Height of cabin

Discussion in 'Stability' started by scanner, Oct 24, 2008.

  1. scanner
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    scanner Junior Member

    Hi,
    I recently purchased a 13ft length by 5 ft beam bondwood runabout. My intention is to build a "cabin" on it. Now woodwork I am competent with, boat building well lets say I haven't been down that road yet.
    What I would like to is how hight can I build the cabin and still remain safe regarding stability.
    The boat btw will only be used for river and a max of 500-600 meters off-shore in calm weather. I realise that the weather can change quickly but that will be its basic use. Ideally the roof of the cabin would have to be approx 1 meter above the gunwhales.
    Also i bought the boat with a 25hp johnson outboard... I am gathering that this will not be too powerful for the boat in question.


    Any and all help/suggestions would be greatly appreciated


    scanner
     
  2. PAR
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    PAR Yacht Designer/Builder

    Burdening such a small craft with a cabin seems silly to me. Practical, useable headroom (sitting) is 4', which would be a condo sitting on top of a 13' long boat. You might be able to get by with less height, say 42", but this is tight, even for those of average height. A cuddy cabin may be possible, though the headroom in these is even more limited. The intent of this style of enclosure is to provide dry stowage and possibly a place to dive into when a sudden rain squall rolls through.

    The best way to proceed is to sketch up an accurate profile of the hull and place a cabin that looks "about right" in scale. Remember you need room for a cockpit, splash well, maybe controls (if not tiller steered). The next step would be a plan view of the addition (viewed from overhead) to arrange side decks, carlins, etc.).

    With this information in hand, you could scale directly off your drawings or as many do, use stiff card board or cheap, thin plywood to make templates directly on the boat.

    Build the cabin light, as excessive weight, especially on small boats is a real performance killer.
     
  3. alan white
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    alan white Senior Member

    Normally any enclosure aboard a small boat like yours will be a fabric one. Fabric enclosures are very light and don't affect the stability measurably.
    What is affected is windage if the enclosure is of any appreciable height.
    A hard cabin, as PAR said, will be low to be practical (if any cabin on such a small boat could be practical).
    A small cabin for stowing things and occasionally to climb into if need be, is called a cuddy cabin. Usually, cuddys are proportioned to the boat so as not to look clumsy, meaning not high and unseamanlike, but low and snug.
    Also, while it might seem as though a cabin makes for a safe place to go if the weather kicks up, the opposite is actually true in a small boat, which could trap its occupants if the boat capsized. And capsize is far more likely if the wind could contribute to turning the boat over.
    You have said, however, that the boat won't be experiencing any real exposure. If so, use common sense in any case and build a comprimise cabin that takes into account all factors discussed.

    Alan
     
  4. scanner
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    scanner Junior Member

    Thank you both for your reply,
    Aesthetics were of no great concern to me within reason of course, however the safety factor is.

    The "cabin" as I called it was mainly for shelter from the elements, rain and sun in particular. I should have explained it a little better.
    From the sole to the gunwhales is 25". It has a bench type seat approx 16" above the sole. Leaving 9" to the gunwhales. Taking into consideration the remarks posted by Par in regard to suitable headroom, that would require a structure of approx 40" above the gunwhale.

    The structure I had in mind was basically as Alan stated.
    I was considering a plywood/glassed top with drop down (fabric) sides for protection should it be needed.

    In the conditions the boat is to be used as per my previous post.

    Would it therefore be safe to assume that a structure as described above with a height of approx 40" above the gunwhales, would present no problem as far as stability is concerned, .

    Regards and thanks

    Scanner
     
  5. PAR
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    PAR Yacht Designer/Builder

    Without knowing what we're dealing with, suggestions about stability would be purely guess work. The headroom is measured from the sole. I'm currently sitting in a chair that is 16" off the floor. I'm 5' 9" (okay, well I was, when I was in the army a number of decades ago) and this places the top of my head at 52" above the floor.

    There are mechanics of ergonomics that need to be understood before a reasonable seating arrangement can be had. This is amplified when space requirements or other limitations impede the decision making process.

    As a rule, the higher you sit off the floor the less narrow the seat needs to be, for reasonable comfort. This is why a bar stool can have a very small top, but remain comfortable. Conversely, a low slung sofa that is 12" off the floor may need a 24" (front to back) seat so your upper legs have support.

    In a short headroom situation, like inside a cuddy, it's assumed there will not be a fixed seat, or at least it's very low to the sole. Sitting cross legged on the floor, my butt to head height is about 36".

    Sit down, with a tape measure at hand and get some basics for your needs.
     
  6. alan white
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    alan white Senior Member

    I'm guessing more like 20" above the gunwale at the midship centerline point for overall height. Better to make the sides solid (plexi in frames) and the top of fabric.
    That way the sides become useful coamings when the top is stowed (rolled forward and perhaps a couple of stainless pipe frames removed from pockets just inside the coaming/sides.
    Now you've got a crowned fabric roof a few inches higher than the side coamings, which would put the sides at only maybe 12" above the gunwales (so the crown is fully 8" from side to middle).
    This should give you sitting headroom within (hopefully on a board sole a couple of inches above the bottom of the boat).
    This is assuming the bottom is relatively flat so that you sit just as deep at the side as you would at the centerline. So 25" plus 12", making 37" at the sides and a full 45" at the centerline, minus the thickness of the boards that make up the sole.

