Sea Sled madness. It’s in my brain.

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

  1. fallguy
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    fallguy Senior Member

    Another way to reduce the spans and flex is to widen the glue surface. A 12mm ply bulkhead/frame does not afford much glue and is easy to end up weak. Cleating with a 2x2 reduces the span and increases the glue surface and allows for any final height changes.
     
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  2. Ad Hoc
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    Ad Hoc Naval Architect

    On the one hand, this is possibly built like a brick... on the other hand, you never know where excessive loads come from. In addition, if I recall, you do intend to use this as a "workboat" too...yes?
    Thus bearing that in mind, i would first place long.ts shown in yellow:

    upload_2021-4-21_12-0-27.png

    This to be in line with those existing long.t bhds shown. So this provides a load path for any loads and importantly gives you long.t strength. Ordinarily you don't need to consider long.t strength on such small boats, but too many transverses does not gives you any long.t strength. So when carrying a load and if you fall off a big wave, the hull - could - concertina. Thus to avoid local damage, the long.lt provide the strength/stiffness you require, in such rare events.
    In addition, it also helps when you wish to have her up on the hard during winter times or cleaning etc. It gives a load bearing location.

    So, i would first place the longt.,s, with flanges in the yellow locations You do need to ensure that with each step long.t down to the next longt., there is a load path. By this I mean the web, of the upper long.t has at least 50% of the web depth at the end, connecting to the next web lower down, at that join. Otherwise, where does the load go?
    I would continue those longt.s, if you can, going all the way aft.

    Once that's done, depending upon the panel size (aspect ratio) you can then add transverse members, which do not need to be huge, at locations shown in red. Again with a flange, so as you note, you can screw the sole to it.
    Does this help?
     
  3. fallguy
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    fallguy Senior Member

    I think marrying @Ad Hoc's build concerns and my 16" or your adjustment to some other number is a good blend.

    got pictures going aft?
     
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  4. DogCavalry
    Joined: Sep 2019
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    DogCavalry I aim to misbehave.

    20210424_093453.jpg
    Here's a section at the last step. Bulkhead 2. The longitudinals from B1 to B2 that form the sides of the steps are 30cm tall, at B2. Aft of that there's only 3", 7.5cm dpth available under the cockpit sole to carry on the long.ts Ad Hoc recommends. But note the apex of the tunnel. That should also carry longitudinal loads aft. And the sole is ⅝" thick, plus glass. Can it also be considered a part of the load path?
     
  5. fallguy
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    fallguy Senior Member

    D0401E95-4DFA-495B-8695-C666AF8AC21D.jpeg
    I don't profess the same expertise as AH, but I say no or not without the stringers.

    The thing with structures in boats is areas of high strength that drop off quickly seem to be the most trouble.

    So if you hit hard, the boat ain't gonna break at the step that is tabbed to the hull; it is gonna break where the area of great strength vanishes.

    And this generalization carries throughout the hull. Once you grasp the concept; you'll basically not be able to forget it.

    Ask yourself, not if the boat would break, but where, and this points out the answers. I have a spot on my boat I am nervous about because I cut the BH tabbing back to the BH and I am worried about the variation there..for example.it was a rookie mistake that was pretty hard to repair

    ps...this is the 'concertina' remark in red I think
    Pss...did the best I could with the arrow, but the load is head on and the way the load transfers to the sole is by the short stringers
     
    Last edited: Apr 24, 2021
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  6. BlueBell
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    BlueBell "Whatever..."

    Of course it is part of the load path.
    Figuring what the loads are is a whole other matter.

    Is there a reason for the step to be so high or is this your preference?
    Same with the soles to either side...
    Perhaps it's to keep with the step spacing or match the sole height in the cabin.
     
