Light floats

Discussion in 'Multihulls' started by bruceb, Dec 2, 2012.

  1. bruceb
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    bruceb Senior Member

    I intend to build a set of light "racing" floats for my 24' tri. Glass/foam core to keep them sort of cheap. Max unsupported panel size is about 2' x 6', and I am not very concerned about impact damage, just strong enough skins to "keep the water out";) . What are 8.5 rule tris using? I can deal with bulkheads and other structure, but I would like to know what sort of skin lay-up is the minimum I can use. I consider these almost disposable/expendable, and I want them to be VERY light.
    Thanks for any input, Bruce
     
  2. Gary Baigent
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    Gary Baigent Senior Member

    Bruce, show us your drawings/sketches. Are you going full length/high buoyancy or the small float plus foil way?
    You can build the latter very light; just have bulkheads and separate compartments for strength and collision/sinking float avoidance.
    Sid's small floats in 3mm tensioned ply and glass weighed some ridiculously low number, like less than 10 kgs each (before glass and carbon attachment to the beam).
     

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  3. Corley
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    Corley epoxy coated

    I agree with Gary that it's going to be tough to make them much lighter in sandwich construction, if your not too concerned about impact strength then ply is the way to go. You could also do a very light tortured ply skin and vac bag a core to the inside of the skin for extra stiffness.
     
  4. Samnz
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    Samnz Senior Member

    basically if you want to keep them the same size you cant build them any lighter in foam and glass unless you use a very fragile laminate. Female molded vacuumed carbon Nomex can be built lighter than 4mm ply but other than that 4mm ply is probably the lightest panel possible. The floats on Capricorn were 4mm ply with cedar stringers and weighed 70kg each. Foam and glass F82 floats which are about 1m longer weigh around 100kg each!

    From the 70 or so races and 5 years I owned a sistership to your boat and racing against faster boats the whole time my suggestion (and what I was going to do if I didnt build a bigger boat) would be to build longer higher volume floats with a very light laminate and try not to add weight to the boat.

    This way the devastating light air performance would not be hurt, but she would be a rocket in higher winds.

    A fairly generic laminate in foam and glass for an 8.5 is 12mm H80 core with 400gm DB on inside and 600 DB on outside. I used 400gm inside and outside and 15mm H80 core on main hull which has very few bulkheads and ringframes. Stiffness is ok and its a massive unsupported area.

    If I were you I would build it from H60 foam, 12mm - 15mm thick and with a 300gm boat cloth on outside and 200gm boat cloth on inside. No ring frames and as few bulkheads as possible. any areas that need to be walked on would need heavier foam/laminate. my seat and galley was build with this sort of laminate and its surprisingly stiff/strong.

    There would be a chance this sort of ultralight laminate could fail and you certainly wouldnt wanna hit the dock but if all the surfaces are round it'd be very stiff and light.

    I think a 7.8m long float with around 220% buoyancy would be ideal. You'd be hitting 12 to 13 knots upwind and over 20 reaching easily. If they were the same weight and dihitheral she'd be just as quick in the light airs.

    Gary's idea of small floats and foils is also valid. If you enjoy messing about with foils then maybe try that option!
     
  5. Samnz
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    Samnz Senior Member

    you can see my new 8.5 Tri floats in lines plans and sailing upwind in the images.
    The design of the float was done by Phil Maxwell and they are about as good as you can get in tortured ply in my opinion. The photo is courtesy of Stephen Craig.

    After sailing the new boat its interesting the difference between its float shape and the Bucc 24 float shape.

    If building out of foam and glass I would change the float shape quite a bit knowing what I know now.
     

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  6. bruceb
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    bruceb Senior Member

    Light floats 102

    Sam, what i would really like is have your boat up here for "testing". Want to put it in a crate?
    Thanks to everyone for the info, now I have to decide. My current 1/4" ply floats are heavy- one is about 200 lbs, the other one a little heavier. To keep my "stock" rating, I need floats that at least look like my hard chine Buc originals, but they don't have to be as heavy, or even the same size. I have made several test panels of various combinations, and 4mm ply is hard to beat- and really easy to build. I wouldn't mind something even lighter and take my chances, but it doesn't seem enough lighter to make the effort worth while. They will definitely end up a few feet longer, as I don't seem to be that good with a ruler;) but "look" standard.
    Sam, what would you change if you built different floats in foam/glass? Those already look good.
    Gary, even if the float size were open, I am not to sure about small floats on my Buc. I think it is too heavy, and coming out of tacks and mark roundings would stall and bury the float before it got up enough speed for the boards to provide lift. Your boats weigh less, my main hull with equipment is probably around 1000lbs plus the floats. It doesn't usually lift out of the water before my floats are almost in.
    Sam, are there more details of your new boat posted somewhere?
    Thanks, Bruce
     
  7. Samnz
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    Samnz Senior Member

    I would have a narrower deck and a harder turn of the bilge if I could, i.e flatter sides with more volume low. as is the floats are very forgiving they dont nosedive and when they do they pop up really quick plus they were easy/cheap to build and light so its all good.

