Plywood Kayak Hull Help

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

  1. ChrisMacD
    Joined: Feb 2021
    Posts: 9
    Likes: 2, Points: 3
    Location: SW Florida

    ChrisMacD Junior Member

    I've been fishing the Atlantic here in central and south Florida using several different kayaks for some years now. Using both paddle and peddle kayaks.
    Recently I've been dissatisfied with with what the commercial market has to offer for kayaks designed for going offshore. Hobie actually discontinued the Revolution 16, which I think is one of the best offshore kayaks. Every manufacturer seems to focus on freshwater bass fishing type kayaks.
    Being somewhat handy and knowing my way around tools, I thought I might be able to build something that suits my needs. So I went ahead and ordered some plans for a plywood kayak that I thought would do what I'm looking for. After carefully reviewing the plans, I came to the conclusion that it's not quite what I want.

    I promptly started researching about kayak design and specifically plywood kayaks. Reading lots of forum posts on kayak building and even looking at non kayak related plywood projects.
    I found and installed all the software that I stumbled across and spend several weekends playing around trying to come up with something. Being a manufacturing engineer (glorified machinist, lol) in the aerospace industry, I'm not used to just dragging geometry around trying to make a nice shape.
    I need numbers!! LOL

    So after all of that, this is what I am looking for:
    I am looking for someone to help me with creating the shape of the hull up to the deck-hull joint.
    This will be a Sit on Top type kayak. Fairly certain that I could handle designing the deck as it not really related to the performance of the kayak in the water.

    The design criteria:
    16'-18' in length
    Max beam of 32"
    Stability should be somewhere between a touring kayak and and a well behaved SOT kayk
    Must be able to build from plywood (Developable panels)
    4-5 panels up to deck-hull joint
    Speed is what I am really after

    Solidworks is my daily driver when it comes to software. I would need a model of at least half the hull in a format I could open in SW.

    I am planning on incorporating a Hobie Mirage Drive. Already purchased one of the drive wells designed for their inflatables to take a mold from and lay up a fiberglass copy.

    I am willing to compensate for someone's effort.
  2. rwatson
    Joined: Aug 2007
    Posts: 5,920
    Likes: 320, Points: 83, Legacy Rep: 1749
    Location: Tasmania,Australia

    rwatson Senior Member

    Should be easy.
    Buy and Download these plans for $150, and just scan and "trace" the digital plans.
    Then you can "stretch and fair" the Plan and Elevation of the lines in Solidworks, and create an offset table.

    Kayak hulls are no "black magic", and the difference between "fast" and "slow" designs is probably about a Knot or so, and easily altered.
    Fast = minimum rocker and maximum length
    Slow = Beam and Cross section.

    I presume that like Rhino, SW can
    1) Generate a curved line from two Plan and Elevation lines (using the table of offsets or the Plan and Elevation Lines)
    2) can "Unroll" surfaces to give you

    F1430 Sit On Top kayak – Bedard Yacht Design

    If you ignore the dreadful music, this is an example of the process. I am sure there are other better videos out there
    bajansailor likes this.
  3. clmanges
    Joined: Jul 2008
    Posts: 547
    Likes: 118, Points: 43, Legacy Rep: 32
    Location: Ohio

    clmanges Senior Member

    A plywood SOT ... there is one (and only one) that I've ever heard or know of:

    Sea Island Sport: Wooden Sit-on-Top Kayak That You Can Build!

    This isn't the fastest hull shape possible, and may be problematic modding for the Hobie rig. You might contact them about that. Click on the 'Construction Gallery' button and you'll see what the hull shape is. It is self-bailing, though, and almost within your dimensional requirements.

    I'm personally not fond of that hull shape; I suspect they used it for its initial stability, though there may be other structural reasons. You might get some ideas from it, though.
  4. Kayakmarathon
    Joined: Sep 2014
    Posts: 116
    Likes: 48, Points: 28, Legacy Rep: 10
    Location: NewEngland

    Kayakmarathon Senior Member

    Stability between a touring kayak and SOT would put the beam between 24 and 32 inches, but for speed I would use 22 inches based on a 17 foot kayak I built 20 years ago. To get the best combo of stability and speed use a 28 inch beam and a rounded vee midship section. The static beam waterline will be about 24 inches, but the flare will provide secondary stability very quickly. A rounded vee will give the hull more stiffness by reducing flatness.

    Would you please clarify 4-5 panels? Do you mean from the keel to the gunwale, or gunwale-keel gunwale. I've generated some hull cross sections with 3 panels from keel to gunwale (6 total hull panels), and the shape is almost as smooth as wood strip. I've generated one hull and built a model with 2 panels keel to gunwale. The deadrise was shallow at midship, and steep at the bow, so bending the panel took some force.

