Side airbox size on 12' sailer

Discussion in 'Sailboats' started by hospadar, Apr 20, 2012.

  1. hospadar
    Joined: Apr 2011
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    Location: Michigan

    hospadar Junior Member

    I'm building a little 12' scow, basically a stretch PD racer (12' LOA x 4' beam, using the PD racer stations spaced out to 18" per station instead of 12" - bow transom is a little lower slope, but basically the same idea.)

    I'm going to use full-length side airboxes, and I'm trying to decide on their width. I'm thinking about going with 3.5" airboxes (so I can use a 1x4 for the top of the airbox instead of chine logs & plywood), which would give probably about 250 lbs of floatation per box (freshwater - Michigan).

    These are probably a little skimpy, but they're still big enough to sit on, and I want to maximize the area inside the boat (I'm tall, I want to do a lot of rowing around lakes and fishing with my friends packed in when I'm not sailing).

    Will this be enough flotation to recover from a capsize in medium-rough water? I don't care if she takes a little water on after a capsize (and requires a little bailing), but I must be able to recover her without getting out of the water.

    I don't know what the final weight of the boat is going to be, but I would think that it certainly woudn't be much more that 250lbs, probably a bit less. Perhaps with a (wet) rig, in windy/wave-ey conditions I might need more floatation though.

    Also there will be a little foredeck/compartment (the size of which will be determined by the mast placement, since the mast will attach to the aft end of the foredeck). So I'll have a little more floatation in the bow, but I don't plan on a stern deck, since I'd like to keep the transom flat for space/simplicity/motor attachment).
     
  2. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    A boat that size does not usually weigh even close to 250. Well not unless you are using extra thick skins and frames. However you are talking about using one by fours and the like. So it might get unnecessarily heavy. Both you and the boat will like it better if you keep it as light as is practical. I'd be devastated if my 12 foot boat weighed any more than 150.

    You are doing well to consider the self rescue capabilities if/when you dump the boat. Decide if your re entry will be from the side or by crawling over the transom. Re entry from the side is easier, on a good day, but it can be problematic in the blow that dumped you in the first place. Side entries have the potential for causing the boat to turtle unless you can manage to get the boat rotated so that the entry side is on the weather side. Righting the boat from there is not difficult but if the sail catches wind as it comes up it will capsize again with you under it. The initial capsize will usually be to the lee side unless you have been hiking so hard that it goes over to windward. That can happen if the main sheet was cleated and you did not react quickly enough.

    The side boxes that you describe are probably big enough but when the boat is righted there will be water to bail. If you make them bigger, like 5 inches wide, the turtle tendency will increase but there will be less water in the boat when you get it right side up.

    I would favor transom re entry in which case the side boxes would not be used. Instead there would be fore and aft boxes............. Lets see what the other guys say.
     
  3. hospadar
    Joined: Apr 2011
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    Location: Michigan

    hospadar Junior Member

    I made a summer breeze (12' skiff, similar to a goat island) and found transom entry to be easiest (that boat was crushed by a tree, which is why I'm building a new one). That boat had no boxes at all (so I never really even bothered attempting side entry or capsize recovery), but the weight of the boat gave plenty of leverage for me to get in on the back transom. I could always pull myself in over the bow in this new boat if need be since there will be a sizable compartment there (the foredeck will be a sealed compartment).

    As to the use of 1x4 (or 1x5 or 1x6, etc), I realize it'll add a little extra weight, but it'll make it easy to attach anything (cleats, handles, oarlock sockets) to the side decks without having to reinforce the insides.

    Another reason I was thinking to try and minimize airbox width is to provide plenty of clearance for oars, so the oarlock sockets (probably I'll actually be using bolger-style single thole pins raised up from the side decks a bit on a piece of hardwood). I want to keep the oars from rubbing on the gunwales/inwales (it'll probably only happen by accident, or when someone is lifting the oars way too high while feathering, but still).

    I'm imagining if I had a really wide airbox, when someone returns the oars with the blades too high up, they'd rub along the inside of the airbox.

    Maybe I'll go another inch or half inch wider, every inch of width gives me another ~60lbs of lift.

