Building a prototype

Discussion in 'Wooden Boat Building and Restoration' started by alfbennett, Mar 2, 2010.

  1. alfbennett
    Joined: Mar 2010
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    Location: Seabeck, WA

    alfbennett Junior Member

    I am planning a Boat with my Father (me 34 him 64)

    We are working on a design for a 12-14 foot Jonboat.

    Design requirements include
    Carry 3 250 pounders, dry storage, stable on flat water, row pole or power. Sounded like a job for a jonboat.

    We will be working on a scale model in hardboard this weekend, and the prototype later on.

    The construction method will actually be sort of ply on frame sort of stitch and glue. (Ply on frame for now because I feel more confident, and I can mess with the design without starting over if I run into an issue)

    Now I have read TOMES of arguments about epoxy/polyester. I get it... epoxy.

    However, for the prototype, being just that. I want to use less expensive material in case it all needs to go to the scrap yard.

    Sooo.... I was thinking, Would it be possible to coat the boat in polyurethane sealer as a temporary coating for the wood, then take it off later if we decide that the prototype worked out so well that we should just keep the boat?

    I am prepared for the floggin
     
  2. Boston

    Boston Previous Member

    actually ac plywood exterior with almost any kind of paint would be fine for just a prototype that you want to use for just a short test run
    keep it simple and cheap
    that way you can cut it up and do it over if you need to
    also you might get in some valuable practice time with the epoxy if you just use it instead of a cheaper substitute on the prototype
    you can always bond more epoxy to it which is an important consideration also epoxy fills voids better than your other choices

    I'd skip the poly coating and just go with some kind of exterior house paint
    and dont leave the prototype in the water for to long

    just check your water line maybe and check that its stable (in shallow water)
    with all you are wanting to carry in it

    best of luck
    B
     
  3. lewisboats
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    lewisboats Obsessed Member

    That's a lot of weight for a 14 footer. To have reasonable draft you will need to go WIDE with it...say 5 ft across the bottom. To hold all this weight you will need to build it beefy so it will be heavy too. I would expect the hull to weigh on the order of 200+ lbs. With folks, motor, cooler, gear, gas, anchor(s) added to the weight of the boat I would expect you would end up displacing around 1200 lbs. With a waterline of around 11.75 ft, a bottom width 5 ft and sides flaring out at 15 degrees, transom at 15 degrees and bow transom projecting out at 30 degrees you get a hull of about 14 ft. This could go +or- depending on how much shear you want but the important waterline would be about the same. This hull shape would draft 5" at a bit over 1200 lbs if everything was positioned to float it level...in fresh water. Seeing as these things almost always float a bit nose up...I would expect it to draw about 6" of water at the stern fully loaded. I would make the sides at least 16.5" high(vertically) to give almost a foot of freeboard. You would need to notch the transom for a short legged motor. Like Boston says...paint would be fine for the coating and you can always sand it off and epoxy coat later if you want. If you keep up on the paint store it protected from the weather you probably don't need the epoxy coat. If you are doing some form of S&G you will want the epoxy for the fillets...Nothing does fillets like epoxy.
     
  4. alfbennett
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    alfbennett Junior Member

    I was using that free hull designer application (kind of confusing at first)

    I did come up with a 15 deg flare, that much we did have in common.

    A couple of things I did to the design was to
    1.) I started the sheer more forward

    2.) put less bend in the chines after the forward chines - this got me some better displacement.

    unfortunately, though I was trying to design around the wood that will be used, so I designed it with 48" beam at the chines. and

    My displacement according to the hull program at 1000 lbs was around 5.5"

    Pythagorus tells me with a flare of 15 deg, and needing about 16" of free board, my planks should be about 16.6 ", I could concieveably put the sheer to a shallower slope, and raise the gunwales (I have seen folks spell gunwales wrong soooo many times just because it is pronounced gunnalls)

    Since you are making me think this through more (and I think like a programmer, which is more distracted than most might think - kinda like chacing a rabbit in a field of rabbits)

    I might even be looking at the wrong kind of boat.

    Duties - Fly Fishing, Trout Fishing with the kids, and a duck boat.

    Mebee I should be planning a flat skiff instead....


    Anyway, I am too far into it to do a 180.... Well more of a 90... on the planning.

    Perhaps I should lower the bar to 2 of us heavy dudes, and if it works out build 2

    And since my mind is wandering... It would be cool to be able to latch the sterns when at anchor in a shallow salt-marsh and make one big Comfy blind!