    Alan
     
  7. scanner
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    scanner Junior Member

    cabin height

    Alan and Par,
    Thank you both for your input. I think my method of putting across my question has misled you both as to what I was proposing. I have attached a drawing of the idea I had and to which my question was in regard to. My apologies if I have wasted your time. I fully understand what you were both saying. The impression I seem to have given was in regard to a full blown cabin rather than a structure for shelter from the elements should it be required. Basically I was trying to find an alternative to buying a bimini top.

    Thank you both again for your input.

    regards

    Scanner
     

    Attached Files:

  8. PAR
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    PAR Yacht Designer/Builder

    Now you're cooking with gas. That's a nice, light, seemly well proportioned structure and just the ticket for a small boat.

    Of course windage will be an issue, but everything has it's trade offs. Again, think light weight.

    The top (which should have a bit of crown) can be hard or soft. Soft is the classic bimini material or cotton duck. Hard can be one of several materials. Plywood is easy to work with, though not especially light in thicker sizes. 1/4" would be more then fine for your needs and it's light. Seal it, paint it and have fun. If you wanted to get real anal about weight then a composite or cored roof could be built, though there'd be more effort involved in building one.

    The aft supports could be a couple of 2x2's or maybe some aluminum tubing. The windscreen can be as simple as a piece of plexi glass with a basic wooden or aluminum frame. You may find that an opening windscreen is desirable. This will add complication, but not too difficult a problem to over come.

    With your seat height at 16", you'll need a fair amount above the sole for the top. Find something which is 16" high and sit on it. Then measure your seated height, plus a few inches so you don't bump your head. Height off the sole will be in the mid 50 inch range for an average height person.

    If looking for a term for this addition to your boat, call it a hard dodger.
     
  9. tom28571
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    tom28571 Senior Member

    All good advice. the biggest problem I see with the proposal is getting in and out of the seat. You will need something strong to grab and take your weight to move about. the lack of a back on the seat is also an issue. You will need to use the steering wheel for support and I'd favor some kind of foldaway seat back strong enough to take bouncing your body against.
     
  10. scanner
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    scanner Junior Member

    cabin height

    Hi Tom,
    Your comments are valid and normally I would agree with you, however taking into consideration the size of the boat, the structure is not designed to move about in. It is purely for usage as shelter should the situation arise. The boat will never have more than two people aboard it and with the seat, as depicted in my drawing, which was already fitted, at approx 16"w x 16"h it is easy enough to step over or at worse sit on it and swing ones legs over it.
    As for the back support, again taking into consideration the conditions the boat will be used in, I can't see it representing a problem, however as I stated your comments are valid by all means, the exact structure of the seat, should I decide to change it, is yet to be determined and your suggestions have been taken down in my notes.

    Regards

    Scanner
     
  11. scanner
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    scanner Junior Member

    Hi Par,
    Thank you for your comments, yes the "roof" I have considered would be light ply fixed to curved formers made out of meranti, and with a coat of flow-coat on it to seal and add a little strength, without, hopefully adding too much weight. Also the frame work and supports would be 2" diam anodized aluminum tubing. The windscreen, which was present already, is plexi-glass or perspex, as I know it as, in a wooden frame. however to give myself access to the bow anchor, it would have to be removed and replaced with a 3 section windscreen the middle section to open for access to the anchor.

    Regards

    Scanner
     
  12. tom28571
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    tom28571 Senior Member

    Hi Scanner,

    I was not clear on the moving about part. I meant getting into and out of the seat with a "low as practical" cabin top. At advanced years this could be at least a nuisance for me now but not so much when I was younger. Like wise the wish for a seat back. I like about 36" over a seat for comfortable sitting but can get by with a couple inches less if the body never bounces.

    I have a 13' runabout with an older Evinrude 25 on the transom. Its capable of over 30mph and will move me about in any significant chop whether I want or not.

    Meranti is 24 per cent heavier than occume plywood.

    Good luck with the project.
     
  13. scanner
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    scanner Junior Member

    Hi Tom,
    I must admit I hadn't thought of making the formers out of marine ply. Thank you for the suggestion, it is definitely something I will look into. I understand what you mean now by moving about, I thought you meant moving about in the covered area. Yes I understand that there would be some "bending" necessary on my part to get under the cover, I myself am starting to get on a little, but to keep the structure as low as is practical for what I want it for it is something I will have to do. I am expecting to get around 20 knots out of the johnson 25hp this one has on the back, not that I am intending to push it that far unless it is necessary, so your proposal for back support is by no means of no value. In regard you liking about 36" over a seat for comfortable sitting, are you talking abut the width of it ?

    Regards

    Scanner
     
  14. tom28571
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    tom28571 Senior Member

    Scanner,

    I design trailerable cruising powerboats and like 36" of height over the berths (seats in your case) for good headroom for a 6 footer. This allows for a bit of room over head without rubbing for a persons a bit taller than that. Everyone's concept of enough room on a boat is different though.
     

  15. scanner
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    scanner Junior Member

    cabin height

    Hi Tom,
    I'm with you now, yes I agree in regard to the 36" .. I estimate that at 5' 10" approx 39" above the seat would be comfortable for me, my buddy who comes in at 6' 2", who will also be venturing out with me on the boat would require just that bit extra for comfort. Things always look better on paper and in theory, so all measurements are subject to alteration. Being able to "pick the minds" of those who have more experience at these things is the beauty of forums like this one.

    Thank you Tom, Par and Alan for your suggestions and help, believe me nothing has gone unheeded.

    Regards
    Scanner
     
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