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Thread: Paddleboard hull Design: and do "ribs" touch the skin?

  1. #1
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    Default Paddleboard hull Design: and do "ribs" touch the skin?

    I'm working my SOF paddleboard design. For simplicity of the joints I want it flat in the middle and so have the choice flat everywhere or can do something different at the bow and stern. I was thinking of slight V at front of the bow for breaking waves, transitioning to double convex around the "calm" waterline which transitions to flat at midsection and then transition a deep V at the tail. The double-convex in front provides for speed with some stability while the deeper V at the back is good for speed and tracking.

    I've seen foam/fiberglass boards with this style of hull but am not sure if the double convex will work on skin-on-frame since the skin effectively follows the convex hull. I've seen Dave Gentry's Lunai which is a single V but wanted to explore the double-convex since its more efficient and allows better turning of the board.

    double-convex.jpg
    I've attached a simple drawing of what I was invisoning in the cross section, with the thin line below the top representing the skin. My stretch test for my fabric show less than 5% stretch with 200lb load and the convex regions wil be filled it it stretches about 9%. (The white holes at the apex of the convex regions will have a flat support bar running the length of the boat incase it does stretch more than expected over time). The first question is if the skin stretches enough to fill such a void am I just asking for trouble by having cross members that are in contact with the skin?

    I did note that in most of the SOF kayak pictures I could fine the longitudinal members (often called ladders) on which present the majority of the skin contact support along the water line with the ribs far enough offset that the skin seems like it would just form long channels along the ladder. It does not seem like it would ever touch a rib. (but the plywood-based SOF kayak I had growing up had ribs were in contact with the skin and so do some I see online, e.g. https://folk.school/wp-content/uploa...ame-boat-2.jpg). Got me wondering, should my cross members (rib/thawlart) members be shorter/****ted line in the bottom version so the longitinitual members (three darker sections) are the only parts in contact with skin hence cause drag by having a lump, or should I maximize support for the skin by having both logitinitual and cross members in contact with skin?

  2. #2
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    Default Re: Paddleboard hull Design: and do "ribs" touch the skin?

    Skins will not touch the ribs which are "sunken" away from the skin. Unless your skin installation is so loose it will be useless.
    The skin will deflect from water pressure, so you will have a double conave shape on the bottom.
    This will not be useful for turning ability, it will act like 3 mini keels to keep the boat going straight.

    You don't want the lower skin to touch the ribs, since this will make cross wise ridges in the bottom which will slow down the boat.

    On top you are going to need something more stiff than skin cloth to stand on. Put some longerons up to the skin line and then some plywood to stand on (or some other stiff material).

    Where and how are you going to stitch on the skin?

  3. #3
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    Default Re: Paddleboard hull Design: and do "ribs" touch the skin?

    Quote Originally Posted by upchurchmr View Post
    Skins will not touch the ribs which are "sunken" away from the skin. Unless your skin installation is so loose it will be useless.
    The skin will deflect from water pressure, so you will have a double conave shape on the bottom.
    This will not be useful for turning ability, it will act like 3 mini keels to keep the boat going straight.

    You don't want the lower skin to touch the ribs, since this will make cross wise ridges in the bottom which will slow down the boat.

    On top you are going to need something more stiff than skin cloth to stand on. Put some longerons up to the skin line and then some plywood to stand on (or some other stiff material).

    Where and how are you going to stitch on the skin?
    Thanks for the feedback. The potential for crosswise ridges slowign it is exactly what I was afraid of and why I asked. You saved me from making a dog of a board.

    Yes there will be a plywood deck; I discussed the overall design in a my first post/ thread, but had too long a post so went for more specific narrow questions instead of adding to an already too long initial post.



    I'll be doing a two-part skin, one on top and one on bottom. The skin will be stapled on rails on each side, with the edge/seam 2 inches or so above the flat water line with my weight on the board. Then I'll have a silicone-glued rub-strip over the double seam board with a few brass screws.



    Note: I may have gotten the the terms concave/convex backwards because what is concave/convex depends on what part of the board is facing "up".

    I agree double concave helps tracking if the board is flat, with a 3 keel like function, and helps with stability compared to a V. I felt that on fiberglass boards double concave/convex it turned better than the others I tried and I reasoned it helps with turning because when you shift weight it allow one to drive one rail edge in for the turn while lifting the other side out of the water, which makes better/faster turns than a single convex but less tippy than a pure V shape that tends to fall too much into the turn. It may also be that the it directs the flow into the rear fins which increase the fins "lift". But then again fiberglass boards I tried had a very shallow double convex, quad fins and more narrow rails so it may not translate to the SOF design (though I will have narrow rails). I definitely won't have quad fins. But all of that is really conjecture as I've not seen definitive reports on it, maybe I'll go looking for more formal analysis instead of "sales speak" from board vendors.