    As to turtle prevention, I'll probably be using a wooden mast (be it solid or hollow, it'll be sealed either way) and maybe a float on top of that, so I'm not terribly worried about going turtle. This boat isn't going to be built for top speed, rather for fun and max. safety/ease of use. (otherwise I wouldn't be building a brick ;)
     
  4. gggGuest
    Joined: Feb 2005
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    Location: UK

    gggGuest ...

    As I think I said recentl in another thread, from a safety point of view you really ought to have enough buoyancy that you can sit on the wreckage out of the water with one compartment punctured...
     
  5. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    GGG I agree that you'd ought to have enough bouyancy to support the boat, rig and passengers. However, to support the passengers out of the water would be rather difficult on so small a boat.

    Hospadars boat is to be wood which has some positive bouyancy itself. Probably enough to have the unladen boat floating, if slightly awash. His side boxes at 250 pounds each will provide more than adequate support for a person of normal size and weight. That person is going to be wet however.
     
  6. gggGuest
    Joined: Feb 2005
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    Location: UK

    gggGuest ...

    Why should it be difficult to support a person out of the water on a boat that size. I've sailed on 12' two handers that will support both crew for the last thirty years.
     
  7. sharpii2
    Joined: May 2004
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    Location: Michigan, USA

    sharpii2 Senior Member

    Foam, Foam, and more foam.

    I am suspicious of 'air boxes'.

    There was one fellow who built a boat that had three of them.

    The boat had a dagger board (another thing I'm suspicious of) and hit a shoal, or log, or something.

    The trunk tore a huge leak in the hull and all three air boxes failed as well.

    This was on a small lake during reasonably warm weather and the water was shallow enough for this fellow to wade to shore, with his sunk boat in tow.

    The biggest hazard with a tall, thin air box is seam failure. water can come in at a surprising rate, especially if the top of the box is not air tight.

    I'm more into separate compartments, which serve double duty. Emergency floatation and a place to put things. When you're putting your stuff in one of these compartments, you have a day to day incentive to make sure it doesn't leak.

    If I were going with just three and a half inch wide floatation, I would go with foam.

    You can't beat it's reliability. And it's probably lighter too.

    If I wanted to keep the thin air boxes for structural reasons, I would divide each into three separate water tight compartments, with an inspection port for each compartment. Then you have effectively six compartments, not just two.
     
  8. hospadar
    Joined: Apr 2011
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    Location: Michigan

    hospadar Junior Member

    I see your point sharpii, and I may divide the boxes into multiple compartments, but I'm not too worried about airbox failure. Anywhere I sail is going to be safe enough where even if my boat was totally destroyed somehow, I'd be OK.

    That said, dividing them into compartments is a good idea, I may to that, or at least add some foam into them (although I will probably leave some open space for storage)

    I'm not terribly worried about leaks, all seams will be chine logged, pl premiumed, bronze nailed, and fiberglassed. Some will have epoxy+fiberglass fillets. There will be some through-holes for pivoting leeboards and some other hardware, but they shouldn't permit much water.
     
  9. sharpii2
    Joined: May 2004
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    Location: Michigan, USA

    sharpii2 Senior Member

    Sounds like a good plan to me.

    The boat I mentioned earlier was designed to be put together with epoxy, taped seams, but was assembled with with glue taped seams, instead.

    My experience from the pre epoxy days has been bad, with taped seams over chine and clamp joints. The wood expands, as it takes up water, and splits the fiberglass.

    I suppose, if you saturate the the chines and clamps themselves with epoxy, you may not have that problem.
     
    Last edited: Apr 23, 2012

  10. lewisboats
    Joined: Oct 2002
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    Location: Iowa

    lewisboats Obsessed Member

    I went with 8" boxes with my 10 foot version "Eider Duck". If I go over I doubt I'll even ship water because righting it will put the inside of the boxes above the waterline before they get level again so any water will drain out before it rights itself. I glued an 8" x 1.5" x 8' strip of closed cell insulation foam on the insides of both the inner wall and the hull wall. Here are a couple of pictures showing it. The framing is Cedar and the panels are 3mm so it is pretty light...it is for my daughter.

    [​IMG]

    [​IMG]
     
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