Small Kayak - Under 13 feet - Stability?

Discussion in 'Stability' started by millionswords, Dec 16, 2008.

  1. ancient kayaker
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    ancient kayaker aka Terry Haines

    All high-risk ideas should be tried as experiments away from the hull being built! If lashing will hold the ribs then you can go with that. With wood, the ends of the ribs fit in narrow slots under the gunnel. These slots do not weaken the gunnel to any significant degree because they are in the center of the wood and bending stresses are taken up by the outer layers of the wood. The slots serve to stop the ribs moving around. To avoid weakening the gunnels, the ends of the ribs are usually tapered down to reduce their cross section to allow use of smaller slots. The problem with bamboo is its hollow so there's no center in the gunnel to cut a slot in, and if you taper the rib there is no center left there either. Perhaps a small dowel can be put in the end of the rib which can fit into a small hole in the gunnel. There is not a lot of stress on this joint so the dowel does not have to be large.
     
  2. millionswords
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    millionswords HomeMade Kayak?

    wonder what should be the length of the Chine Stringer? when mu Gunwales are about 13 feet length...
     
  3. robherc
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    robherc Designer/Hobbyist

    I'd start out with it longer than I think I'll need it, bend it in & see how it looks. If it's too long/too much bend, you can take it out, cut some off and try again. Remember, it is always easier to cut a little more off, than to put a little back on! :p
     
  4. millionswords
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    millionswords HomeMade Kayak?

    I measured and cut my Chine Stringers today.
    The gunwale are about 13.5 feet. And My chine stringers are cut at 11.7 feet.
    It looks perfect to my eyes, some one suggest if the ratio is fine.

    The Height above base for the Kayak planned at about 28 inches, so I'm cutting my Rib Stringer at 10 to 11 inch long, will connect the ribs from Gunnel to Chine stringer and the next set of ribs to chine to keel.

    Did not take any pictures today, just cut them and no lashing, will post with pictures when I progress with some lashing.
     
  5. robherc
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    robherc Designer/Hobbyist

    Sounds good, can't wait to see them lashed in. :)
     
  6. millionswords
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    millionswords HomeMade Kayak?

    A new way to miter the Ribs found!

    Report on day 005

    Day 004 went without any lashing or construction, today I had cut some rib and started, but later thought I will improvise on the cut.

    I used a hole saw to cut it like a half circle (Miter) to let the bamboo Gunwale and Chine stringer rest on it while I lashed them tight.

    It works wonderfully, but I need another hand to help me out, so I'm waiting for a friend to come in. Issues I faced was, the ribs need not be of the same height all along the Chine/Gunwale, so Wonder how I would calculate?

    Not resting the frame on a StrongBack is posing me problems, but my space constraint keeps me from making a strongback or may be I'm too lazy?

    Need to cut more Ribs, about 16 to 18 of them for the Gunnel-Chine Stringer portion and Miter them all - sounds a lot of work!!

    Some pictures of the miter and a test lashing...
     

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  7. alan white
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    alan white Senior Member

    Looking good there million. It's beginning to become a boat.
     
  8. robherc
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    robherc Designer/Hobbyist

    Million:

    If you're wanting to make sure all of your stringers are EXACTLY the same length, try measuring them all & placing dots to start the drill bit in the center of your hole saw on. This way, you should be able to get them all the same length +/- about 1/8" (3mm) or less.


    ...Just my $0.02 worth
     
  9. ancient kayaker
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    ancient kayaker aka Terry Haines

    It's hard to tell from a photo but it looks rather tall. It would be a good idea to study a few photos of kayaks to get a feel for the overall shape, which has evolved over centuries.

    There are many variations of size, design and style related to use and sea conditions where the style was developed. Because kayaks are so low in the water they are higher at the bow to allow them to rise to a wave. Some have bows that curve well above the rest of the boat; this looks nice but is unnecessary for flat water use.

    The gunnels slope gradually down from the bow to midships where the cockpit is located. From midships to the stern the gunnels may rise a little or remain somewhat flat, but the stern is never as high as the bow. The foredeck may be domed or ridged, so water runs off to the side before reaching the cockpit. The foredeck typically slopes up to the coaming at the front of the cockpit then the coaming slopes down from front to back. This allows you to slide your legs into the cockpit. The side decks either side of the cockpit slope down sharply to deflect water and to provide clearance for the arms and paddles. The after deck is very often nearly flat but may have a slight camber to support weight; it is never ridged as you often have to sit on it while getting in or out. The Inuit people used these boats for hunting seal and small sea animals which could be carried on the after deck so it is kept low.

