SOF ribs

Discussion in 'Wooden Boat Building and Restoration' started by duluthboats, Sep 29, 2009.

  1. duluthboats
    Joined: Mar 2002
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    duluthboats Senior Dreamer

    I may be rehashing old news; if it’s there I can’t find it. Most SOF hulls call for green or air dried steam bent ribs. I want to produce a SOF frame from off the shelf items. If I use kiln dried wood for the ribs will my only problem come during the bending or will I have a weaker ribs? Or is there a good replacement for the wood?
    Gary :D
  2. Petros
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    Petros Senior Member

    Hi Gary,

    I can help you with this one. I suspect if you are doing true steam bending green wood works best. You either have to harvest it and mill it yourself, or buy it from a specialty wood supply (and it is usually pretty costly). However I have found that any suitable wood for steam bending can be boiled (instead of steamed) even when dry. Boiling is far less sensitive to length of time of course moisture content, and with the wood being in the hot water puts all the moisture back.

    Once the wood drys out, either from steam or boiling, it should have the same strength. The less moisture content of the wood btw, the stronger it gets. That is why it is best when your frame is good and dry to put a finish on it. Some people like leaving their kayak frames bare, but the finish preserves the wood and slows moisture penetration, helping to maintain its strength.

    I like using salvaged lumber to build my kayaks, and the wood I collect can be over 100 years old from old buildings being torn down, it is VERY dry. The best and easy to find steam bendable wood is white oak because it is so common. I have a stack of old salvaged flooring, I have also found it in shipping crates from the east coast, and in junk furniture and old benches. I set old boards up in the table saw and adjust the angle of the blade to get the rib to have the grain going flat with the broad dimension of the rib. Than I plane and lightly sand them, cut them to length, and put each end in turn into a large turkey sized pot of water on a hot plate. after about 15-20 min I put the heated end in my bending jig (made from scrap wood) and bend it. Than I put the other end in to let it soak while I bend the next one. I typically have 4 or 5 ribs soaking at once, and I work my way down the line. Once I get both ends bent, I dip both ends back in the hot water for about 10-15 sec each, and than place it the mortises in the gunwales so it holds its shape. If I have to I hold it in the correct position until it cools down to lessen the amount of spring back. The water temp is just below boiling (my hot plate is not large enough to quite get the large pot boiling) and it work great, better than steam bending in my exeriance.

    The old floor boards are very dry and hard, if boiling works for them, just about any wood you buy can also be boiled. I find boiling far more forgiving than steaming. I can even bend wood with grain run-out and other defects without splitting by boiling it. That would never work with steam bending green wood, the grain has to be near perfect in my experience.

    As far as wood goes, Alaskan yellow cedar is my favorite bending wood, you usually have to go to a cedar yard to find it. Already mentioned is white oak, which even places like Lowes or Home Depot will have white oak to buy in planks. There are other woods that can be steam bent, but they are not as easy, I have seen doug fir and western red cedar steam bent too, but those do not work for me.

    Since you live close, let me know when you want to try it and I can drop by and show you how I do it. I even have some old white oak scraps I can give you to play with and I will bring my bending jig.
  3. duluthboats
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    duluthboats Senior Dreamer

    Thank you Peter it is kind of you to offer to help. I have steam bent a lot of green wood, some an inch thick with mixed results. The ribs I’ll be bending are thin and the bends small. I’m trying to make the whole build easy for someone with limited skills. My main concern was that of kiln dried wood being steamed or boiled would be weaker from the process.
    You have encouraged me to do some experiments. I’ll let you know how they come out and will for sure invite you to see the build once it gets going.
  4. mark775

    mark775 Guest

    If the bend isn't too great and is localized, dry heat works well and one can glue (depending on glue it may not matter) and finish sooner.
  5. alan white
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    alan white Senior Member

