Plywood Kayak Hull Help

Discussion in 'Boat Design' started by ChrisMacD, Feb 4, 2021.

  1. clmanges
    Joined: Jul 2008
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    Location: Ohio

    clmanges Senior Member

    This boat was just painted; no glass, and lasted 13 years with next to no maintenance. Built of birch plywood on frames.

    The White Boat https://hvartial.kapsi.fi/white/white.htm
     
  2. mitchgrunes
    Joined: Jul 2020
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    Location: Maryland

    mitchgrunes Junior Member

    Interesting. But I'm guessing the epoxy primer paint and polyurethane paint didn't hold the boat together, nor seal between ill fitting pieces like West Systems Epoxy glue/filler and Fiberglass would - you did that in a more complicated way.

    But - should one apply an epoxy primer before applying epoxy and fiberglass in stitch and glue boats? I.E., does the primer improve adhesion or lifetime for real epoxy?

    West Systems free online educational materials for using their Epoxy, at
    Instruction manuals, book, videos & workshops - WEST SYSTEM Epoxy https://www.westsystem.com/instruction-2/
    in particular this "book":
    The Gougeon Brothers on Boat Construction
    where it says they use a thinned epoxy is used to penetrate the surface of the wood or plywood, to stabilize and strengthen the wood, which they claim leads to a lighter boat. Does "epoxy primer" do that too?

    For some strange reason, their free educational materials are biased towards using their products, and towards Epoxy in general. :) But it and the other educational materials there may give the o.p. some ideas on how wooden boats are built. The book will appeal to that guy's engineering and carpentry background; I can't fully follow the technical lingo myself.

    Incidentally, my skin on frame boat, that I bought from someone else, also used polyurethane - to coat the nylon skin. It lasted many years, left out in the sun, but is now starting to crack and the outer polyurethane coat is peeling. (Hint: never paint your boat black. It gets hot in the sun, which probably isn't good for coating lifetime. It may even smoke in the sun.) OTOH, the uncoated wood frame inside the boat is rotting and starting to collapse. It still floats, and doesn't leak (or if it does, I think more water comes through the spray skirt, because it is a sit-inside boat).

    For a sit-on-top boat, if we model this on surf skis, you need a near perfect seal, because buoyancy comes from a sealed air chambers. (Maybe wouldn't hurt to use a little foam, so it would float somewhat even if it leaks.) But you may want air holes on top, to accomodate air expansion. In fact, bulkheaded air chambers in fiberglass sea kayaks have such holes near the top of the bulkhead. I'm not clear how air holes work in a sit-on-top boat - how you prevent water from coming in if it washes over the boat. Could one use a flap valve? It makes sense to look at store-bought boats, and other people's boats, to see what they did.

    BTW, used boats often sell for a fair bit less than new ones. A composite "performance sea kayak" will go fairly fast, if it has a 21" or less width and it is 16 - 19' long - dimensions should vary with the paddler - and maybe even less than that at the waterline. A racing sea kayak or racing surf ski would go fast with relatively little effort - but no way would it be stable enough to fish - though outriggers formed by adding paddle floats to a paddle might stabilize it some - people use them to stabilize the boats to re-enter sit-inside boats at sea. (Expedition kayakers often sleep at sea using such outriggers.) I personally find it hard to stabilize racing boats, even while paddling, and I used to whitewater kayak daily starting in the early 1980's. Also, racing boats are designed to minimize paddling efforts at racing speed, and not to do so at normal paddling speed. But sometimes people sell their old race boats, somewhat cheap, if something better becomes available.

    Also, BTW, sit-inside boats can be more stable, because you lower the center of gravity by sitting on the bottom. But they are harder to re-enter at sea.

    P.S. I apologize for my beginner oriented comments about paddling and taking lessons - I didn't read the o.p.'s post carefully enough - apparently he is an experienced kayaker.
     
    Last edited: Feb 11, 2021
  3. clmanges
    Joined: Jul 2008
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    clmanges Senior Member

    I've seen (on the 'net) an example of how that's done. Saw somebody's photos of his racing cat; I think the main hull was basically a surf ski. The main hull and the ama both had these funny-looking little things sticking up out of the back of them; looked like little radio antennas, a few inches long and slightly curved. I asked what they were and he said they're breather tubes. Probably have a fine enough bore to keep out water in anything but prolonged submersion.

    Maybe a cat is what the OP should be thinking of: blistering speed, but all the stability he could want for fishing.