    I wish you in my neighborhood so I could help you make a mess in your garage.
  5. ChrisMacD
    Joined: Feb 2021
    Posts: 9
    Likes: 2, Points: 3
    Location: SW Florida

    ChrisMacD Junior Member

    Check out JEM Watercraft - Boat Building Accesories
    They have several different SOT plans available.
    I actually bought some plans there but like I've mentioned above, it's not quite what I am looking for.
  6. ChrisMacD
    Joined: Feb 2021
    Posts: 9
    Likes: 2, Points: 3
    Location: SW Florida

    ChrisMacD Junior Member

    28"-29" for the beam is what I was thinking.

    Keel to gunwale is what I meant with 4-5 panels. I think it would be easier to achieve the desired shape with more panels but correct me if I'm wrong. However, I am aware that it could make construction more difficult.
  7. portacruise
    Joined: Jun 2009
    Posts: 1,361
    Likes: 132, Points: 63, Legacy Rep: 218
    Location: USA

    portacruise Senior Member

    "Speed is what I am really after"

    AFAIK, all HPB records for endurance and burst speed have been made with water propellers which should be considerably more efficient than Hobie fins. The air propeller records have come into question because no correction was applied for an allowed Tailwind. Studying some of those boat hulls may yield some ideas, a search of "Adventures of Greg" and "Rick Willoughby" should turn up something. Rick has some fast trimaran designs that should also be stable for fishing purposes and has developed special high efficiency folding propellers for HPB, and may still do some Consulting.
  8. gonzo
    Joined: Aug 2002
    Posts: 16,353
    Likes: 1,405, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    At 32" beam you are describing a canoe rather than a kayak. Kayaks should be just a bit wider than your hips. There are many plans available online. Probably checking the reviews is a good way of finding out how well they perform.
  9. ChrisMacD
    Joined: Feb 2021
    Posts: 9
    Likes: 2, Points: 3
    Location: SW Florida

    ChrisMacD Junior Member

    I have tried both types of peddle drives and prefer the Hobie Mirage Drive. The straight back and forth motion on the Hobie drive feels more comfortable over a long period of time than the circular "bicycle" motion on propeller drives.
    Catamarans and trimarans would be too stable for the purpose of this kayak. We spend a lot of time near shore, sometimes in pretty decent swell, so being able to "lean" into the waves a little is beneficial. Not to mention all the points the line could get hung up on.
  10. ChrisMacD
    Joined: Feb 2021
    Posts: 9
    Likes: 2, Points: 3
    Location: SW Florida

    ChrisMacD Junior Member

    I'm fairly certain that I have looked at all the available plans that are out there. That's the reason I'm here.
    32" and wider is standard for most commercially available rotomolded kayaks labeled as "fishing kayaks".
    I would like to create a hull similar to a Stealth Profisha 575 but using plywood.
  11. gonzo
    Joined: Aug 2002
    Posts: 16,353
    Likes: 1,405, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

  12. Kayakmarathon
    Joined: Sep 2014
    Posts: 116
    Likes: 48, Points: 28, Legacy Rep: 10
    Location: NewEngland

    Kayakmarathon Senior Member

    When I looked at the website for that kayak, I immediately thought of a surf ski, and the beam looks narrower than 32 inches you mentioned before. The specs for that boat is a beam of 24 inches, which is the same as a standard touring kayak. If you have pretty good balance and also want to paddle for fitness; then the Stealth Profisha 575 would be a nice all-around kayak for flat to 2 foot water conditions.
  13. Mr Efficiency
    Joined: Oct 2010
    Posts: 10,401
    Likes: 1,035, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    When a big "Noah's Ark" comes up in your berley trail, you will be cured.
    rwatson likes this.
  14. messabout
    Joined: Jan 2006
    Posts: 3,269
    Likes: 411, Points: 83, Legacy Rep: 1279
    Location: Lakeland Fl USA

    messabout Senior Member

    Chris, send me a personal Message. I can recommend a designer who might be interested in your project. He is a wizard with SW and is a small boat builder himself.

    I am a small boat guy as well. In the past I was an enthusiastic Kayaker. Too old for that stuff now. E mail ................

  15. mitchgrunes
    Joined: Jul 2020
    Posts: 168
    Likes: 48, Points: 28
    Location: Maryland

    mitchgrunes Senior Member

    For me that would be about right - at least for a sea kayak. (Whitewater playboats are different.) And, as far as I can tell, about 12:1 length to width ratio - counting only the part in the water!, and staying low to the wind - makes things reasonably fast and easy to handle, if you know what you are doing.

    But for someone fishing - especially for a sit-on-top in open water far from shore - stability matters. They want to be able to cast a line, and have a strong large fish pull on it without going over. Though - maybe one could use an outrigger (like a paddle or stick with two float bags, under a bungie cord?), but you still want speed to get there, and to be able to fight the winds and waves to get back if you have to. (This is speculative. I don't fish.)

