a question on canoe building

Discussion in 'Boat Design' started by yoram, May 27, 2011.

  1. rwatson
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    rwatson Senior Member

    Thats quite a lump of wood, way overkill.

    What do the plans call for ?

    Dont forget, the longtitudinal strength is a combination of all the stringers, not just the keel.

    Keep it light.
     
  2. yoram
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    yoram Senior Member

    rwatson you are right. it is over kill (and keel too...). i was thinking to use it because it is the nearest piece of wood to a keel we have here. i decided to go and buy some wood today and i checked all kind of widths. up to 2-2.5 cm (1.8 cm thick x2-2.5 width) it still bends but more then that, (3-4 cm width x 1.8 cm thick) it is very hard to bend.
    i also noticed that fir is very weak even a piece that is clear of "eyes". i understand that the strength comes from the unity of all components but a piece of 1.8 cm x 2.5 even 3.5 cm still could break under moderate pressure, knowing that the stringers and the keel are supported by the cross sections every 50 cm or so.
    it is only my feeling, not my experience since i have none but it looks a bit to fragile with this size of wood. please tell me that i am worrying for no good reason.
     
  3. yoram
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    yoram Senior Member

    the plan calls for 1.5''x 3/4'' gunwales which is 3.81 cm x 1.9 cm and 1'x3/4 (2.5x1.9 cm) stringers and keel.

    the piece of wood in the photo i have posted is 1.9 cm maybe 2 cm x 5 cm. probably this 5 cm width that prevent it from bending. if i cut it in half on its longitudinal line to make 2 pieces of 2.5x2x400, i have a feeling it would be too fragile. i saw a guy in a demo video clip jumping on his sof kayak. with this wood, it would break.
     
  4. peterchech
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    peterchech Senior Member

    Yoram no offense but don't try to reinvent the wheel. Those scantlings are PLENTY. If you want a boat that you can literally jump up and down on, get a plastic kayak that weighs 60 pounds, is slow and starts at $500. Even a s&g plywood kayak won't last long if you try to jump up and down on its deck while on land.

    That said, the strength of SOF is far more than its individual pieces. That keel will only be stressed between two bulkheads, maybe what, 3 feet apart? Furthermore, SOF gets its strength from its skin. The skin transmits loads into compression, in which direction wood is strongest. Without the skin, that stringer might break, but with the skin it will be quite strong. Furthermore there is a great deal of flex in the frame, especially if it is lashed together not glued/pegged. MANY MANY MANY kayaks have been built in SOF, for thousands of years (literally). The scantlings from the plans are more than adequate, trust me.

    If you are truly concerned about it, increase the skin to 12 oz ballistic nylon (dyson carries it too), that stuff is bulletproof (literally was used in the first bulletproof vests) but more expensive and unnecessary in my opinion. And lash only, no epoxy on the lashings, that will allow more flex. But again, unnecessary IMHO.

    Just make sure when you rip that piece in half, that you try to avoid those knots I saw in the pic. If you can't avoid them, scarf them out. If they are small and loose, I have had luck poking/drilling out the knots, cleaning up the hole and filling with epoxy/filler. I can't tell from the pic, but if they are small and not loose/hard, you may be able to leave them in, but do some testing first to see if you hear cracks or see the knot poking out when you bend the piece of wood.
     
  5. peterchech
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    peterchech Senior Member

    PS fir (if it is the same species that we have here in the US) is one of the strongest softwoods. It is far stronger than western red cedar, and those scantlings from Yost are for cedar. So...
     
  6. ancient kayaker
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    ancient kayaker aka Terry Haines

    If you don't want to scarf you can always use butt blocks to join short lengths and to add strength around a knot.

    On the question of strength, for a rigid boat that runs up on a rock or log, the point of impact has to withstand the instantaneous load which can be quite high. However, with a SOF boat the point of impact can flex - absorbing the energy of the impact. It's like the difference between dropping a ball and dropping a glass . . .
     
  7. yoram
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    yoram Senior Member

    thanks you guys and no offense taken. on the contrary, i am very happy to read your comments.
    i do not think i would use this piece of wood from the photo for the keel. Peter, your advice about the knots is very good. i have never done that but seems like very good point. also the idea of ancient kayaker about butt blocks seems good and easy to do. i will use something else that i could bend, probably the same wood i am using for the stringers so it would be 2.4 cm wide and 1 cm thick. it is hard wood that i can not identify but seems very flexible and strong. when cut into 2.5 cm, it is possible to bend it.

    here is the clip with the guy jumps on the kayak.
    http://www.youtube.com/watch?v=aWLI5rwZijc&feature=related

    i have manege to find polyester skin here in a fabric shop. it costs 13 $ for 1x1.6 meter. it is more then double the price of George Dyson but it is good to know that it is available here. now i have to look for the right paint. i wrote to a big paint company "Hemple" and lets see what they recommend.

    so now i have got all the stringers, and i will try to work on the sections in the next days. i make it hard on myself by not buying the right stuff even though it doesn't cost much. i have wood for it just missing wood for 2 sections. it will come soon.

    how do i find the right angle of the bow and stern in relation to the base line? it is not in the offset. any ideas?

    and again, thank you guys for your patience and help.
     
