

Canoe all stripped, ready for glass, etc..

Discussion in 'Strippers, Stitch-n-Glue, and Other Wood Composite' started by joecool911, Jun 14, 2009.

  1. joecool911

    joecool911 Curious about Wooden Canoes

    I have a 20' (Outback 200) canoe ready for glass. This is a stripper canoe without the wood stems...Gilpatrick style construction. I have a few questions...

    My hull surface is about 64" at the widest point. To avoid confusion, it has a 42" beam but when you add the side height would take 64" wide glass to cover it at the center (I already own the glass). When you go back 24" in either direction the 60" glass will cover the boat completely. My question is how is the best way to glass this boat? I am thinking of a football piece plus two layers of 6 oz.. Seems like to avoid the oil canning that the glass coverage is important in the area of the center of the boat at the gunnels.

    Next. We are large people. That is one reason why we chose to build the 20' canoe. I have read about and worry about the bottom flex when standing in the boat. I read Todd Bradshaw's post about partial ribs on the bottom. I am wondering about ordering some white cedar ribs that are intended for use in canvas canoes. Maybe steam bending and adding some ribs to the inside for the standing areas. I am kinda worried about ending the partial ribs and having stress points at their ends. Seems like 3-4 ribs that go clear to the gunnels would be stronger. That way there would be extra support and stress distributed to the entire hull profile. How would I attach them without fasteners? I would not plan on glassing over the ribs.

    Are there any non-perminant solutions to the standing and flexing problem...some kind of mat that would distrubute the weight out?

    Looking forward to your responses and being part of this community.
  2. Todd Bradshaw

    Todd Bradshaw Sailmaker

    The idea of half-ribs causing a weak spot (called a stress-riser) at their ends may seem logical, but in real life, it doesn't really work like that. The reason is that as you get close to the turn of the bilge, the hull starts to get drastically stiffer from the curved shape alone. Ribs (or no ribs) have very little effect on that area, in or near the curve. It's already so stiff there that if you flex it much, it will most likely break, rather than flex and recover. As long as you taper the outboard ends of your half-ribs a bit, don't expect there to be any structural weakness at their ends. It's just not going to happen because what's next to them is already far stiffer than the flatter parts of the bottom.

    You can certainly use ribs which run all the way up the sides if you really want to, but the major effect that they will have is to additionally stiffen the sides (where you don't likely need more stiffness) and add excess weight. You see this sometimes on super-light Kevlar competition boats, where the sides are built "bleach-bottle-thin" and just have to keep splash water out. These boats usually have foam-cored bilge cookies covering most of the bottom and additionally, small ribs running up the sides of the hull.

    I think the permanent solution to the bottom-flexing problems on big strippers starts with proper scantlings and I sometimes shake my head when I see the layup schedules for some of these boats. My 22 footer has a narrow, flat bottom and flared sides. At its widest point, the flat between the beginnings of the bilge curves is only 27" wide. The bottom strips are 5/16" thick and in addition to the half-ribs, it has full layers of 10 oz. fiberglass covering the hull, over additional, bottom-and-chine-covering layers of 10 oz. cloth, inside and out. The areas under the stems are gradually built up to be six layers thick on the outside in the areas where the boat might ground-out on rocks. This may sound like overkill, but I wouldn't want it any more flexible down there than it already is. The same construction that would be used on a 16' touring canoe often may not be adequate for big strippers in terms of either strength or stiffness. I built my big boat for a potential Quetico trip situation with five or six paddlers and their gear. That's a lot of weight and a lot of momentum if you happen to accidentally nail one of those rocks that they hide just under the surface of what seems to be smooth, open water. That much force would blow right through a typical strip canoe hull. If you were building a big freighter canoe, designed for an outboard and higher speeds, you would have a similar problem. Sooner or later, you get into conditions where you are pounding your canoe through waves and that puts an incredible amount of stress and flexing force on the boat's bottom (and transom). In your favor are the facts that your design, though long and wide, maintains some arch in the bottom (and the stiffness that such a curve provides, compared to a truly flat bottom) and that it's actually more of a really big paddling canoe than something along the lines of a true freighter.

