Can I cut a window in a keel box.

Discussion in 'Boat Design' started by Ooks, Nov 28, 2015.

  1. Ooks
    Joined: Oct 2013
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    Location: Brisbane, QLD, Australia

    Ooks Ian

    All - looking for advice on an idea I have been working around.

    I have a custom built mono 36ft - based heavily on a Mumm 36, but with a lifting keel. It has been through a few hands before mine and still not seen the water.
    I am trying to make it more comfortable for light cruising and the keel box makes life a bit difficult as it really is in the best position for a central table and I have not found a good alternative location.

    So while sitting on her staring for inspiration it occurred to me that I may be able to cut a window out of the walls of the keel box and open it up enough to make it OK to sit round as a table.

    The keel box is constructed of vertical hardwood (I presume) pillars fore and aft of approx 100x50mm. SS C section in attached internally to these and 10mm marine ply forms the faces. I also have a deck mounted pulley system to raise the keel.

    From looking at it I think the faces don't really provide much structural purpose. I do need the top section connecting to deck to transfer loads from the raising mechanism to the pillars, but the large panel than is giving me grief I suspect serves no purpose.

    I can provide photos of my boat, but I scoured the internet and found a couple that probably illustrate quite well my line of thinking at the moment.

    The first shows a full keel box with lifting mechanism at top - mine is mounted a little above the deck. The second is from a much larger yacht, but shows what I am considering.

    As I said the design is custom - I have managed to contact the guy who did it, but all drawings etc were lost some time ago, so I don't have much to work with form there.
    The keel itself is a 1.9m deep with a constant profile and lead in the bottom 500mm. Weight is 800kg. the whole thing is built on a gal steel frame.

    Seems like a good way to get more useable space from the yacht, but I would love some advise/thought from people more experienced than I.

    Cheers.
     

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  2. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    Ooks; This thread has been languishing for several days with no reply. I will credit my peers with the careful judgment to leave this question alone. The question can be described as a "hot potato" with all sorts of ramifications.

    Think of the structure as a playground teeter totter, see saw if you prefer....I do not speak fluent Aussie. . The retractable board is on one end of the lever and the board case is on the other. The pivot point is at the garboard area of the hull. The deck and its reinforcements keep the top of the case in its assigned position. There is a huge cantilever load on the upper end of the case. Sorry to tell you that the skin of the case is a major component in the strength of the overall structure. The skin is the outer most fiber of the lever, and thus, by far the most important in terms of, cantilever like, beam strength analysis.

    If the top of the case, having the window opened, is left unsealed, then there is the possibility that it might cause disastrous problems with flooding if the boat is ever on beam ends. If sealed then the board could not be fully retracted.

    I strongly recommend that you have a local engineer or naval architect examine your boat to give you a real case bit of advice. The reason for that is that there are a lot of variables that can not be perfectly described on a forum such as this.

    With all that nay saying, I do understand that a window would be a desirable feature. A shelf top would be a fine place to rest your Martini glass or other uses. That convenience may not offset the structural problems that could result from the modification.

    Cheers.
     
  3. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    I saw this thread, but had nothing to offer, because I don't know enough about this boat's structure to reasonably advise. I agree it would be a nice upgrade and the settee area will feel less cave like, but . . .

    The logical thing is to contact Bruce Farr (> info@farrdesign.com <), maybe he or his team can help on their blog (> farryachtdesign.wordpress.com <)
     
  4. rwatson
    Joined: Aug 2007
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    Location: Tasmania,Australia

    rwatson Senior Member

    I am assuming that the "cutaway" picture a proposed solution, not your existing setup.

    If this is the case, there is no reason why it couldnt work in your situation. Messabouts points about getting professional help are very important, but mechanically it should work fine.

    Repeating also, the sealing of the top of the case while underway would be important, as pressurised water up the case could make for a dangerous flooding opportunity.

    Also, instead of a big open rectangle, you may want a vertical beam in the centre of the opening, opposite the top of the "chord", as the bevelled edges at the leading and trailing edges may not have enough strength.
     
  5. Ooks
    Joined: Oct 2013
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    Location: Brisbane, QLD, Australia

    Ooks Ian

    Thanks all for your replys, pretty much gave up on it after a few days of silence. Useful comments all - I was not thinking of the faces as forming part of a box beam, which it essentially is, so that does make me think a bit more on it.

    This is not a completely new idea though, many trailer sailers employ a similar idea, some actually do away with the pillars entirely, or have something that appears only to transfer vertical load to the lifting mechanism. when extended the structure of the keel only extends about 600mm above the floorboards - so I was thinking that is where the strength is more crucial. A very similar setup is on the Beneteau first 27.7.

    [​IMG]

    and also on an Andrews 28

    [​IMG]

    So I think the take away advise is talk to a designer before I do anthing. I have spoken to Farr in the past, but they do not know anything about the boat - suspect the Farr provided some inspiration, but definitely not one of their boats.

    Thanks again.
     
  6. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    The builder then, would be the next place to start. Maybe Cookson in NZ? The HIN should offer some help.

    Yes, some trailer sailers do use interesting arrangements. I have a design that eliminates the tall case and compression post, using ring frames and other techniques to absorb the loading issues, but on smaller boats, you can get away with these. This is because the loads are relatively light, compared to the material properties typically employed. This isn't the case once you get into this size craft, where laminate schedules will need to be "up to" the tasks, rather than "more than sufficient" because of the minimum values available. In other words, a small boat is usually "more than" stiff or strong enough, because the laminate minimums, will be over what's necessary, while larger craft need to have finer pencil point used in engineering, to insure the laminate is "up to" the loading expected.

    In short I think this can be done, though a solid understanding of the boat's construction will be necessary, to make any significant (this is) changes.
     
  7. Ooks
    Joined: Oct 2013
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    Location: Brisbane, QLD, Australia

    Ooks Ian

    Hey PAR - thanks for an emphatic and clear response - just what I was after!
    I was sort of coming to that realisation myself, but very helpful to have it put so clearly. I will slowly walk away and put down the jigsaw...
     

  8. schakel
    Joined: Jul 2008
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    Location: the netherlands

    schakel environmental project Msc

    I know ships like that and a window in the keelbox will let the boat unable of total retrackt of the keel and therefore "falling dry" on the shore will be not horizontally. If that is not a problem you can cut a window. If you want to fall dry horizontaly not.
    This is my humble opinion of course.
     
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