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Boat Forum / Building / October 2007Marine Plywood?
My Potter 15 rudder is made of 4 pieces. The top is a solid core about 10"x15", with two pieces of plywood screwed to it on either side measuring 10"x30". The solid blade mounts between the plywood, below the solid top. One side of the plywood is falling apart, and the other side isn't doing much better. I went to the lumber yard today to buy some plywood to replace the bad parts. They have Marine Fir and Marine Okoume. They told me that neither will be durable; they have to be covered with fiberglass. If that is true, it certainly explains why the original is falling apart, as it is not covered in fiberglass. I bought a piece of fir, which I will cover with spar varnish just to get me through the end of the year. This winter I will rebuild the whole thing. (neither the top core nor the rudder blade are looking real good either). What is the story on plywood? Is nothing suitable for marine use without being fiberglassed? Would solid wood (3/8") substitute? Any advice on how I go about doing this so it is durable would be appreciated. > What is the story on plywood? Is nothing suitable for marine use without > being fiberglassed? Would solid wood (3/8") substitute? Any advice on > how I go about doing this so it is durable would be appreciated. Thinking about it, how about making the top core and one side out of one piece of solid wood, and then using a second piece of solid wood for the second side? I could partially resaw the core/side to remove most of the wood (where the rudder blade goes), and rout out the balance. That way it would all expand evenly, since it is all the same wood oriented the same, without any pesky plywood. Would it be strong enough? > My Potter 15 rudder is made of 4 pieces. The top is a solid core about > 10"x15", with two pieces of plywood screwed to it on either side measuring [quoted text clipped - 22 lines] > without any pesky plywood. > Would it be strong enough? Toller, One should very much wonder about the individual that told you that marine fir would not be durable enough. The difference between the marine PW and others is 2-fold. 1- PW graded as marine has no voids in the core. 2- The bonding adhesive has very low water permeability (it used to be better than exterior, but common exterior now usually uses the same process. Time was all small boats that were not clinker plank (even some that were) were made from marine plywood, and some are still out there. I have made rudders and centerboards from marine PW. Fiberglassing PW will just assure that it never has a chance to dry out (Oh- sure it is completely encapsulated - with ZERO porosity or permeability - Right - ) You can make PW cheek pieces and replace those on the rudder. What you make them from is not of much consequence in actual fact. The plan you outline in that last paragraph sounds like a whole lot of work. Matt Colie Lifelong Waterman, Licensed Mariner, Congenital Sailor and Third Generation Boatbuilder Or, you could simply buy a replacement from West Wight... http://www.westwightpotter.com/ ..maybe even upgrade to their 'Kick Up Beaching Rudder'? The issue with Douglas Fir plywood - marine or otherwise - is that is 'checks' - or cracks along the outer ply layer. Water can ingress along these cracks, and the rot process begins. The solution is to cover the plywood with fiberglass (stops the checking) and epoxy resin (stops the water). The application of these materials should be considered carefully, meaning that additional research into 'fiberglass and epoxy plywood encapsulation' should be undertaken before starting. Note that any screw / bolt holes in or through the rudder / tiller are an area of concern. Common practice (for bolts) is to drill an oversize hole, fill with a thickened epoxy slurry, allow to cure, then re-drill with the appropriate sized hole. In this way, water never gets to the wood. For screws, much the same except allow the crew to penetrate the wood for about 1/2 its length, i.e. 'drill oversize' for 1/2 the length of the screw. Anyway... good luck! MW A few thoughts: Ply, since it is laminated, is much more resistant to warping than solid wood. It's also able to take impact and strain without splitting as solid wood might. Provided you can keep the water out, it's a very stable material. Ply is, pound for pound, really strong. Good marine ply is void-free and uses waterproof glues. I build stitch and glue kayaks, where the 4mm (3/16") ply is encapsulated in epoxy resin - about 4 coats on each side. Our 'yaks take a pounding on our rocky beaches, and I've yet to damage the ply itself. Provided you epoxy seal both the inside and outside surfaces with multiple coats of resin, or even fiberglass cloth soaked out with epoxy, making sure you seal the end grain really well, there's no reason the ply won't outlast any solid wood, and possibly you :->)) Make sure you use epoxy, and not polyester resin - the poly doesn't stick to things or hold them together nearly as well. BTW - that Potter is one sweet little craft - used to admire them greatly in our sailing days, before we downsized and simplified things to kayaks and canoes. > My Potter 15 rudder is made of 4 pieces. The top is a solid core about > 10"x15", with two pieces of plywood screwed to it on either side measuring [quoted text clipped - 15 lines] > being fiberglassed? Would solid wood (3/8") substitute? Any advice on > how I go about doing this so it is durable would be appreciated. > My Potter 15 rudder is made of 4 pieces. The top is a solid core about > 10"x15", with two