Cabin Sole as a structural member?

Discussion in 'Sailboats' started by Pylasteki, Apr 3, 2008.

  1. Pylasteki
    Joined: Apr 2008
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    Pylasteki Junior Member

    Hi Guys,

    I'm rebuilding my 1961 Pearson Triton and am wondering how much of a structural role the cabin sole plays in strengthening the hull. The sole is made of 3/4 or so plywood with a layer of fiberglass across it connecting the port and starboard settees, tabbing it in. The only problem... it looks like a single layer of finishing cloth.

    I'm planning on installing a water tank in the bilge under the cabin sole, and would like to replace the sole with something a lot lighter than 3/4 plywood, as well as gain a little head room.

    A few stringers will be built out of foam and epoxy cloth, then the sole will set on top.

    I'm thinking about building a cored sole out of 3/8ths balsa encased in polyester resin and cloth. Once I've installed the water tank it'd be shot with release agent and made sure the angles don't lock it in to the thickened epoxy will be filled around the edges. Then on top of everything a few layers of epoxy and cloth between the settees and sole.

    What do you guys think about this plan? For the strength of the boat would it be best to go with thick plywood?
     
  2. alan white
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    alan white Senior Member

    The Triton is very robustly built, like many early glass hulled boats, back before much was known of the medium. When in doubt, overbuild.
    I disagree with the use of balsa for the sole. Use plywood which has its own strength and save a lot of work and costly epoxy. That hull is probably over an inch thick down there. 3/8" balsa is really a weight-saving material. It depends entirely on glass for strength. You do not have a light boat, so any effort to save weight will gain almost nothing for all the cost.
    3/4" plywood is probably overkill in fact. Half inch ply might work if the spans are not too great.

    Alan
     
  3. PAR
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    PAR Yacht Designer/Builder

    If you want to save some weight, consider a honeycomb core sole panel(s), tabbed into the hull shell (Pro-Core, Nida-Core, etc.). You'll save weight, but have much more 'glass work to do (sandwich construction). Lift out floor boards will weigh just ounces in this stuff.

    The Triton is one of the few yachts that has it's laminate and structure engineered. Most yachts of this era, just applied what they knew was more then enough material to insure a strong (usually way too heavy) hull. The Pearson Triton's were the better built boats (and slightly lighter then the Aeromarine versions), though Alberg still designed a pretty heavy laminate schedule, it's not as thick as most. Do you know which one you have? The Pearson's had a cored deck and wooden comings, while the Aeromarine was all 'glass.

    You could save some weight by doing as Alan suggests and using 1/2", especially if you installed some 1/2" ply stringers (on edge) under it, well tabbed into the hull.

    Frankly, the weight savings from 3/4" plywood to honeycomb isn't that significant in a 3 1/2 ton boat, so I wouldn't get all excited about saving 50 pounds in the sole.
     
  4. Willallison
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    Willallison Senior Member

    Just as an aside - I'd be very hesitant about trying to fill the void with foam.
    1. You almost certainly won't be able to completely fill it
    2. When water gets in there - and it will - it will be very difficult to get rid of
    3. The moisture will shorten the life of evrything around it - especially your new tank
     
  5. Pericles
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    Pericles Senior Member

    Will,

    Modern 2 part closed cell flotation foams, such as these below are much less of a problem than earlier products, plus there continue to be cases where silly boat builders squirt aerosol cans of insulation foam into voids! Not a good idea. :)

    http://www.spray-insulation.co.uk/buoyancy foam.htm

    http://boatbuildercentral.com/products.php?cat=56

    OTOT, in the present case under consideration, lift out floors would be a contra-indication for the use of buoyancy foam under the sole for this sailboat.

    Regards,

    Pericles
     
  6. Willallison
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    Willallison Senior Member

    Well, this is true, but it only requires the teensiest bit of moisture to be trapped in/on the surfaces (suposedly) encapsulating a tank and they will soon be consigned to history. (I'm assuming we are talking metal tanks here of course...). The best thing to surround any tank with is nice dry air - and lots of it IMHO
     
  7. Pericles
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    Pericles Senior Member

  8. Pylasteki
    Joined: Apr 2008
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    Pylasteki Junior Member

    Thanks guys,

    I wasn't clear, I'm not going to foam around the tank.

    What I'm thinking about doing is using sheets of polystyrene to build the supports for the sole. 6 inch wide, tapering down to just above the tank. Then fiberglass over that and tab into the hull. A lot easier to work with than wood, as my saws and stuff aren't on the boat, and she's still afloat.

    I was thinking balsa, just because I'll be doing a deck recore in a few months and was planning on using it. Any thoughts as far as weight goes between balsa and some of the others? Price steered me toward balsa, but I haven't seen what wholesale pricing does to the others.

    Pylasteki is being built into a cruising boat, with racing sails and running rigging. Every pound I can remove before loading supplies the better light air performance in the end. Right now she's sitting about two inches higher than her boot stripe, as I've removed the inboard engine, fuel tank, and associated hardware, as well as most of the interior trim, and ice box.

    http://pylasteki.blogspot.com/

    Zach
     
  9. Pericles
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    Pericles Senior Member

    Try not to use expanded polystyrene EPS, as it shears/crumbles easily. Foam your stringers with 2 part buoyancy foam between two planks or a row of building blocks, lined with PVC sheet. Pin the mold securely to the ground with wooden stakes (sounds like a Dracula film) as the foam is powerful as it expands.

    Then carve to fit. Trust me, I'm a foamologist (whatever that is).:D :D :D :D :D

    Best of luck,

    pericles
     
  10. Pylasteki
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    Pylasteki Junior Member

    Is the shear/crumble problem while you are working with it?

    Foamologist... I like it, nobody can call you dense when thats your line of work! :D :D :D

    Zach
     
  11. Pericles
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    Pericles Senior Member

    "Is the shear/crumble problem while you are working with it?"

    Yep, before, during and afterwards, (in service). Ever unpacked a new computer and been showered with EPS packing?:D :D

    Ne'er spoil the ship for a ha'p'orth of tar. :D :D Very nice website BTW.

    Pericles
     
  12. Pylasteki
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    Pylasteki Junior Member

    Thanks, I try to keep that website picture heavy and content rich. The more pictures of destruction the better from my point of view... the sticky resin covered hands have a hard time snapping the after photos. :D

    Yeah I've torn down a few computer boxes, with foam that doesn't fit the box after you remove the computer. I swear, they are designed and packed by rocket scientists. Clearly 12lbs in a 10lb bag. :D
     
  13. Pylasteki
    Joined: Apr 2008
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    Pylasteki Junior Member

    PAR and anyone else:

    I've been seeing a few plywood dividers running athwartship standing on end in the bilge... glassed to support keels, and form water tanks.

    How does one do that without building in a hard spot? I've been reading a bit about using foam behind bulkheads...

    Thanks

    Zach
     

  14. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    You boat has hard spots, but the laminate thickness is sufficient to accommodate the point loading without "signs" becoming visible on the hull's exterior. They did it with well done tabbing, using lots of over lap and flange width (a tab is really a laminated flange). This spreads out the point loading. Of course, it also costs more in materials and labor too, which manufactures were quick to cut back on.

    If you want to place foam trapezoids around the perimeter of your new bulkheads, partitions and stringers, then go for it, but honesty, I don't think on that hull you should be especially concerned about it.
     
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