  7. bajansailor
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    bajansailor Marine Surveyor

    Would it be feasible to run a longitudinal stringer on the centreline along the apex of the tunnel vee ?
    That would be very little additional material, and it would half the size of the unsupported panel of the cockpit sole above Anne's left foot.
    Looking at the section you drew in the sketch above, one could perhaps say that it is a bent strut in compression - and bent struts require very little additional load to buckle when compared to 'straight' struts.
    But a stringer along the apex should be very effective in helping to prevent the 'bent strut' section from buckling.
    Ok, I know that the analogy with a 'bent strut' is probably too simplistic, and it ignores the effect of the surrounding structure, but I think that apex longitudinal stringer would help to put strength in to this area.
    And the more strength the better, as I don't think anybody can predict accurately the size of the impact loads that you will experience locally in this area when travelling at speed, in 'rough' weather, with a full load of bricks on board.

    You could even run another pair of longitudinal stringers further outboard - the starboard one would be approximately on the outboard side of Anne's right foot.

    This area of the hull is going to take the most abuse when you are underway at speed, and I think it is probably better to err on the side of caution (some might say overkill) rather than possibly having to try to repair fractured stringers later down the line.

    This does have the disadvantage of creating multiple small sealed (ie inaccessible) compartments though - but these small spaces could perhaps be filled with foam before putting the lid (cockpit floor) down?
     
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  8. TANSL
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    TANSL Senior Member

    The problem with adding reinforcements to solve local problems is that the overall idea is lost from sight, with the synergies that this entails, and a cluster of reinforcement pieces is reached that, fulfilling their mission not too well, greatly increase the weight of the set.
    But given the current work situation, there is little that can be done but patch things up.
     
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  9. fallguy
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    fallguy Senior Member

    this is a way of saying a couple stringers would have been best...I ran it through the tansl translator...

    :wink:
     
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  10. DogCavalry
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    DogCavalry I aim to misbehave.

    The scantlings as determined by Dave Gerr state unequivocally that stringers are unecessary with strip plank composite. The end grain crush strength of the glass wood assembly is in the region of 3000 psi. Adding another central stringer would give strength where the structure can already manage around 15 tons of compressive load. Probably too small an increase to measure.

    I feel the structure has a whole has adequate strength, but distributing a concentrated load just over the cockpit floor that may equal the weight of the rest of the boat is challenging. Fortunately the load will always be downward on the sole, so the cockpit section can be treated as a truss, where the upper chord is aways in compression. At B2 the center is very shallow, but B1 is close to there, and it can sustain huge loads in shear.
     
  11. DogCavalry
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    DogCavalry I aim to misbehave.

    And I don't know. You guys are making me nervous....

    Oh well, fear of death concentrates the mind wonderfully.
     
  12. DogCavalry
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    DogCavalry I aim to misbehave.

    20210404_154523.jpg
    I guess it's more like 15cm under the sole. Well, we do what we can.
     
  13. fallguy
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    fallguy Senior Member

    Crickeys John...a couple 3" stringers is pretty ez and you get the sole span reduction. Sort of a no brainer ask me.
     
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  14. BlueBell
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    BlueBell "Whatever..."

    Agreed.

    Plus, it's going to offer added support under the treads so you can use 3/8 ply instead of 5/8 or 1/2".
    Please consider a small overhang on those treads.
    Like a sacrificial edge and an I-beam combined.

    Although, if the centre stringer's only going to transfer load to a weak point
    (the forward bulkhead on the cabin) then maybe it's not a good idea.
    However, if you can transfer the load through that BH and under the cabin sole to the transom, all the better.

    You're doing great work John, carry on.
     
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  15. DogCavalry
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    DogCavalry I aim to misbehave.

    20210424_174601.jpg 20210424_174156.jpg 20210424_173332.jpg
    Here are the inside and outside of the cabin bulkhead- B3. The inside not really a weak point. Although the discontinuity in rigidity is clear. Huge fillets and much glass.

    Through the door at the apex of the tunnel, just at the first stair, there's a heavy black line. That's where the cockpit sole lands. Can't go down any. Up maybe.


    The first image shows where I think to install transverse bulkheads, up to the sole. I guess that makes them floors. I can surely install long.ts between them to B3. They are on 11" 28cm centers. That's why, Fallguy, I'm not concerned about maximum span. After the top chord is on those transverse components, max span for the ⅝ plywood will be 8" 20cm.

    And a shot through the door someone asked for. Ad Hoc I think.
     
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