  8. peterchech
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    peterchech Senior Member

    haha I have wondered about the stems for the stern and bow myself. I read in a forum somewhere that you can kind of do as you please with these areas. On my build I will try and estimate the angle as accurately as possible from the plan. But I have read that the angle isn't that critical. Neither are his lengths of the boat. You can make the kayak a foot longer if you like by giving it a greenland stern/transom with lots of overhang. Or by keeping the same bow/stern but making the stringers a tad longer. It seems to be more art than science with Yost designs, and I personally like that fact.

    As far as paint, I have read that pretty much any oil based paint will work on the poly skin. Urethane varnishes such as Zap give a tougher coat supposedly. I only have experience with Corey's Goop from skinboats.org, lots of people swear by it and I have to admit it's pretty good stuff.

    Look up Dave Gentry on the woodenboat forum, or on his own website:
    gentrycustomboats.com

    He has build more sof boats than anyone I know, and often uses just oil based paint over polyester.
     
  9. yoram
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    yoram Senior Member

    Hi guys

    I was building the frame for the Seabee in the last few days in between doing all kind of other things. I must say I am having a great time doing it despite all the mistakes and using all kind of “left over” material which is not the best for this kind of work. I will send some photos.

    I just have to sand the frame and paint it and the frame is done. I wanted to ask about the skinning process. I bought polyester skin. I saw a few clips on youtube how to stitch the skin and I foresee many difficulties for me doing it. I was wondering if it is possible to staple the polyester to the gunwale, the skin that covers the hull goes inside and stapled to the inner side of the gunwale and the deck skin over lap it and stapled to the outer side of the gunwale and then I will attach 2 rub rails on top of the outer staples to hide them.

    Have anyone tried it? Do you think it might work? What kind of problems might come with this way of skinning?
     
  10. yoram
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    yoram Senior Member

    I have built the frame with stringers that were actually pieces of 80 cm glued together with butt blocks with simple carpenters’ wood glue and when I became a bit worried about it, so I added screws, 2 on each side of the joint, that would be of course on the inner side of the stinger. I used those pieces because I had this wood here and it was about to be thrown away and I hate when wood is thrown away. I feel like I have to build something from it.

    So here is a photo of the wood and the joints.
     

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  11. yoram
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    yoram Senior Member

    I also found some pieces of plywood in the dumpster in the wood shop from which I could build the cross sections. Of course I cut it and then I saw that I cut 2 of them wrong but they had more pieces in the dumpster. My working skills with the jigsaw are not that great but I have learnt that patience is really important when I cut and I should go slowly and carefully to make a better job. Unfortunately I realized it towards the end of the building process so some of it looks really off.
     

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  12. yoram
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    yoram Senior Member

    I have built a strong back from a 5x15 cm wood 4.5 m’ long and put metal angles, the one used to hold shelves, for holding the cross section. First I worked on my balcony but then we had rain almost every day so I moved into the work shop that was finally cleared.

    i placed the cross sections before completely cutting their inner part just to have a look if it all make sense.
     

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  13. yoram
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    yoram Senior Member

    I have built the coaming with all kind of 20x120 pieces that I cut and glued and had 2 pieces of plywood (found them here in the school) that were enough to make whole coaming.
     

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  14. peterchech
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    peterchech Senior Member

    Wow great Yoram! I'm impressed by your use of scrap materials, definitely influenced by the inuits who built their kayaks out of whatever they could find, which was driftwood!

    I don't know what you mean by "painting" the frame, I wouldn't put actual paint on it. I would just epoxy coat or else varnish heavily the end grain (edges) of the plywood, and just oil the rest.

    Stapling then covering with a rubwhale is fine, that's what I did. Only issue might be the bow and stern. I originally was going to put a sort of false stem over the staples to cover them, but ultimately got lazy and decided against it. It works fine. Just be sure to use stainless staples!

    Looking forward to more pics
     

  15. yoram
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    yoram Senior Member

    Attaching the stringers was relatively easy. I did it with glue and screws. I tried to lash it but I did not have the right thread and had trouble with the knots so I decided to go the easy way for me. Attaching the stringers to bow and stern was hard. Especially the stern and then I saw that I placed it ½ a foot shorter then the plan. That was after already cutting the stringer to match the ½ foot shorter size. So I had to add more length to them with butt blocks. Then it was relatively easy. I had to saw the edges in an angle to fit the shape and also to have more contact surface for the glue. I used epoxy putty here as a glue from left over epoxy that I had from building a canoe.

    when the bow and stern are placed according to the plan, the stringers are reaching them in a natural way and there is not much need of bending.

    i have placed photos so you could see all the mistakes and not such great work so other people who want to build could say, if this guy does it, i could do it too.
     

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