    With doubled six-oz. cloth inside and out on the bottom, it certainly wouldn't bother me at all to put some half-ribs in for extra insurance, but it's tough to say how much flex to expect without actually trying out the finished boat. Simple, glassed-in ribs or half-ribs would seen to be a much more secure and hassle-free way to permanently reinforce the boat than messing with inventing new technology. For a canoe of that size though, it's certainly not over-built with six-ounce and your personal pattern of useage will determine whether or not the construction is stiff and strong enough for your purposes. If the scantlings are adequate to keep the bottom from bouncing, some folks could probably paddle it for decades without a problem. Other folks might destroy it in a week. If nothing else, while using the boat I would keep in mind that it's a fairly lightly constructed hull for a big canoe and one you want to be somewhat careful with.

    I'd probably build to plan at this point, put it in the water and see how it behaves (I might hold off on varnishing the inside until I was certain that it didn't need more reinforcement in there). If you can move around and bounce the bottom much, it would be worth thinking about adding some ribs, half-ribs or maybe even a keelson, pogo-sticked off the thwarts to stiffen the bottom.
  3. Canoez

    Canoez Paddle Bait

    Sadly, I'm afraid that it does work like that and I think that statement is misleading.. From an engineering standpoint, the hull may get "stiffer" from the turn of the bilge, but the materials remain unchanged (unless you add extra glass or thicker wood) and are no stronger - the hull just distributes forces in different directions. Also, the additional stiffness does not mean that there aren't stress concentrations in that area. I was playing with an FMEA CAD model of a canoe hull with full-thickness half-ribs and found that having them end low on the bilge results in very high stresses. If you want end them either in the middle of the bilge or up at the gunnel, the stresses are much different. (lower) (AMENDED TO ADD - all of this depends on how you load the canoe - is it just paddlers or is it paddlers with gear in the boat...)

    As Todd said, tapering the ends to about 1/3 or 1/2 thickness at the ends over 1/3-1/4 of the rib length seems to be a very good idea if you are going to try to end the half ribs low on the bilge turn. This will give you a stiff hull center, but should limit the stresses distributed to the turn of the bilge. From what I saw, the thinning and the use of a large piece of overlaying glass to distribute the load seems to be key.

    As I said in my earlier post, professionally made boats that I've seen with failures at the end of the half-ribs that end basically at the beginning of the turn of the bilge seem to bear out the results of the electronic model. These failures are either delamination of the area at the end of the rib itself, or stress cracks showing on the outside of the hull in this area. As I said in my previous post, this may be thier choice of lay-up materials, poor construction methods or poor engineering of the boat.
  4. OP

    joecool911 Curious about Wooden Canoes

    Maybe it would be prudent to add a piece of biaxial glass as a football shape or 10 oz. woven to the inside and outside prior to the two layers of 6 oz full coverage. Then use some White Cedar rib material and make half ribs on the bottom surface. I like the looks of the wide/thin white cedar. I really dont want to try to glass them in. That sounds like a pain.

    I am talking closely spaced half ribs that cover the entire bottom like the spacing that is done with canvas canoes, but taper the ends and stop them before the bilge turn. This way your feet would be supported by the ribs no matter where you step. This would seem to be a much different concept than a few half ribs spaced a foot or two apart. It is my belief that the stripper canoes have more strength longitudinally due to the orientation of the strips and wood fibers. Having a network of half ribs spaced apart about the distance of their width would provide for distributing more weight in the weaker direction. I think it would look nice too.

    I wish now I would have used thicker cedar strips. But that option is void at this point.

    Thanks for your help here.
    Last edited: Jun 15, 2009
  5. Todd Bradshaw

    Todd Bradshaw Sailmaker



    I don't know about your cad modeling examples, but out in the real world, it works exactly as I stated. In addition to strippers, the technique of cored bottoms, cored half-ribs, cored keelsons, etc. has been used on composite boats since the Late 1960s (Indian Brand, Old Town, We-No-Nah, Mad River, Hyperform, etc.) and I have yet to see a boat damaged at the end of a half-rib due to a stress riser. In most cases, the taper on the ends or edges of these production ribs and panels is often very short, in many cases just a 45 degree angle (as seen on most competition kevlar boats).