    A bit more detail here
    http://crew.org.nz/forum/viewtopic.php?f=37&t=16046
    and here
    http://crew.org.nz/forum/viewtopic.php?f=37&t=19717
    and the original
    http://www.blairboats.com/demon-85.html

    I guess it changes things for you a wee bit currently having 6mm ply floats and wanting to keep a similar look.

    I think you should copy the original float plan, scale them up 15% and stretch them to 7m long (that way theyl look the same but have heaps more volume)

    Make the panels out of the 15mm H60 foam and boat cloth as flat panels,(quick and easy to do on a bench) build hulls exactly as they were designed to be built. Will be faster on all points of sail in all breeze
     
  8. cavalier mk2
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    cavalier mk2 Senior Member

    I'd say keep the ply but only epoxy outside no glass and no epoxy inside. It might be interesting to see if a 3mm panel with 4oz glass both sides is lighter.
     
  9. oldsailor7
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    oldsailor7 Senior Member

    Cav, you have to have the epoxy inside and out if you are to be able to control
    water content of the wood and prevent so called dry rot.
    I would say use 4mm Luan ply or similar, to spec 1088, two coats of thin epoxy on the inside, three on the outside, with glass tape only on the seams. Polyurethane paint, for U/V protection and appearance.
    Absolutely no need for overall glassing and it's weight penalty.
    No fairing or excessive sanding needed.
    Very light but bullet proof.
    I proved this on my very successful B28. :D
     
  10. redreuben
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    redreuben redreuben

    Gaboon ply and Paulonia framing timber, epoxy and paint finish.

    mY 2c
     
  11. oldsailor7
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    oldsailor7 Senior Member

    There is no doubt that two layers of 4oz glass cloth set in epoxy, polyester or vinylester, after it has been filled and smoothed, is going to weigh more than 1mm of plywood simply painted with epoxy. Foam/glass sandwich is however a completely different matter.
     
  12. cavalier mk2
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    cavalier mk2 Senior Member

    Well if he wants the ultimate light wood he'll skip those inside epoxy coats and dry sail or keep heaters at the dock for race night. I suppose you were figuring 5mm total thickness for the 3mm sandwich but I think it could be thinner with minimal coats and why would you fill the inside? Set the glass in one thick coat. Wet the wood, lay and wet out the cloth, done. Perhaps just inner glass on the bottoms.
    It's time for Bruce to get these things roto molded and make them out of recycled plastic....
     
  13. messabout
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    messabout Senior Member

    My careful experiments yield the following: 4mm luan with 6 oz. glass on both sides weighs 0.55 pounds per square foot. Thickness of the sandwich is 5mm. This makes a stiff panel but not quite as stiff as naked 1/4 inch fir at about one pound per ft.sq. Six mm okumee weighs about 0.66 pounds per sq ft before applying anything wet, like paint or resin.

    I'd trust the 4mm with glass sandwich for a minimally durable ama. Not to be trusted without glass. For impact resistance and weight compromise, glass on the inside only will be a stronger deal than glass on the outside only.
     
  14. oldsailor7
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    oldsailor7 Senior Member

    I'm with you Reuben.
    From nineteen years experience of building and sailing 29 boats of all types of plywoods and finishes, plus foam/glass sandwich constructions.
    Glass fibre coating is simply not necessary, inside or outside, with lightweight plywoods and modern epoxy resins.

    Modern multihull sailboats are lightweight structures, like wooden aircraft, not like armoured tanks. Any slight damage is quickly repaired with epoxy paste and a spatula, plus a dab of paint to touch up.
    It is a fallacy to think that a glass/resin covering will protect the hull from damage. Scuffs and abrasions maybe, but epoxy alone will do that.
    One 40 ft boat we helped to build was coated with glass cloth and polyester resin on the outside only.
    It rotted out from the inside in only a couple of years.
     

  15. cavalier mk2
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    cavalier mk2 Senior Member

    Its trendy to think of ply as a core. Messabout is right that glass on the inside makes it more rupture resistant. My tests have shown that a layer of glass can ad strength beyond the weight penalty. Testing would be very important. I'd be worried about a bow rupture in a nose dive if it went in deep. I really do think rotomolded amas would be good for a class. Volume would offset the costs and light weight would be there.
     
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