Cutting back bulkhead

Discussion in 'Boat Design' started by Ben Sherratt, Apr 29, 2023.

  1. Ben Sherratt
    Joined: Apr 2023
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    Ben Sherratt Junior Member

    Hi all,

    I’m refurbing my 38’ sailboat and I want to “trim back” the aft bulkheads to open up the space more. I’ve attached pictures with the lines I intend to cut and wanted to know if this will compromise the structure or how I find out if it is structural? I can get the marina structural guy over but I can save a bit of cash if it’s a simple solution.
    Fire over any questions if you have any.

    Ben
     

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  2. willy13
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    willy13 Senior Member

    Is there any rigging hardware attached to these bulkheads or near the bulkheads?
     
  3. Ben Sherratt
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    Ben Sherratt Junior Member

    No rigging hardware attached. The nearest chain plate is about 1.5m forward.
     
  4. fallguy
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    fallguy Boat Builder

    Not a good plan. The way you have it drawn, there is widening of the span above, and if you walk above it; probably good chance of failure, or static failure sagging..

    You could probably use posts and beams and get away with an aesthetics change at less risk to vessel. And the posts can be quite fancy looking.. you could probably keep the top of the existing panels for the beams and keep them tabbed at the hullsides.
     
  5. willy13
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    willy13 Senior Member

    20F7914F-D8EA-4378-BD26-0CB15AF5F35F~2.jpeg something like this would be better compromise. Use curves, not sharp edges.
     
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  6. mudsailor
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    mudsailor Junior Member

    I’d leave a minimum of about 250mm/10” all the way round and have no radii smaller than 150mm/6”
     
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  7. wet feet
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    wet feet Senior Member

    Its unusual to see a full bulkhead in that location.The exceptions normally being when one is required for a heads compartment or as an end partition to a small private cabin.It is much more common to have most of such a bulkhead extending vertically to the height of a galley worktop or a chart table and then to have a post with a circular section extending upwards to the coachroof.This provides both a sense of connection with the rest of the interior and the post provides support to the coachroof.The outboard portion would normally extend as far as the inboard edge of the side deck and be securely bonded in order to transfer the loading of a genoa sheet.
     
  8. Ben Sherratt
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    Ben Sherratt Junior Member

    Would attaching a SS pole from the hull to cabin top reduce the need of leaving a plywood strip at the top? There isn’t a support beam directly above the bulkhead but there is about 13mm FG I could go into

    Thanks for your suggestions
     
  9. wet feet
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    wet feet Senior Member

    It makes life easier to leave a relatively thin portion of the bulkhead attached to the coachroof-say 6-7 inches deep.You can,with the aid of a tame fabricator,use a stainless pole with a U shaped plate at each end to bridge the gap between the lower and upper portions of the remaining bulkhead.The U shaped bracket can simply be bolted through the bulkhead and will be a handy thing to prevent the mouldings lozenging when the boat is in slings.The strip of bulkhead along the coachroof is a useful place to join smaller headlining panels then the wide open spaces would require.Any cutout in a bulkhead is a weakening factor and ought to have rounded corners to reduce the stress raising potential.
     
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  10. wet feet
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    wet feet Senior Member

    reduced bulkhead.png I had a few minutes and did a rough drawing of a potential solution.The blue line is what a cutout might look like (no sharp corners) and the green line is potential metalwork.You will have to determine details and if you haven't read any of Ian Nicolson's books on boat modification,you should.
     
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  11. TANSL
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    TANSL Senior Member

    Better take the metalwork to a bottom floor, calculate that metalwork in compression and calculate if the floor is capable of supporting that compression load transmitted by the metalwork.
    I don't want to complicate what doesn't have to be complicated. Probably none of that is necessary and a bulkhead would suffice, just as wet feet has drawn it, with the proper thickness. It all depends on the deck load and the frame on which the bulkhead was initially placed. If there was already a frame there, and it was the same as the others, you probably won't have to worry about how you cut that bulkhead.
     
  12. Barry
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    Barry Senior Member

    What is the thickness of the bulkhead?
    It appears to be glassed into the top deck and side of the hull? Yes No?
     
  13. Ben Sherratt
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    Ben Sherratt Junior Member

    Thank you for this. This would be a perfect solution for the port side as I only want to drop it to counter height. Would it work if the cut was lower (seating height) on the SB side of I used curves and included a pole?
     
  14. Ben Sherratt
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    Ben Sherratt Junior Member

    These bulkheads are 16mm thick. The port one is glassed from the bottom and under the side deck but stops before going on to the roof. The SB one is glassed up to where the seating was and then nothing all the way up. The tops of the bulkheads sit in a moulding where they are adhered but not glassed.
    More photos attached.
     

    Attached Files:


  15. wet feet
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    wet feet Senior Member

    The principle would work,but you need to determine the best way to leave sufficient strength.As for dropping the bulkhead to counter height,much better to leave it five or six inches higher to contain plates,cups,jars or bottles on a bumpy day.
     
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