    Yes, the materials generally remain unchanged through the curve and any abrupt thickness transition in a cored construction makes a stress riser at its end or edge, but as I mentioned, this curved area at the turn of the bilge will usually break on impact, rather than flex and/or distribute the forces anywhere - and whether or not it has half-ribs or a cored bottom panel doesn't seem to make much real difference. With any material except maybe single-layer polyethylene, if you hit something hard with that section of the canoe, you are most likely going to do some damage because flex in that area is quite limited due to the curve. The end of a nearby half-rib or the edge of a cored bottom panel is usually nowhere near as stiff. The boat that I showed in the photo with the funny little half-ribs was built in 1976 using redwood and polyester resin and is still doing just fine. The typical crew weight is usally about 1,000 pounds. If you can tell me when I should expect it to break at the ends of the ribs, I'm all ears. Stress-riser or not, similar ribs and floor-coring have been standard practice in the canoe industry for decades and don't seem to be causing any serious problems. In a perfect world, all cored canoes would have smooth transitions in coring thickness throughout the hull, depending on the desired thickness for any particular area, but that hasn't happened yet and isn't likely to happen any time soon. In the mean time, there are thousands of boats out there working quite well, despite some rather abrupt changes in hull thickness.

    Joe, biaxial cloth would strengthen the bottom, but you might have some clarity problems that you wouldn't like, so you might want to do a small test first. You might find that one more bottom layer of six-ounce would be easier and look better if adding more glass is the aim. Stiffness gain is more a matter of increasing the distance between the load-bearing outer skins (hence the addition of ribs, cored-floors or thicker bottom strips). The glued-in wooden rib thing on a composite hull has been done by a couple manufacturers and can work, but does create a bit of a maintenance and life-span problem. They look best varnished, but are sealed better when coated with epoxy and it's hard to do it neatly enough in the boat to really look nice. I suppose you could coat them, one by one on the workbench, sand them, glue them in and then varnish, but I haven't tried it. I prefer the coring on my composite boats to be the glassed-over variety.
  6. OP

    joecool911 Curious about Wooden Canoes

    OK Mr Bradshaw and I also appreciate the input from Canoez.

    Sounds like the best solution may to be the addition of another layer of wood on the bottom to put more distance between the layers. So what if I added a layer 1/4" or so of alternating colored wood material to the bottom and taper the ends as it meets the bilge rounds? I like the looks of the wider cedar rib material. I could rip some approximately 2" wide or so and glue them to the existing cedar strips. This would give me about 1/2" bottom thickness. Would not weigh all that much more. This is a big canoe that cant be thrown around by one guy very well anyway. I think it will weigh about 100# when done? This way the inside glass will form around all the wood and make finishing easier.
  7. Todd Bradshaw

    Todd Bradshaw Sailmaker

    I imagine that it's possible, but don't know anybody who has tried it on canoes. It's not uncommon on some cold-molded sailboats to start with a layer of strips and then laminate layers of 1/8" veneer (sometimes even thin plywood) over them running diagonally or crosswise. They do the laminating using slightly thickened epoxy to help fill any small gaps between layers.

    If you could get your inside bottom sanding even and fair, it could certainly be done and it should be quite stiff. True strength in a cored composite has to be a balance of the core and the fiberglass skins though. To take maximum advantage of a 1/2" thick core, you would probably need more than just a couple layers of six-ounce, but the standard layup would still likely be pretty tough. I have accidentally run over a few rocks in tripping-sized strippers with two layers of six ounce inside and out that sounded so bad that we actually pulled over to check the bottom. The result was usually just a deep scratch in the filler coats. The bottoms were stiff enough and the glass was sufficient to keep the core from flexing and cracking (which almost always happens along a weak grain line, rather than the glue lines between strips). As your canoe gets bigger, wider and more flexible on the bottom, this "normal" layup (3/16"-1/4" strips and doubled 6 oz. cloth) will eventually not be adequate to survive such a collision, but I don't know just where the limits are.

