Sea Sled madness. It’s in my brain.

Discussion in 'Fiberglass and Composite Boat Building' started by DogCavalry, Nov 11, 2019.

  1. DogCavalry
    Joined: Sep 2019
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    DogCavalry I aim to misbehave.

    Seems very little for a day's work. 20210511_204135.jpg 20210511_204110.jpg 20210511_204048.jpg 20210511_202841.jpg
     
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  2. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    The hull structure in this area should be fairly slam proof when underway - even bullet / rock proof with all these bulkheads - and I think Ad Hoc has mentioned before about ensuring that you have paths for the stresses in the structure to 'escape' to the rest of the structure (where you don't want to have any stress 'bottlenecks', if one can use a bad analogy like this).
    A general question - how would one try to calculate the stresses in a timber structure like this - would you apply the same principles as for a steel structure, but with very small unsupported hull panel sizes (between the bulkheads), while calculating second moments of area of the frames / bulkheads?

    Edit - impressive photos above! A lot of time, effort and determination has gone into that structure, even if you think that the photos do not show much.
     
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  3. fallguy
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    fallguy Senior Member

    I like the plan, but the thin plywood I trade for a piece of 1708.
     
  4. DogCavalry
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    DogCavalry I aim to misbehave.

    I hoped you would know, Bajansailor, I certainly don't. Forward of this area is little enough. I must give thought to what comes after. The rest of the boat must carry this, almost cantilevered.
     
  5. DogCavalry
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    DogCavalry I aim to misbehave.

    A web down to the hull in 1708?
     
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  6. fallguy
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    fallguy Senior Member

    Gotta be honest, I am not a huge fan that you did not add stringer where I drew the black. They could have been notched for the cockpit timbers. I had assumed the floors were going to the hull.

    My gutcheck has you doing it wrong. Don't shoot any rockets into my city.

    Sorry, my finger sketch is not great. It's late. You'll read me.

    @Ad Hoc April 24, 2021

    11B3DAAA-E303-49CE-B004-0A6D448655A1.jpeg
     
  7. fallguy
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    fallguy Senior Member

    Yeah. Instead of the ply. I think it is lighter is all.

    1708 is 50 oz/yd laminated or 11# for a 4x8 sheet, same benefit

    disregard this post, I thought we were talking about the half lap joint
     
    Last edited: May 12, 2021
  8. BlueBell
    Joined: May 2017
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    BlueBell "Whatever..."

    I saw that video, impressive.

    Nice half lap mitered joints!
    1/4" ply or 3/8"?
     
  9. DogCavalry
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    DogCavalry I aim to misbehave.

    Fallguy, your city is safe from me. Challenges strengthen and clarify my thinking. I'll put big longitudinals between these transverse floors. The axis of planking gives good fore and aft strength. I want the floors to evenly distribute the load to every plank so there's no hardpoints.

    ⅜ plywood, BB. ¼" would adequately carry the load, but it's fussy to work with, at least to a wood-butcher like me.
     
  10. DogCavalry
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    DogCavalry I aim to misbehave.

  11. fallguy
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    fallguy Senior Member

    Oh, I get it now. I misread the plywood add. I thought you were going under or alongside the stud to bolster the deck joint which seems a bit xtra. But you are talking about making the transverse floor and actual floor to the hull. Forget what I said about glass. I was only referring to the half lap joint potential weakness and I thought you were putting a piece of ply under it vs a piece of glass and a fillet.

    As for the structure, I certainly have no c.v., but good gutcheck.

    You have to be thoughtful, first. And you have been.

    John mentioned girder loads. I recently saw a boat that had cracked right in half at the gunwhale. It was an 80s Chris Craft. The one with the openable transom. It gets really complex this loading business and the only way it can really be done is with CAD and modeling software. That CC broke because the open transom lengthens the engine loads on the sole and hull and any floors about 3' and someone pounded her hard.

    Perhaps the guy with the c.v., can pencil it, but even for him; chance for errors are high and it would be really difficult.

    One thing you can do in your brain is imagine the boat ten times bigger.

    Walk around and ask yourself how can I break it. You are a smart fellow. You know that if your boat is a beam and you load the middle of the beam; it'll want to sag. You know if you put a pallet of bricks somewhere forward and hit a massive boat wake, the forced on the sole will be tremendous.

    I have one spot on my Skoota that I am worried about stress cracking. Time will tell. But it is at the intersection of a bulkhead and what was the hull and the sole where I foolishly cut the tabbing of the hull back to the bulkhead. Keep in mind, I cut about 16" of hull back and had no instruction on the hull dim, but even Richard did not like what I did. However, by design we did it further back for a doorway similarly, just not on the BH.

    one thing you must avoid is point loading...and it is easy to mess up. If your floors are too big in one spot and you don't have room for glues, that spot is a point and will tend to push harder on the boat than any of the others. Put 500 pounds above it and you can have a 500 pound point load delam your hull skin. Then some water gets in and walla, rot.

    Other than that, I think you overbuilt the section by one floor.

    When we designed the roof for the cabin of my boat, I told my son I wanted to have it hold 100# point loading without breaking or sagging more than x amount. So we engineered the design and I can go up on the roof, but I would never jump up and down on it. All you are doing now is transferring cargo loads to the hull. But for a planing hull, you have to at least account for some slamming or dynamic loads. And again, how can I break my boat is an underlying question. If you dumped a truckload of rock in the forward cargo section, where would she fail? Right now, as is, the hull would crack below the step, in front of or near the next bulkhead back.
     
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  12. fallguy
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    fallguy Senior Member

    On the bulkhead edges, I ran cleats with foam. You probably would do something similar.

    Take a piece of 1/2" or 3/4" plywood and rip a 45 degree angle on what will be the bottom. Rip the flat section so you have about 2" of 45 and flat. Then bond that to the BH sides and tape a 4" 1708 tape onto the 45 and flat and 2" of it to the BH. There is no reason to run floors there.
     
  13. fallguy
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    fallguy Senior Member

    You really want to avoid hardpoints here and either use putty or consider padding these. Not worried about printing through, but heavy loads delaming the inside skin if anything moves. Easy to pad with a 2x4 ripped to half inch pieces. I generally used putties to avoid it, but anywhere I had real concerns I used foam.

    00E22AA4-4C75-4933-B7CD-EA5F862CCCD7.jpeg
     
  14. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Just a thought - re the photo above, DC had mentioned plywood bulkheads attached to these frames, which (I think) would come down to the hull shell (?).
    Instead of using plywood, would it be feasible to instead use more of the same timber stock, and run a length of timber along the hull shell (as a transverse frame), going from the far left orange arrow to the bottom right of the photo?
    You could even have a vertical timber at mid-span to create a truss frame.
    This would have a lot more surface area of timber in contact with the hull when compared to a ply bulkhead.
     
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  15. fallguy
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    fallguy Senior Member

    like a floor joist...also would reduce hard pointing
     
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