    You can use kiln-dried wood providing your radii aren't extreme. Since you won't likely be bending anything thicker than 1/4" thick, your success will depend more on your ability to steam the wood properly.
    The trick is to soak the wood thoroghly before you steam it. For SOF ribs, this will only take a couple of days submerged.
    The steam box and generator has to be capable as well. However, for 1/4" stock, I've used an electric fry pot with a funnel instead of a lid. I got lucky as the funnel was exactly the same size as the pot diameter. then I used a plastic vaccuum hose to connect to a 6" x 6" x 36" long (in my case) box with a removable solid wood plug or door at the end.
    10 minutes is enough with thin stock. The powerful fryer element boils water quickly. I put about 2" of water in the pot and it's producing steam within ten minutes (after which, as said, the stock becomes rubbery within another ten minutes).
    Ash bends very well, even if kiln-dried, and I'm sure many other woods can work as well (though ash is a great bending wood in general).
    Breakage will occur, often more a result of grain run-off than improper steaming. I lose maybe 10% at most, so I'd suggest an extra 20% to have enough ribs.
  6. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Steam bending success depends on several factors. The usually suspects for breaks are "user" errors. Stock selection is the common problem. Grain run out is the big one and it's orientation along the bend. You can live with a fair bit of run out, if it's not in a severe portion of the bend. Backing up the bend with a sheet metal strip also is a tremendous help in this regard, keeping the wayward grain from bursting through during the bend.

    Green lumber will bend best and some species are clearly better at bending then others. You can bend lumber that has been dried, both naturally and in a kiln, but the percentage of breakage will rise. Some breakage is just bound to occur, particularly if you're not accustomed to selecting bending stock.

    Use the prettiest pieces for the most severe bends. Back the bend up with sheet metal, don't over cook dry stock, pre-soak if necessary.

    I've been using a tool my other half bought for cleaning grout lines in tile floors. It kick's butt on small stock and produces steam within a few minutes. Look into floor steam cleaners as an option to a boiler.
  7. Petros
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    Petros Senior Member

    When I steam bend the old salvaged oak after soaking it for several days, I usually break about half of my ribs. When I boil them I break none. Even with significant grain defects and runout, boiling the old dry wood results in very few breaking.

    For kayak ribs I rip them 1/4" thick, and than plane the ends to get each end for about 8" down to 0.20 inches thick to get a better shapped rib (it helps keep the bottom of the kayak more flat, more stable). This is the method Chris Cunningham recommends in his book, I just a joiner, but I have also used a table saw to thin the ends. I place them about 8" apart, going out to about 10" spacing near the ends. Usually have about 16 to 20 ribs per kayak, depending on length.
  8. gonzo
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    gonzo Senior Member

    You can bend wood with a torch too. I prefer a helper to keep on wetting the wood while I heat it. This works specially well for repairs or small jobs where setting a steam box is not convenient
  9. woodrat
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    woodrat Junior Member

    I've built three SOF kayaks, and ribbed two of them with black locust, one with Oregon white oak. The first one was with locust stock that had air dried for a couple of years, but then was left out in the rain for months. Not too big of a deal bending that. The next one was a couple of years later, same air dried locust but not left in the rain and I broke a lot of ribs on that one. The third one was with some oak that had been sitting in the shade in quartered log form for a couple of years, and I ripped it down to size on the sawmill and table saw. Those went pretty smoothly once I got adequate heat in the steambox.

    A friend of mine who builds a lot of SOF kayaks now buys 1/4" bamboo plywood and rips his rib stock out of that, and it works very nicely.

    I recently found a nice fat willow log on the beach nearby and if the tide hasn't taken it back by now, that will be my next batch of rib stock. Having a portable bandsaw mill makes this easier... ;-)

    As to red cedar, I have been trying to get that stuff to bend into a 15" circle, for making drums out of, and every piece, no matter the temperature or time in the steambox, gets compression cracks all over the inside face.
  10. duluthboats
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    duluthboats Senior Dreamer

    Bambo plywood? More tests, I'm thinking.
    Thank you WR
    Gary :D

  11. nordvindcrew
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    nordvindcrew Senior Member


    I built my SOF boat by laminating in place two 1/4" X 3'4" pieces of Alaskan yellow Cedar using polyurethane construction adhesive. No problems except for the first and last frame which had to be cut in the middle and installed in two pieces attatched to the stem and stern knee laminations. I over built for strength because the boat is used in rowing races and sees a lot of stress. Biggest mistake was covering in 12 ounce canvass and using house paint. Nylon would have been much lighter. At 16' 10" OA she weighs in with floorboards seat and foot brace at about 90 pounds. Not light, but too light doesn't work out on the water with a 20 knot wind, there, heavier, much heavier, is better. In experiments, I found the the " mahogany" sold here on the East Coast as a material for house decks is very workable: straight grained, no knots and available up to 20' long . I used it for the stringers in my boat and it made the turn on the bottom stringer from flat to verticle with no fuss at all and no breaks. If I build another, it will all be built of that material but with lesser scantlings.
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