    This is just the first of three boats I have in my mind to build over the next several years.

    Jon boat for hunting and fishing protected waters.
    My own personal fishing/sailing vessel safe for the Puget sound and rowable (Dory or semi-Dory, haven't got that far on this one)
    A larger open skiff that I can take my 5 kids out on the sound for fishing during summer months.
     
  5. alfbennett
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    alfbennett Junior Member

    Oh,

    for the transom, the design already calls 1" ply braced by framing and using internal triangles to make it extra stiff, so I think we have a beefy transom covered.

    And thank you for making me think more about the design, and the advice. I have worked with epoxy to repair scuffed fiberglass, and am comfortable with it, so I will just use a quart or two for the prototype to glass the seams and fillet the joints.
     
  6. lewisboats
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    lewisboats Obsessed Member

    You got the right kind of boat...you just need to go a bit longer. Try for 15 ft, start your bottom curve at just to the fore of the mid point and bring it up with a smooth rounding to about 7.5-8". Freeboard is the distance from the waterline to the shear...not the chine to the shear...that is topsides height.
    I don't understand
    ...the gunwales run along the shear line.

    As far as beefy is concerned...I was thinking more along the lines of the bottom...almost a half ton of weight...most of it able to move by itself on two feet...you want 1/2" or thicker stuff to keep things from flexing too much. You also want stringers and a few cross members to add to the stiffening.
     
  7. alfbennett
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    alfbennett Junior Member

    Oops,
    I meant the rocker at the front... sory, don't know what that is called. could put that at a shallower slope

    sheer is between the chine and the sheer-clamp.

    My Dad would probably slap me if he read that post (he used to sail in another life)
     
  8. lewisboats
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    lewisboats Obsessed Member

    Actually I would go with all I could get out of 2 lengthwise sheets of ply...probably around 15.5 ft. You could get away with 4 ft on the bottom and about 5.5-6" draft so go with 18" wide panels for the sides to give a hair over 17" vertical height. Most small to mid sized motors really like a 1.5" thick transom to mount to...so consider a doubler along the top of the transom or a pad to fill in the thickness.
     
  9. alfbennett
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    alfbennett Junior Member

    I just tried to rescale the length to 180" and all my lines blew up...

    well, it is free software. DangFlab it!

    The transom, where the moter mounts should come out to about 2" thick already, do you think that will need to be thickened? The construction method should make sure it is really stiff though.
     
  10. lewisboats
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    lewisboats Obsessed Member

    Shear is the curve of the upper edge of the side of the boat. Also called a Shear line. The gunwales (inner) and the rub rails (outer) follow the shear line to protect the edge.

    When people say that a boat has a "Sweet Shear" they are saying that the curve is pleasing to the eye from all angles...something not always easy to get. A boat with a lot of shear normally has a higher shear aft that dips low and then swoops up to a high point at the bow...a very hard thing to get "right".
     
  11. lewisboats
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    lewisboats Obsessed Member

    Just what software are you using?
     
  12. alfbennett
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    alfbennett Junior Member

    Chine Hull Designer 1.91

    I just did a quick rebuild to recover from my exploding lines, and it all looks good, but now it will show the the displacement lines and not the values, what a pain in the butt.

    I can't upload the .hul file for you to take a gander at unfrotunately.

    I have it at the full 15.5' (which is actually incorrect. I will need to do the calculations to figure out how much over-all length I am loosing to the curvature (sheer now I know) of the design and put those numbers in still.
     
  13. alfbennett
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    alfbennett Junior Member

    just got it to show the stinking waterline level with 1000 lbs again. it is now at 4.19" nice
     
  14. lewisboats
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    lewisboats Obsessed Member

    It was off the top of my head...you might need to shorten or lengthen by a tenth to fit it in the two sheets. If you scarf you will need to subtract that from the length also. You should also give about 1/2" on each end's edge as fudge factor so in reality you might end up with 15' 3" or 4" as the max length. The method I am assuming is either wood butt blocks or Glass butt connections.
     

  15. lewisboats
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    lewisboats Obsessed Member

    Don't forget that now the boat will be even heavier...more like 230+, and you can put a bigger motor on it so I would look at the waterline at 1200 lbs and be satisfied with that.

    Signing off...gotta go to bed...Job #2 in 6 hours then off to #1 until 7 tomorrow.
     
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