    I also like to play a little with leans and edges, but that isn't his goal for this boat.

    And again - one doesn't want to get sea sick. If you sit in a very thin boat for a prolonged time in choppy beam waves, that is possible, even if you watch the horizon. At least it is for me. :( I tried using an old-style 4m slalom boat, which was cheap, because they aren't raced any more. Aside from the fact it wasn't as fast as a sea kayak, I did get sea sick in beam chop. I'd never gotten seasick in whitewater, but I guess that is because whitewater is constantly changing.

    That said, one should always remember that a boat that is stable in reasonably flat water, tends to become completely unstable when a wave or waves break over the side of the boat. Because flotation stabilizing follows the surface of the water, and when that surface turns over, the boat turns over with it. With too much flat water stabilization, it becomes impossible to stop it rolling. One should never completely rely on a flotation stability to keep the boat upright - but should practice hip motion and paddle stabilization techniques, and re-entrys in close-to-shore small waves first, which are supposed to be reasonably easy for sit-on-tops. Especially if he plans to fish solo (which lots of people will say you should never do, especially far from shore), and has to rely on self-rescues from the inevitable flips. If the original poster hasn't prior experience with kayaks, I suggest he take lessons, because kayaking in open water can be dangerous if you don't know what to do. Expect waves that are many times the height of the boat, and under some conditions, they will break over the side of the boat.

    Plus, it helps a lot to know how to paddle efficiently (e.g., using core muscle rotation instead of elbows and shoulders to the extent possible). In addition, he may need to paddle back to shore strong enough to go 5 or 6 knots if it was in calm conditions, if serious weather hits, to fight wind and wind and tide generated currents. (Which, to me, IS a good argument for a fast boat.)

    There are other tradeoffs. A lot of fishing craft have live wells. So while I want full length waterline, and a boat that is low to the wind, despite the wet ride - for speed and lightness - he might want to avoid having water constantly splashing into and maybe washing fish out of the well. So he needs to clear the surface more, and needs a bit of an upswept bow to part the waves. Also, whereas I would prefer a thin fabric skin-on-frame sit-inside - maybe 25 pounds for a 16'" x 16' boat for me (though I admit I am still making do with a 19" x 19', 30 pound boat, with a lot of that length not in the water) - also for lightness - I wonder how good an idea that would be around fishing hooks. Plywood would perhaps be better. (I suppose some fish might even attack the boat, which wouldn't be good for a light fabric hull, though I assume he isn't fishing for big sharks from a kayak :) ) So what if his boat is 45 or 55 pounds, or more? - though it is true that light boats ride better over large waves, and therefore - IMO - handle better in storms.

    I'm not a competent builder, and can't tell him on how to build. I notice that a lot of beginning plywood boat builders use "stitch and glue" techniques, because they are fairly fast and easy, and because companies like Cheapeake Lightcraft, one of the companies that makes kits (though not especially high performance ones - but maybe good enough for what he wants), and sells supplies, advocate it. So do several books. Then they fiberglass/epoxy the boat to slow down rot, hold everything together and seal the cracks. (Older construction techniques frequently required that there be no cracks, so the boat wouldn't leak.) The idea, as best I understand, is you drill holes through the wood, pull wire through them to temporarily hold the pieces together, and then fiberglass/epoxy (e.g., West Systems Epoxy is pretty waterproof) over. Once the epoxy sets, the fiberglass and epoxy hold the boat together, you can cut the wire, and do a final epoxy coat to seal the holes, and sand to make a smooth fast hull. Or something along those lines. (Wet epoxy is toxic and carcinogenic, so one has to protect the skin, eyes and lungs.)

    There are expensive marine plywoods that rot less. I'm not clear why you can't fiberglass (or at least epoxy paint) all sides of the wood, to keep out the rot, instead. Does anyone know?

    One possibility would be to make little models, with different shapes, and test how much effort (e.g., pull on a fishing scale, or stretch of a rubber band) it takes to move them. Theoretically, that doesn't quite work, because of scaling effects, and it is hard to test wind and wave interactions. (E.g., one of my boats is moderately fast, unless it moves through bow waves that are at least 2 or 3' high. Then the bow bounces around, and wastes a lot of energy. Because it was designed for someone heavier than me, has too high volume ends, and because it the keel line is rounded and upswept at the bow, rather than a shape that cuts through the waves.) Also, you have to be realistic. You are designing the boat to usually go about 3 or 4 knots, with minimum muscle power, with bursts around 5 or 6 knots if you really need it, cutting through its own bow waves (i.e., going somewhat faster than "hull speed" - maybe 1.5-2 x as fast as the waves?), not a high powered speed boat which surfs or hydroplanes its own bow waves, and other waves.
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.