    Is a double bottom needed in this case? I doubt it, but it could be done. We can assume that the designer has built and at least water-tested one of these boats. The reason that I like the glassed-in half-rib scenario is that you can test the boat built to plan, see whether or not the bottom bounce is tolerable and then glass-in the ribs if you think you need them. They add minimum extra weight and work and their function is pretty much strictly to prevent the eventual fatigue caused by bounce and to help insure that the bottom is never likely to flex so far that the strips crack along the grain lines down the middle of the bottom. Glassing them in is the most secure and the most maintenance-free method of attaching them.

    I was an Old Town dealer for many years and I'm reminded of the time I called them to complain that there was an incredible amount of bottom bounce in their big, wide, deep 17'2" Royalex "Tripper" model (one of their most popular canoes at the time). The canoes' bottoms humped-up and wiggled in use, almost like the bottom of my whitewater raft, which is really inefficient from a hydrodynamic point of view. Their reply was "Well some folks put a sheet of 3/4" plywood inside over the bottom". My reply was something like "One 78 lb. canoe and one 30 lb, sheet of plywood equals one 108 lb. canoe - oh goodie!"

    At the same time, their fiberglass boats were using a large, cored section of end-grain balsa blocks as a stiffener between the glass layers and were quite decently stiff. The balsa comes in sheets of small blocks, held together with a light cloth layer on one side and will will bend to fit shallow contours pretty well. They would gelcoat the mold, lay up part of the glass schedule, then lay the sheets into wet resin and finish with a fairly substantial fiberglass layer over the balsa on the inside of the hull. It was pretty tough, but you have to be sure balsa is very well sealed by the glass layers or it absorbs a lot of water and delaminates. Modern cored boats use closed-cell foam instead. It's not quite as tough, but it's lighter and doesn't absorb water if damaged or not completely sealed. This is a sheet of the balsa blocks. This one is 3/8" thick, but it was available in other thicknesses as well. I suspect Old Town's sheets were more like 3/16"-1/4" thick, but never cut open a boat to find out. The edges of the balsa-cored sections, as I remember, has a short bevel cut into them, but there was a small level change, rather than a really smooth taper.

    Attached Files:

  8. Dan Lindberg

    Dan Lindberg Ex Wood Hoarder


    Adding a second layer to the wood core sound interesting, and likely would make a very stiff bottom, but if you plan to hit any rocks, a bit of flex isn't all bad.

    Being a beliefer in multiple layers of thinner glass, if it was me, I'd put on 3 or 4 layers of a tight weave 4 oz cloth, per side, and then as Todd suggests, test paddle it. If it isn't to your liking, then you could add stiffeners to tune the canoe.

    Standard plain weave 6 oz and heavier just take too much resin.

  9. OP

    joecool911 Curious about Wooden Canoes

    One last idea before we put this to rest.

    Could I take the canoe out of the molds...cut the molds down the thickness of rib material plus glassing schedule...steam bend some ribs on the molds and then install them in the canoe? My molds are 12.75" apart. Then use the same material to make half ribs in between the real ones. Only part I am unsure of is getting the steam bent ribs to make contact to their whole mating surface to get them glued in. I would do this after the glassing was done. Coat them with epoxy prior to installation.
  10. Dan Lindberg

    Dan Lindberg Ex Wood Hoarder

    Sounds like a lot of work for little return.

    If your stations are 3/4 thick, how are you going to keep the steamed ribs laying on them correctly? I would think that with that narrow of support it would be very difficult to get them to lay correctly.

    FWIW, on W/C, I bend my replacement ribs over the outside of the canoe, after carefully measuring inside and outside dimensions to locate the ribs. My only concern doing this for you is what would the heat do to the fresh resin.