    Oh, here's how Hurricane does it:

    The “Vent Hole” – Hurricane Kayaks https://hurricaneaquasports.com/our-kayaks/sit-on-tops/the-vent-hole/
     
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  4. rwatson
    Joined: Aug 2007
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    Location: Tasmania,Australia

    rwatson Senior Member

    Umm, I don't know of a single plan or kit from Epoxy and Glass that doesn't recommend complete encapsulation.


    My experience is that scale models are pretty useless for Kayaks/canoes. You might as well build the full size thing for all the value you would get from a smaller model. As you said, the scaling effects work against accurate model hull responses, for such small craft.
    The biggest problems, is that canoe/kayak shapes cannot expect any kind of radical performance difference by utilizing subtle hull shapes. They are either long and thin, or wide, with or without rocker. It pretty straight forward because by default you have a low power source, and basic handling characteristics. As you said ..

    The quest for "speed" is a pretty cut and dried design exercise.
    I can remember this out of condition old man keeping up with a young, very athletic paddler - but he was in a ten foot "sit on", and I was in a 16ft Canadian (not narrow) boat.
    I also remember the huge increase in speed that my 21 foot Kayak with zero rocker provided compared to a 16 ft version.

    As far as speed to travel, and buoyancy while fishing, you would have to go a long way to beat this idea. Totally doable in Ply and Glass - retractable "scissor" sponsons.
    [​IMG]

     
  5. mitchgrunes
    Joined: Jul 2020
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    Location: Maryland

    mitchgrunes Junior Member

    Cool! I suppose you could set up pumps or bailers for an extended trip in open water. I.E., have some sort of plug between the closed air compartments and the open part, that you pull to let out the water. I've never done an extended multi-day expeditions in open water, or taken a sea kayak into waves over 6 - 6.5'. When we took a few weeks to paddle around Florida, we camped on land every night, so we could empty out the boats.

    You mean a catamaran, like
    Hobie Tandem Tri | Hobie kayak, Kayak boats, Tandem kayaking https://www.pinterest.com/pin/47147127338037895/?d=t&mt=login ?

    I think cats are only fast for sailcraft, because the stability lets you fill a bigger sail without flipping. For a kayak the pontoons would add a lot more drag, and slow you down, as well as get in the way of efficient paddling - where you start the stroke as far forward and as close to the hull as possible. The paddle would run into the cross-beam pieces that hold the pontoons. (Of course, the o.p. wants to pedal instead of paddle.) You minimize drag with one almost half-cylindrical hull cross-section, like Huki racing boats; the wetted surface can actually be thinner than your hips for a sit-on-top. (Though Greg Barton advocates something more stable than a half-cylinder.)

    When I suggested two paddle floats on a bungeed paddle, that is a temporary catamaran. But you stow the paddle and floats when you aren't fishing. (After all, a spare paddle is a good thing.) I have wondered whether very thin small pontoons, behind the paddler, could act like training wheels, and help a beginner learn to stabilize a relatively unstable craft.

    Oh wait - inflatable float bags might be problematical around fishing hooks. Maybe one bungees or ties on closed-cell foam pieces instead?

    I wonder if sculling craft are more efficient than pedaled or paddled boats?

    Of course, you could always just use a motor. :)
     
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  6. clmanges
    Joined: Jul 2008
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    clmanges Senior Member

    My apologies; my old brain misfired. What I had in mind wasn't a true catamaran, but something like this:

    OC1 Canoes — Puakea Designs https://puakeadesigns.com/oc1canoes

    With the outrigger for stabilization, the length-to-beam ratios (at waterline, even) can be outrageous, like the ~14:1 typical of these models. A rudder becomes necessary, though, unless you don't mind using up an acre of water surface for a course reversal.
     
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  7. mitchgrunes
    Joined: Jul 2020
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    mitchgrunes Junior Member

    If you are that picky, the pictures of "catamarans" that I described or showed were actually "trimarans".

    Some time ago I looked into Hobie pedal drives. They were sufficiently wide, heavy, clumsy and inefficient that a reasonably strong paddler could easily beat them with a kayak paddle in a racing kayak, and a strong rower could easily beat them in a racing scull. But it makes sense for people with a disability that prevents them from paddling efficiently to pedal. Also, perhaps a person who rides bicycles a lot, and has very fit legs, might be better off in a pedaled boat.

    Look at recent race results for the Blackburn Challenge (20+ miles):

    www.blackburnchallenge.com/CARC_Race_Results.asp

    It looks like the fastest single person times in any given year are for surf skis, kayaks, or sliding seat sculling craft. Neither pedal craft, nor outrigger boats, come close.
     
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