pieces of plywood screwed to it on either side measuring > 10"x30". The solid blade mounts between the plywood, below the solid top. Those side pieces are called the rudder cheeks. I've replaced mine on a 21 foot mahogony strip saiboat. They get a lot of stress when running before the wind with waves comming under the transom and trying to change your course on you. I just used ordinary fir plywood without any voids in it. It should be thick and stong. The thickness might be determined by the hardware used to hang the rudder off the transom (pintles and gudgeons) although you can rout the plywood to accept narrow hardware. > One side of the plywood is falling apart, and the other side isn't doing > much better. I went to the lumber yard today to buy some plywood to replace [quoted text clipped - 10 lines] > being fiberglassed? Would solid wood (3/8") substitute? Any advice on how > I go about doing this so it is durable would be appreciated. >> My Potter 15 rudder is made of 4 pieces. The top is a solid core about >> 10"x15", with two pieces of plywood screwed to it on either side [quoted text clipped - 10 lines] > transom (pintles and gudgeons) although you can rout the plywood to > accept narrow hardware. Rudder Cheeks eh? What is the piece between the cheeks called? Mine are 3/8" ply, routed down to 3/16" at the hardware. What did you put on them to waterproof them? > Rudder Cheeks eh? What is the piece between the cheeks called? > Mine are 3/8" ply, routed down to 3/16" at the hardware. > > What did you put on them to waterproof them?- Paint. For the edges anything you have handy will do. They are not going to be under water. If I were doing it today I'd spread polyester resin along the edges where it can soak in to keep moisture from penetrating. Epoxy, polyurethane varnish, or even linseed oil if that's what you have at hand. I'd also smear polyester resin on any places which would chafe, for example if the tiller is inserted between the cheeks (I don't know what that's called), or the rudder rotates between the cheeks ("kick up" rudder) . Polyester is good for abraision resistance. I use it on my home built boats where parts might rub. I coated the inside of a dagger board trunk with it. Just the resin, no fibreglass reinforcement. Or epoxy if that's what you have. >> Rudder Cheeks eh? What is the piece between the cheeks called? >> Mine are 3/8" ply, routed down to 3/16" at the hardware. [quoted text clipped - 14 lines] > dagger board trunk with it. Just the resin, no fibreglass > reinforcement. Or epoxy if that's what you have. I have epoxy (and linseed oil and polyurethane varnish) but hesitate to use it since it is not UV stable. I have never used polyester, but have heard it doesn't adhear worth a damn. That is not your experience I take it? > I have never used polyester, but have heard it doesn't adhear worth > a damn. That is not your experience I take it? A lot of things - including the 40' ply and fiberglass Newporter ketch - were made with polyester resin. Does it stick as well as epoxy? No. But it isn't exactly easy to get off either. When I had a sizeable sailboat I used it (with fiberglass) on anything I made of ply - trunk cabin deck, dog house, hatchs, etc. - and never had any adhesion problem in the 20 years I owned the boat. Consider the number of cars that have had dings filled with Bondo. Bondo is polyester resin and talc. Have you seen many chunks of Bondo along the roadside?  SignaturedadiOH ____________________________ dadiOH's dandies v3.06... ...a help file of info about MP3s, recording from LP/cassette and tips & tricks on this and that. Get it at http://mysite.verizon.net/xico "dadiOH" > Consider the number of cars that have had dings filled with Bondo. > Bondo is polyester resin and talc. Have you seen many chunks of Bondo > along the roadside? Alas, yes. "dadiOH" > Consider the number of cars that have had dings filled with Bondo. > Bondo is polyester resin and talc. Have you seen many chunks of Bondo > along the roadside? More than I can remember. You can spot a "Bondo Job" from a 100 ft in a snow storm. Lew "Toller" <Toller@Yahoo.com> > I have epoxy (and linseed oil and polyurethane varnish) but hesitate to > use it since it is not UV stable. Almost nothing is UV stable, including paint, wood, epoxy, varnish, polyurethane, and wood. However, sealing plywood edges with epoxy, lightweight fiberglass (with the appropriate weight fabric and a curve that it can take), covered with primer and paint, is as good as you can plan for. As long as you maintain the paint for appearance, it will probably serve to preserve the epoxy & fiberglass. I figure that a decent job of it, aside from direct scrapes and dings, should be good for 5-10 years of regular use. If you're looking at the leading edge of a kick-up rudder that meets the ground routinely, you'll need more than one layer of glass, and regular touch-ups of paint. > My Potter 15 rudder is made of 4 pieces. The top is a solid core about > 10"x15", with two pieces ofplywoodscrewed to it on either side measuring [quoted text clipped - 14 lines] > being fiberglassed? Would solid wood (3/8") substitute? Any advice on how > I go about doing this so it is durable would be appreciated. You can buy a marine plywood as the following: Species: hardwood(red hardwood like Okoume is better) or birch Glue:best Phenolic glue The core should be void free . Mason Pan http://www.plywood.cc/ Okume is not hardwood it is softwood, as such it is often covered in fiberglass. In France we use Moabi plywood with f/g but I'm not sure if its available in your area. >>My Potter 15 rudder is made of 4 