    As for getting them to lay flat against the inside surface, you probably can't, thickened resin, peanut butter, will be your friend. :)

  11. Todd Bradshaw

    Todd Bradshaw Sailmaker

    Agreed. At this point, I'm getting the feeling that your mods are more likely to screw up a perfectly good boat than to improve it. The biggest mistakes that beginners tend to make on strippers are alarmingly often the direct result of getting creative with something that they don't know enough about and not simply sticking to the designer's plans and instructions which are right there in front of them. I still firmly believe that your best bet, by far, is simply to build the boat according to the plans, toss it in the water and see what it does. If the bottom then flexes more than you think is proper, it's a pretty simple fix that has been successfully used on thousands of high-end canoes over several decades.

    If you really like the look of wooden ribs, I'd start reading some of the excellent books on traditional wooden canoe construction and planning your next boat project. Composite/ribbed hybrid canoes have a pretty poor track record and are probably best avoided - especially after putting in as much work and cash as you already have on this one.
  12. OP

    joecool911 Curious about Wooden Canoes

    Thanks for all your help. I will follow the design specs with two full layers of glass and a football piece inside and out. Then evaluate for half ribs after finished. Will keep you posted with pics. Probably will have other questions too, but thanks for now.
  13. Steve Willoughby

    Steve Willoughby New Member

    Hi I have been viewing this forum for a long time finnally decided to post. All this talk about oil canning with strippers got to me. If a stripper oil cans with a covering of 6 oz. glass inside and out its not the fault of the building process , it is just a bad design. Build good designs and you will never have this problem. Ted Moores from Bear Mountain Boats builds a 30-40 foot war canoe using the same system he uses to build a 16' prospector. I am not a professional builder but I have built 6 canoes ( one is a kayak). And yes you can add ribs to a stripper after it is built , if you are interested go building forum on Bear Mountain Boats site and search for threads from Patricks Dad , he built a Freedom 15 and put white cedar ribs in it , for any one thinking about building a stripper designs from Bear Mountain are unbeatable in my opinion , some will argue but the proof is on the water. So if you want to build a flat bottom boat , use ply-wood.
  14. Todd Bradshaw

    Todd Bradshaw Sailmaker

    I'll pretty much disagree with that one. As you know, you can build almost any design from strips. Whether it oil-cans or not is not a matter of the design, its a matter of the scantlings used to build it. Some designs and some panel shapes simply need more material than others in order to properly hold that shape. Failure to understand this is what results in a weak boat, not the design itself. There are some perfectly good big canoe designs that don't work very well and are inherently very weak if built with a standard, touring-canoe-weight layup. It simply doesn't have enough beef (core thickness and/or glass) to do the job. It can't handle that amount of displacement or provide the amount of protection from the potential force of wave action or impact under paddle power that most people want.

    Once you have figured out how much material is needed to adequately keep the hull in shape, you are free to boost the scantlings for more strength and durability as you see fit for your particular use, at the expense of extra weight. Light weight is great and cored composites are the best way to produce light, stiff hulls - but they don't automatically generate a lot of strength out of nowhere. It has to be added as needed - and that takes more material.

    I'm reminded of the old joke where a worker designs an incredibly strong, stiff composite panel. Feeling pretty smug, he takes it to his boss, along with a hammer. He proceeds to pound on it with the hammer to show the boss how tough it is. His boss says "That's nice....but what about this?" and he turns the hammer over and with one simple stroke he drives the claw side right through the panel.

    Like it or not, a 30' strip canoe built to the scantlings of a 16' canoe may hold its shape, but it's got a lot more in common with that guy's super-duper composite panel than you probably want to know about.
  15. Steve Willoughby

    Steve Willoughby New Member

    Yes I know many types of boats are strip built. But we are talking about canoes. I realize that if I lofted my 17' Chestnut Freighter and built it the same as my Prospector that the 6h.p motor would kill it. A 30' boat would not have the same scantlings ( have to admit had to look that one up) as a 16 ' boat but that does not mean that both hulls can;t be built with the same material. The mentioned canoe in this thread has an estimated design weight just under 100 lbs.There are designs out there for 20' canoes that weigh 30 lbs. less using one layer of cloth inside and out. 90-100 lbs is not an acceptable weight for a canoe that size built with wood and epoxy. I want to carry a canoe , not trailer it.But yes if a design needs extra strength then you need to add it . but a good design won't need it.
  16. Todd Bradshaw

    Todd Bradshaw Sailmaker

    I never said that a 16' canoe and a 30' canoe can't be built from the same material. Instead, I corrected your opening statement:
    "If a stripper oil cans with a covering of 6 oz. glass inside and out its not the fault of the building process , it is just a bad design. Build good designs and you will never have this problem.If a stripper oil cans with a covering of 6 oz. glass inside and out its not the fault of the building process , it is just a bad design. Build good designs and you will never have this problem."