  4. #4
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    Default Re: Paddleboard hull Design: and do "ribs" touch the skin?

    You've got three threads on your paddleboard, but I will more or less address them all here.

    First off, you are basically attempting to reinvent the wheel. Go here and read all of it - Yostwerks This will give you a foundation for every aspect of your build, minus the sectional requirement.

    Yes, your fabric will work fine and, yes, it is feasible to double up the skin on the bottom should you need to. You can make any angle of bend with it that you desire. No you will not be able to skin with that fabric without wrinkles and, yes, corners and conforming to hull shapes are why. Tom's site will explain how to skin with pvc, which has similar characteristics to your fabric.
    Like PVC skins, it sounds like you will not need to coat the fabric at all, and I would not unless proven otherwise - be sure to include a drain plug in your build, btw, plus foam for flotation.
    We have used adhesives as a first coat on fabrics, followed by paint, for a decade or more. Specifically Loctite's 3X or newer 8X PL Premium construction adhesive. It does toughen some fabrics considerably if you work it into the weave. This is not something I do, or call for, with every boat, though, and I only see it as particularly useful for you for gluing seams, or if you need to attach a second layer of your fabric over the bottom. Titebond 2 or 3 may work just as well according to you. Use staples to hold the fabric tight until the glue dries. Staples are indeed faster than stitching. HeatNbond tape is also an option.
    You need not put rub rails over the seams, though they can be useful under the keel and chines. I do not call for them with my SUPs, but your fabric is relatively delicate. Any of the tougher woods will work fine in 1/4" thick strips. No bedding, in silicon or whatever, is really needed.

    Longitudinal members may be called "ladders," but I have never heard or read of that. They are typically called stringers. From your drawing I posit that you have more than are necessary. Likewise with frames/cross members.
    1/4" plywood for the frames/cross members should be OK, but they will likely bow when you bend the chines and gunwales on. I would double them at the joint between the halves.
    As mentioned by others and shown by Tom Yost, the cross members should not touch the skin, even under pressure.
    Oh - your example boat, above, which is my Shenandoah Whitehall design, does not have the fabric touching the frames! It's just lighting.

    Hope this helps!
    Dave
    Last edited by DGentry; 08-21-2021 at 01:24 PM.

  5. #5
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    Default Re: Paddleboard hull Design: and do "ribs" touch the skin?

    Thank you Mr. Gentry. You provided excellent answers and as a designer I admire those are helpful. Sorry if I violated protocols by having multiple threads, I kept generating new questions. and thanks for answering all in one place.
    I've been to Yostwerks and will revisit and study more. It had a lot of do X then Y style instructions but did not really address variations I wanted to consider or explain why things were to be done that way. I did read Daniel's Carter Beard's classic on boat design including bark canoe.

    I will have a at least 2 and maye 4 small screw top hatches, so I can continue to inspect the frame/joint sections. I figured those would also allow draining though maybe a drain at an end would be easier for draining and is easy to add.

    I was planning on having about 180lb of foam floatation per section (~3 cu.ft. closed cell foam upcycled from from various computer packing packed, potentially wrapped/suspended n some mesh bags to keep them in place and not touching/rubbing the skin).

    I'll try the fabric without paint, though it will look pretty funky since the skin is being made from my academic posters with text/, figures and equations on them, though I guess it would make it a very unique look on the boat and be a conversation starter. I can always paint later if needed.

    I might seem to have excess in stringers, but my simulations show I need it for the strength to support the sectional joints, so I can better spread the stresses around. I do know the stress analysis is flawed since my cad tool does not do wood properly and I took a conservative approach in modeling it. At about .5lb per stringer I can live with the extra weight to ensure its strong enough for now. Might not need as many cross-members and maybe I'll try a few designs/simulations with one less as I decided the joint needed to be made .75" thick boards not just doubled up plywood as there is a lot of stress there, and more importantly I need enough room to attach different thing like a hinge to help align things so solid wood will be better there. So with that I make need less stringer/cross members.

  6. #6
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    Default Re: Paddleboard hull Design: and do "ribs" touch the skin?

    One can have as many threads as desired - it was just more efficient for me to respond in only one of them.
    My link takes you to the wooden frame construction section of Yost's site, which is about building, not about lofting frames.
    Good luck!

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