    There is a balance of forces as a beam wind (from the side) may push either the bow or stern around, causing the boat to head into the wind or away from it. The higher bow counteracts the paddlers body sticking up out of the cockpit which is behind the center of gravity (CoG). A pronounced tendency to head up or down wind is undesirable, I prefer a slight tendency to head into the wind so if I have to paddle upwind I can concentrate on paddling hard and not worry about maintaining my course. People living where there is a prevailing wind blowing them to shore may prefer otherwise.

    The bottom of a kayak is never completely flat although it may have a flat center section. A flat bottom results in a lot of buoyancy low-down which lifts the boat causing the CoG to be too high; this requires a wider boat for stability or it will tend to flip over. Generally the bottom is barely wide enough for the hips, then flairs up and out to the waterline from which comes the stability. Anything that lowers the CoG improves stability, including rocker, which is the convex curve of the bottom viewed from the side.

    Here are some good links:
    http://www.guillemot-kayaks.com/guillemot/information/kayak_design/kayak_stability
    http://www.greenval.com/jwinters.html
    http://gorp.away.com/gorp/publishers/ics/how_can2.htm
    http://capefalconkayak.com/howtoskinakayak.html
     
  10. millionswords
    Joined: Nov 2008
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    millionswords HomeMade Kayak?

    Temporary Chine Stringer lashing...

    here is what it looks like when I test lashed the Chine Stringers.
    I had cut the ribs to lash @ 11 inches, it proved to be too high like AK exclaimed, so I made 8 inch cuts and have them ready for lashing.

    2. I made sure the only way to keep the Chine Stringers in position till lashing is to lash the Bow and Stern Stems first to the Gunwale and then to the Chine Stringers.

    3. Now with the temporary lashing, it helps me hold it in some position without the need for a helping hand[a second person].

    4. I'm yet to start lashing the ribs from gunnel to chine

    I plan to lash the middle ribs first and then start from either side (bow or stern) and proceed to the center. Wonder how it goes...
     

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  11. millionswords
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    millionswords HomeMade Kayak?

    A little more Rib-Lashing

    Update:

    I had cut the bamboo culms at about 7.5 inches long, the culms were about 1inch thick in diameter. I drilled using a "hole saw" bit for the Hammer-Power drill and made the miter ends.

    The Gunwale and Chine stringer rest on this miter, this rally made things easy and lashing was more nicer. Earlier when I did the deck ridge ribs, I had not employed this idea, with a straight cut the lashing was a big problem and they never stayed tight on the Gunwale. I'm thinking to cut them deck ridge ribs once again and lash them with this miter idea.

    The whole structure is a little flexible I feel it will be like this because it is just lashing and nothing else.. [waiting for a friend to help in holding while lashing..guess will make tighter lashes then]


    Pictures: after lashing the Gunwale to Chine stringers with the mitered culms the shape has improved, looks little like a boat [atleast to me :) ]
     

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  12. robherc
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    robherc Designer/Hobbyist

    Hey, 10^6 Swords;

    You're right, it IS starting to look a bit like a boat now; looks like the chines are going to be quite hard, but that's ok. I'm glad you found the hole-saw method works so well for you.
     
  13. millionswords
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    millionswords HomeMade Kayak?

    yes rob the hole saw idea made the whole process a breeze!
    I'm now contemplating to employ the same idea for the deck ridges too.
    Might work on it now and post pictures soon...
     
  14. millionswords
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    millionswords HomeMade Kayak?

    Some Lashing pictures

    Here are some close ups of lashing, and the work in progress..
    After some practice with lashing, and getting a right position to seat the frame, I think the lashing is coming a bit better, and holds the frame well. here are some pictures...
     

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  15. sigurd
    Joined: Jun 2004
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    sigurd Pompuous Pangolin

    tight lashing will be stiff enough if you have the right geometry on the sticks.

    I don't know what direction of flex is bothering you but you can try to add some diagonal sticks.

    1) A cross in the deck,
    2) a cross from stbd gunnel to port chine and vice versa,
    3) a cross from gunnel to chine on the same side
    those are some options.

    Is that coconut fiber? are you putting any oil in it?

    edit: It is also possible to use cord instead of sticks for the crosses.
    edit: with 3) you can force some rocker, which will be good like AK said.
     
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