    ...which is blatently false for a number of reasons, and if you ever build a big canoe this way, you will most likely find this out very quickly. Whether a canoe oilcans or not, or can be built to exhibit a reasonable amount of durability using strips and 6 oz. cloth, has nothing to do with it's design being good or bad, it is a function of the scantlings and engineering of the laminate in relation to the job that they have to do. Perhaps you actually have to ride in one of these big canoes before you start to develop a realistic idea of what kind of flexing forces are present when a canoe with that much displacement (weight of it, and in it, forcing it down in the water) starts running through waves, or how much momentum half-a dozen paddlers and 1,500 lbs. can create when they accidentally hit a rock at cruising speed.

    Strippers are great boats, but the construction has its limits, and not tolerating a lot of excessive flexing without breaking or fatigueing is one of their weak points. Thickening the core by using thicker strips, ribs, half-ribs, etc. is a good means of increasing their rigidity to prevent excessive flex, but if we're talking about canoes that are expected to survive more than a sunny paddle across the lake at summer camp, the load-bearing fiberglass layers need to be kept in proportion to the thickness increase in the core. A thicker, stiffer core with the same old glass layup produces a stiffer canoe, but a boat which is proportionally weaker. The same thing happens if you decide to get increased stiffness on a big canoe by using contoured panels (like a round bottom, instead of a flat one). You increase stiffness but if you don't change the glass layup, you don't gain strength to go with it.

    I'm all for building light boats, but not for building flimsy ones for general use. You may want to hold off on deciding just what a big canoe should weigh and what its layup needs to be until you have actually had some experience building and using them. Underbuilding to save weight is a very common rookie mistake for beginning builders which causes many of the problems commonly seen on the stripper forums. There is no good reason to encourage it.
  17. OP

    joecool911 Curious about Wooden Canoes

    It is my understanding that the sheerline strength is a key component to fight oil canning. Strong in/outwales should help. So don't skimp on weight here regardless of the glassing schedule. If the sheer doesn't flex, it will be hard for the boat to oil can.
  18. Todd Bradshaw

    Todd Bradshaw Sailmaker

    We should be careful here with the term "oil can". If your stripper actually does oil can, it will most likely split open the first time it happens. Strip constructions will fatigue from repeated pounding or flexing, but they may survive it for a while. True oilcanning, in canoe terms, is generally something beyond the flex limits of stripper construction and more like what happens to some plastic boats.

    In any case, a good gunwale/thwart system is always a plus for a number of reasons. It will help by preventing the upper hull from being able to spread, stiffening the entire boat, but though it may be adding hull stiffness down where the rocks are, it doesn't add any strength there. One of the other interesting features of a sturdy gunwale structure on big canoes is cutting down on the fore and aft flex and wiggle that such a long gunwale can sometimes exhibit.
  19. RHaslam

    RHaslam Curious about Wooden Canoes

    So, at the risk of winning "Thread Resurrection Award", I wonder how the Outback turned out? I'm mounting the forms for it now. I've followed this conversation with interest, as it is something I have not thought about before. I stretched out J. Winter's 18.5 ' Quetico to 20 feet, and used one layer of 10 ounce glass on it inside and out. That canoe has seen a few thousand kilometers of travel through the roughest stuff that Northern Ontario can dish out. Sure, it's got a few patches now from white water bangs, but I have never noticed any hull flex.

    I was thinking of covering the Outback with one layer of 10 ounce, inside and out again. This canoe is going to be a family fishing canoe, not a tripper. I'd be interested to know what Joecool did, and how it turned out.

Share This Page