Design / stability of cabin mod - 15' seahog

Discussion in 'Boat Design' started by JimmusUK, Jan 8, 2021.

  1. JimmusUK
    Joined: Jan 2021
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    Location: Worthing, UK

    JimmusUK New Member

    Hi.

    Looking to adjust the cabin on a small fishing boat from closed to open but ideally increase the height. The current design came with a closed cabin with big heavy ply doors and has a step down to such a low roof that it's hard to crouch.

    Is there a system or online calc or just plain old fashioned experience that could guide me as to how much I can raise the cabin without compromising stability to unsafe levels? I'd love to have an open wheelhouse to be out of the wind whilst at the helm but appreciate that may be too much to expect from a tiny boat :-(

    Construction methods would ideally be either all glass (as few layers as possible just to get enough strength or wooden frame plus thin glass or thin ply sandwiched with light glass (200g chop or similar).

    I've seen loads of DIY cabin mods for seahogs but obv have no idea how stable they are.

    I'd love to find someone who's made mods to little fishing boats and can confidently saying adding a few pounds 60-80cm above the gunwhale won't make it unstable or adding juet a few grams a few cm above the gunwhale will compromise stability.

    I understand that I've not given you any calcs like weight, cog, cob, freeboard etc. The boats are quite small, 15' with 5' beam. Manufacturer specs suggest 350kg in weight.

    Any advice / mandatory info I need to provide would be great.

    Apologies if this is the wrong place I don't really know how to use all of these tools that the boat design.net sites seem to offer.

    Thanks
     

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  2. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    Almost everytime someone endeavors to put a full height wheelhouse on a 16' boat; it is wrong.

    If it were not, you'd see lots of 16' cabin cruisers. The problems are numerous and not limited to raising the center of gravity.

    Here is a short list.

    1. Overloading
    2. Affect on trim both static and underway.
    3. Windage.
    4. Dangerous when steering.
    5. Boat capsizes and ends upside down versus in a manner where people have something to stay attached to.

    All that said, some things can be done, carefully and with limits.

    In your case, if the existing cabin is 300 pounds, if you can build a higher cabin that comes in at less weight, you could theoretically end up at a wash. But the only way to do so practically is by making a lighter house. Most likely, using foam sandwich construction and also vacuum to reduce resin content and weight.

    You have a baseline. The existing cabin has a certain amount of weight and portions of it are known distances above the sole inside. You can make estimates of those lever moments and determine whether a mod would have less or more moment.

    A spreadsheet would help. The problem is I am not sure how sticky the UK is about such matters. You would not want to do the work and get grounded for an unsafe vessel, if that is a thing there.

    The way you'd do it is as follows, for example.

    roof existing wt est 15kg, distance to sole 1 meter, 15kgm moment
    Window left side wt est 5kg, distance to sole, .7m, 3.5kgm moment
    Etc
    Total Moments 285kgm

    When done, you'd have a baseline and then be able to estimate a foam house weights and distances. And this calculation would assist you in determining if it was feasible before cutting anything.
     
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  3. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    The normal thing is that the boat has been designed with a certain margin of stability (in a general sense of the word) so it should be able to accept a slight increase in high weight. It would be necessary to do some calculation to be sure but it is always possible to resort to the placement of fixed ballast near the bottom.
     
  4. fallguy
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    fallguy Senior Member

    While I agree with you from a technical perspective, the problem with the statement is it sort of becomes tacit approval for anything. Then we see people building full height aluminum pilothouses on small 16' skiffs which never were intended for any house, let alone a heavy aluminum version. And the danger for us becomes will we be blessing, for lack of a better word, a potentially life threatening situation, by the suggestion safety margins are there.

    Also, if the boat has available loading of 350kg; does that mean for passengers? There would be little available for increased weight and ballasting without making the top light(er).
     
  5. TANSL
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    TANSL Senior Member

    All right, so let's do some math (which I think is what you should always
    to be made). Anticipating that no one will do them, I have already suggested the use of ballast. And, above all, let's use common sense
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    The best solution might be a perspex screen and a vinyl/canvas canopy, with a clear side curtain if that doesn't keep the spray out whilst standing at the helm, a wheelhouse seems like a step too far.
     
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  7. JimmusUK
    Joined: Jan 2021
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    Location: Worthing, UK

    JimmusUK New Member


    This is very informative and straightforward if I can determine the weight of construction, perhaps by cutting out a section and weighing it but it's heavy old chop glasswork that's for sure.
     
  8. JimmusUK
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    JimmusUK New Member

    Yea this is a nice solution.

    The current cabin is so low it would be better cutting the roof off entirely to save bashing my head so much just trying to stow things away.
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    With a canopy that you can fold down on a nice sunny day (there must be the odd one of those in the UK !) you get the best of both worlds, in a lightweight package.
     
  10. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    If I was in your wellies Jim, I would be looking at keeping the current low forward cabin - it 'looks' in proportion (and that is a good thing), and build a simple windscreen, high enough that you can stand and look through it, and have a canvas roof like what Mr E suggests.
    You could add clear side screens as well.

    In the USA these are usually made with 'isinglass' - here is an explanation -https://www.sailrite.com/What-Is-Isinglass-Clear-Window-Material
    One advantage of these is that they do not look as top heavy as a solid dodger / wheelhouse over the helm, and they would be much lighter as well.
     
  11. kapnD
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    kapnD Senior Member

    I’d be willing to bet that a 200# man could easily roll that craft by leaning on the bow rail hard.
    Im with MrE here, lightweight aluminum tubing with canvas and vinyl for the house.
     
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  12. fallguy
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    fallguy Senior Member

    EF00AE8D-484E-4467-AAF8-07BD26197C92.png
    You don't need to cut anything. Plenty of members here can tell you how to estimate those weights.

    we see these a LOT on big Minnesota waters that can be very cold at times
     
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