angle iron imbedded in caulking as they will recieve more wear than elsewhere. Try to avoid, by proper design, water sitting on top of the pontoons for long periods of time. The weak point of pontoons and catamarans is the system/deck that connects them and the tendency for them to twist in waves and wrack/wreck the boat. Some people replace rotted decks, and you might consider building a new deck, with pressure treated plywood topped with rubber roll roofing for flat roofs that comes in wide enough sizes for one piece to cover the whole deck. It is glued down with contact cement or some similer adhesive and then indoor outdoor carpeting is glued to the rubber for a leakproof deck. Sam >> MRusson >I would divide the pontoons into watertight sections for emergency >flotation and skip the foam which would only add redundacy, price, >weight and a good starting point for rot. This makes good sense since i was thinking the same thing. I like the idea of skipping foam. Every old boat i have seen had rot developing around the foam locations. > I would also make each >watertight section inspectable with a hatch or seal big enough to make [quoted text clipped - 3 lines] >White Lightning house caulk as opposed to 5200, as you wouldn't have to >open them but once a year, more or less. I like this idea since it would provide a repair point for each section in the event water somehow got into the pontoon section. Would it be wise to also glass the interior seams for greater strength and a better resistance to rot? This way if water got into the section, it would puddle in a glassed section rather than bare wood. >The bottom corners (chines?) >of the pontoons will have to be slightly rounded for the fiberglass to >wrap around without creating air pockets and they should also be >reinforced with extra layers of glass or something like angle iron >imbedded in caulking as they will recieve more wear than elsewhere. I would think a heavy roving layered in a bi-directional fashion over a rounded edge could provide a stable corner. I could hold back the plywood from butting into the other side and leave a gap at the corner, and perhaps inset a radius tube or pipe etc. > Try >to avoid, by proper design, water sitting on top of the pontoons for >long periods of time. A slight convex angle to the pontoon top perhaps? >The weak point of pontoons and catamarans is the >system/deck that connects them and the tendency for them to twist in >waves and wrack/wreck the boat. Would these "hard points" at which the deck attaches to the pontoons be better if they actually flex around a bit during use? Perhaps a rubber mount of some sort? I wonder if a set of heavy lag bolts secured and epoxied into the pontoon top, and sealed, could be modified to act as a pin rather than a bolt. The deck could sit on top of the pontoons and be pinned into place with rubber cushions under the deck to provide flex and movement. ?????? > Some people replace rotted decks, and >you might consider building a new deck, with pressure treated plywood >topped with rubber roll roofing EPDM roof liner perhaps? >for flat roofs that comes in wide >enough sizes for one piece to cover the whole deck. It is glued down >with contact cement or some similer adhesive and then indoor outdoor >carpeting is glued to the rubber for a leakproof deck. Sam Sam, GREAT info...........:o) M Russon Just be careful when creating "hard points" that you spread the load smoothly out to the skins. Any abrupt change in loading such as occurs at the edge of a metal backing plate will lead to failure in that area. Prime areas for concern include deck attachment points and bulkhead to skin joins. Big epoxy fillets are good; as are stacked backing plates of decreasing size with the largest and limpest next to the skin. Roger derbyrm@NOSPAMearthlinkNOSPAM.net http://home.earthlink.net/~derbyrm > Dear Group, > I am interested in some feedback on pontoon construction. I have [quoted text clipped - 6 lines] > > M Russon >Just be careful when creating "hard points" that you spread the load >smoothly out to the skins. Any abrupt change in loading such as occurs at >the edge of a metal backing plate will lead to failure in that area. > >Prime areas for concern include deck attachment points and bulkhead to skin >joins. Would these need to be a fixed hard point on the pontoon top? Could these be made flexible somehow? Rubber mount points with steel pins perhaps? I can see the pontoon top flexing and cracking with a small hard point with a sub-standard backing plate. Could bulkheads in the pontoons be inceased in size or more placed in the pontoon to help overcome the flexing? I would like to attach the deck directly to hardpoints at the bulkheads of each pontoon. The idea was to have a bulkhead in the pontoon at every other deck mount point. If the deck is 18 feet long, i could space the bulkheads at 24" apart inside each pontoon. I could space the deck supports at 12" on center and bolt through every other one to the pontoon tops. I think that 24" on center at the deck supports is too far apart and would make for a spongy deck. >Big epoxy fillets are good; as are stacked backing plates of decreasing size >with the largest and limpest next to the skin. The larger the better? Thanks Roger..... M Russon Flexible mounts is a whole different subject. The point I was making has to do with transferring the load to the skin. It needs to be a smooth transition. Otherwise the skin flexes at the "step" and eventually breaks. Note that a flexible joint doesn't really cure this since I'm talking about the stresses at the perimeter of the backing plate. Eventually the joint will reach its limit and ask the skin to pick up the load. I believe you can see this in the crazing of the fiberglass hulls around the chain plates and winch mounts on some of the less well designed production boats. Bulkheads are good. They transfer the load to the skin around the whole perimeter of the pontoon rather than just to a spot on top, but do fillet them generously to the skin around the whole perimeter. I'm sure there's a practical limit (diminishing returns), but I don't know what it is. My gut feel is that a 2" radius would be good. (??????) Maybe with a 6" belt of epoxy/fiberglass applied to the inside first. As for having the pontoons move independently of the deck (and of each other), it sounds good, but I'd look for someone with first hand experience. Could you get the same effect by having a flexible deck? What kind of sea states are you expecting to handle? Roger derbyrm@NOSPAMearthlinkNOSPAM.net http://home.earthlink.net/~derbyrm >>Just be careful when creating "hard points" that you spread the load >>smoothly out to the skins. Any abrupt change in loading such as occurs at [quoted text clipped - 27 lines] > > M Russon Roger, Thanks for the reply. I would not be expecting particularly rough water when using the pontoon boat. My biggest concern would be the occasional bumps and potential rough water that would stress the pontoon to the point of cracking any hard point on the pontoon. I have been thinking about what you said concerning the flexible mount and it's relation to proper backing of the hardpoints. It would seem the best answer is to mount the deck securely at each bulkhead and fix it solid to the pontoon. I could construct the deck to "twist" a bit as i believe the factory built pontoons do. The pontoon would of course not roll side to side, but the deck could move with the pontoon. Aluminum has it's advantages for flexibility, but i believe it weakens and cracks over time due to repeated twisting. Having the bulkheads act as the anchor point, and distributing the hull and deck stresses across all the bulkheads at the same time seems the best answer. The deck would literally have to tear the bulkhead apart in order to separate from the pontoon. This would still allow the pontoons to be separated in individual compartments with inspection holes in the top, and allow repair to be performed in the event of a crack somewhere. I believe the worst stresses the boat can see sometimes come from the trailering and hauling of the boat to the water and back. Now you have my brain cranking along....:o)... M Russon >Flexible mounts is a whole different subject. The point I was making has to >do with transferring the load to the skin. It needs to be a smooth [quoted text clipped - 18 lines] > >Roger >derbyrm@NOSPAMearthlinkNOSPAM.net >http://home.earthlink.net/~derbyrm [quoted text clipped - 30 lines] >> >> M Russon |
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