Cutting out my Bulkheads

Discussion in 'Boat Design' started by James Robinson, Nov 4, 2018.

  1. James Robinson
    Joined: Nov 2018
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    James Robinson Junior Member

    This is my first post here so thanks all for your replies.

    I currently own a Kestrel 22 approximately 1978 named Falcon which I sail on Coniston, in the South Lake district. My Dad and I picked up this boat 4/5 years ago now relatively cheap and its been a bit of a father son project for us. Recently my dad retired and has essentially gifted the boat to me. I come from an engineering background and so I love tinkering with this boat.

    My current ambitious plan along with a re-style of the interior this winter is to cut out the bulkheads to open up the interior of the boat and make it seem more spacious. I recently saw the below picture of a new open plan Benetau small cruiser and liked the idea that I might be able to open up the interior in a similar way by cutting out the bulkheads leaving a small amount and then re-inforcing the bulkhead by either doubling or trippling the thickness.

    [​IMG]

    Clearly there is a composite beam in the structure of this boat however I was looking to get some opinions on whether it might be feasable to do something along the lines of the below sketch I modelled.

    upload_2018-11-4_14-58-20.png

    Here is a bad picture of the interior of another boat of the same class showing the current bulkhead openings.

    [​IMG]

    Any thoughts?

    Cheers

    James
     
  2. TANSL
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    TANSL Senior Member

    When a designer places a bulkhead with such an awkward opening he does it, not to annoy the user but, probably, because, for some reason, he thinks it is necessary. Therefore my advice is that you do not modify anything without knowing for sure that what you want to place is equivalent to what exists.
     
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  3. James Robinson
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    James Robinson Junior Member

    I know what you're getting at and I think the difficult thing would be to quantify the difference in strength. The bulk head is only really providing strength in the plane though and I don't imagine there are any dynamic stresses in it. With it being plywood though doing any reliable calcs will be nearly impossible.
     
  4. JamesG123
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    JamesG123 Senior Member

    But sometimes the designer does things to save costs or meet some specification without really considering the user at all. That double bulkhead was probably arranged to sandwich a head or lockers while still supporting the mast step. What the OP is suggesting is a relatively minor change compared to the first example picture.
     
  5. TANSL
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    TANSL Senior Member

    Of course, without knowing the work that the original bulkhead had to perform (the loads it was supposed to be supporting), it is impossible to calculate an equivalent solution.
    I can be wrong but I would beg those who propose a solution that justifies it with valid reasoning.
     
  6. kapnD
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    kapnD Senior Member

    I wouldn’t hesitate to enlarge the existing opening somewhat, at least side to side, as it is certainly restrictive as is.
    Bear in mind that a 22’ boat will never be spacious, you maybe putting a lot of work into an unrewarding project.
    Structural value can be preserved by doubling up, keeping generous margins, and gently radiusing the entire opening, no square corners.
    From your drawing, it appears that the mast steps between the bulkheads?
    Maybe the existing two bulkheads could be significantly reduced and a new, reinforced one added directly below the mast?
     
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  7. fallguy
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    fallguy Senior Member

    If the bulkheads are removed or reduced; you would assume the flip side would be to stiffen the hull, typically with more glass and on the inside.

    If we are talking a few inches; you could strengthen the bulkhead you reduced with more glass as well.

    I see zero good in it and wouldn't bother. A possible exception might be something where you have difficulty getting through an opening and need a few inches.
     
  8. JamesG123
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    JamesG123 Senior Member

    He is trying to go for the "open plan" look, where the whole cabin is one big compartment instead of distinct sections (v-berth, head, salon, galley etc.) .

    Thickening the hull isn't very practical. Its probably the least most efficient way of stiffening a structure. But even a rib-like frame can be made far stronger than is needed in this case.
     
  9. TANSL
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    TANSL Senior Member

    As long as we knew what job the designer planned for that bulkhead. Some functions of a transverse bulkhead can not be assumed by a frame, however resistant it may be.
     
  10. JamesG123
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    JamesG123 Senior Member

    All we need to know is what needs to be supported in the structure. The designer's original intent is irrelevant if you are removing or reworking what was specified.

    I've seen boats where bulkheads weren't even tabbed into the hull. They were there just to hang interior fittings on and maybe take up some compression load.
     
  11. TANSL
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    TANSL Senior Member

    Exactly. You see, we already agree. I would go a little further: all we need to know is what needs to be supported by the bulkhead.
    Have you thought about the raking (or racking, I don't know how it is said in English) of the hull?
     
  12. fallguy
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    fallguy Senior Member

    In any case, the hull sides need mods, with the exception of a 'bulkhead' that isn't structural.

    It isn't okay to just cut them away.
     
  13. groper
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    groper Senior Member

    this is a reasonably easy problem to solve.

    All we need to know is the righting moment of the yacht, mast height and beam so we can calculate the mast compression - assuming this new bulkhead has to take it. The hull itself only needs a stiffener in the form a perimeter ring frame.
    Assuming the worst case scenario the loads are not great - its only a 22ft pleasure yacht afterall not a volvo 70!
    A composite ring frame can be made extremely strong by running a heavy layup of unidirectional fibers around the inner opening and around the outer perimeter which essentially forms a circular I beam - an extremely strong geometry. Provided you calculate the existing plywood properties - which can be approximated to a reasonable level - and the new frame meets or exceeds the old frame - whats the problem here?
     
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  14. TANSL
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    TANSL Senior Member

    One of the problems is that a transverse bulkhead is calculated according to assumptions of loads different from those used to calculate a frame.
     

  15. James Robinson
    Joined: Nov 2018
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    James Robinson Junior Member

    Thanks for all your responses. As I've said, the boat is only a lake cruiser. I don't think I've ever taken it out in anything over force 5.

    From what I can remember, there is a thin beam laminated into the deck head directly below the mast foot and above the deck there is a very chunky looking mast foot, this seems to spread the load of the mast between the two BHDs. I'll be going to the boat end of this week to check on it so I'll try and provide some more photos. I think this might be a bigger project than it innitially seemed. One of the main reasons is so that I can take out the cooker and sink which are directly to the port and stbd as you enter the cabin which will free up seating space in the mid cabin. Moving these two units to the middle of the boat and removing the head which is of no use as I'm 6"5 and would never be able to use it anyway. I would probably only be making the cutout in the top half of the bhd something similar to the Hunter Sonata BHD below but symmetrical.

    [​IMG]
     
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