pieces. The top is a solid core about >>10"x15", with two pieces ofplywoodscrewed to it on either side measuring [quoted text clipped - 22 lines] > Mason Pan > http://www.plywood.cc/ Okoume is hardwood . Hardwood means "wood from broad-leaved or deciduous species of trees (not necessarily hard or dense)" . Mason Pan http://www.plywood.cc/ > Okume is not hardwood it is softwood, as such it is often covered in > fiberglass. In France we use Moabi plywood with f/g but I'm not sure [quoted text clipped - 26 lines] > > Mason Pan > >http://www.plywood.cc/ >Okoume is hardwood . >Hardwood means "wood from broad-leaved or deciduous species of trees >(not necessarily hard or dense)" . Balsa is a hardwood. Yellow pine is evergreen and the wood is quite hard for a pine, of interest if you make tables or floors, and not meaning jack sh.t for framing. The confusion begins with the simple fact that among kinds of lumber pretty much all the hardwoods for sale are in fact harder than the softwoods for sale. All wood is not commercial lumber. There is balsa and there is ironwood, which doesn't float in water.You don't finder either at a lumber yard. Casady Go with rotary cut Sapele. It is reasonably rot resistant, much more so than Occume or Gaboon. It is about as strong as Fir but without the checking issues. It costs about 2 times what Occume or Fir costs. If you plan to keep the boat...go with the best materials and you won't have to do this job again. > What is the story on plywood? Is nothing suitable for marine use without > being fiberglassed? As others have pointed out, you have to be careful fiberglassing plywood. I build driftboats with marine fir and/or mahogany, covered with glass. But only for sides and seats. For the bottoms I use a plastic honeycomb core from Nida Core or Plascore. Plywood that stays in contact with the water will eventually get nicked or dinged. Water will migrate into the plywood and then you're toast--well, soggy french toast maybe. At that point you have to grind off the fiberglass and move to Arizona for a month or two. You could make your rudder with 3/4" Plascore, stiffened up with glass on both sides, with puttied and taped edges. It's a quick and easy way to make lightweight, stiff, water proof parts. > Plywood that stays in contact with > the water will eventually get nicked or dinged. Water will migrate > into the plywood and then you're toast--well, soggy french toast maybe. > At that point you have to grind off the fiberglass and move to Arizona > for a month or two. This must really sound weird to the thousands of plywood sailboats owners. There are plenty of 30 to 40 years old boats still sailing. Well painted marine plywood is very durable. On Oct 27, 7:08 am, Jean-Francois Dockes <j...@nautique-sevres.org> wrote: > > Plywood that stays in contact with > > the water will eventually get nicked or dinged. Water will migrate [quoted text clipped - 5 lines] > owners. There are plenty of 30 to 40 years old boats still sailing. Well > painted marine plywood is very durable. I was talking about marine plywood skinned in fiberglass. The fiberglass will eventually get cut or broken. And then moisture will migrate in the plywood core. This happens a lot faster on driftboats than sailboats. But the process is inevitable, at least for plywood that stays in contact with the water a lot. ..........and even then I was talking about hulls made with plywood that is skinned on *both* sides with fiberglass. Hull that have fiberglass on the outside and a paint or oil finish inside can more easily dry out. I've seen driftboats--with 'encapsulated' epoxy fiberglass bottoms--so saturated with water their weight was almost double. If you do build that way you have to keep up with the repairs, adn patch dinged fiberglass almost right away (not all that hard to do, but you must do it). Or you can build with honeycomb core. It's bullet proof and fool proof, almost. You can pre-fiberglass honeycomb core while it's still flat, on top of visqueen covered saw horses, and then treat it like plywood. > ..........and even then I was talking about hulls > made with plywood that is skinned on *both* sides [quoted text clipped - 12 lines] > flat, on top of visqueen covered saw horses, and then treat it > like plywood. The encapsulation is the problem. Once water gets in, and it will, it can't get out. The best way to handle plywood in my opinion is with paint which is more easily repaired, and allows for some breathing. I have some plywood boats over a decade old that are fine. One in particular, a Payson Skimmer has a glass sheathing on the bottom, but the inside of the ply is paint. I know that about a hundred folks will come in and slam me here, like every other time I say eposy is not waterproof in practice, even if it is in theory so I will not argue. I have built over 60 plywood boats in my life... >> ..........and even then I was talking about hulls >> made with plywood that is skinned on *both* sides [quoted text clipped - 22 lines] > waterproof in practice, even if it is in theory so I will not argue. I > have built over 60 plywood boats in my life... Anybody that comes along to slam you will have to explain himself real well. I repair these problems all the time and even replace plywood deck core with the edges only partly taped down - SO THE MOISTURE CAN GET BACK OUT - Sorry - I guess I'm too used to shouting that at people. If it is a very highly stressed area, like under winches and stantions, sometimes I glass that area and then pull tape to the edges to make the structural beam. Matt Colie |
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