flatpanel construction

Discussion in 'Fiberglass and Composite Boat Building' started by boat fan, Jan 23, 2012.

  1. boat fan
    Joined: Sep 2008
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    boat fan Senior Member

    Hello

    A friend and I got to talking about floating cabins / house boats over the week end , the subject of plywood rotting in a fresh water river environment came up.

    He has a set of Glen L " Mark Twain " ( pontoon / houseboat , 28 ft - 40 ft ) building plans , but thinks plywood is not the best choice of hull material , ( again , the rot factor ).

    I suggested looking into building the same boat using flat panel FRP .
    I`m thinking he could make those flat panels using a formica faced ( or ) plastic lined table.

    Could he then not use those skins the same way as the plywood sheets.?
    The plywood specified for these pontoons is 3/8 " .

    What are the key properties ([SIZE=-1]Tensile Strength ,[/SIZE][SIZE=-1] Tensile Modulus,[/SIZE][SIZE=-1] Flex Strength ) etc that he needs to have calculated / engineered ?

    [/SIZE]
     
  2. PAR
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    PAR Yacht Designer/Builder

    'Glass panels made flat will need a considerable amount of additional reinforcement and/or laminate thickness to compensate for the lack of stiffness of this material. The net result is heavier, more costly and additional building effort then curved panels.

    Plywood can be coated with epoxy and you'll not have issues, particularly in a floating house or barge boat, as these just don't do a lot of passage making that can threaten the sheathing integrity.
     
  3. boat fan
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    boat fan Senior Member

    Thanks PAR.

    I don`t doubt what you say about cost , weight , and building effort.
    Personally , I would still use plywood and epoxy , like you suggested.

    Tom is concerned about potential hull damage ( gouged sheathing , letting water into the plywood) because there are a significant number of submerged logs / stumps , and other underwater obstructions around here.

    The plans don`t show any sacrificial keel stringers or such , what do you think about adding those ( hardwood ) ? Do they mess up the handling ( steering ) of the boat ?
     
  4. PAR
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    PAR Yacht Designer/Builder

    On a houseboat, you can hang a VW bug from your bottom and it will not really affect much, so don't worry about a few runners.

    These can be hardwood, which is the traditional way, but now we have high density plastics, which seem to work better and they don't rot. HDPE would be my first recommendation.

    Couple this with a Xynole sheathing on the bottom and you have a near bullet proof hull shell.
     
  5. cor
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    cor Senior Member

  6. boat fan
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    boat fan Senior Member

    Thank you again PAR .
    I did not know you could buy HDPE in such long lengths.
    Sounds like a really good idea to me. (Most likely cheaper than a VW bug, too ! )http://www.dotmar.com.au/polystone-fender/polystone-fender-uhmwpe-sheets.html

    I would also be interested to know the answer to COR`s query RE : the best way to fix HDPE runners.( It appears that you cannot glue the stuff , so I would assume screwing it through the ply , into backing stringers.

    I would :

    1. Pre- drill runners.
    2. Using runners as template for spacing , pre- drill the hull.( oversize )
    3. Flush - fill holes in hull with epoxy.
    4. Pre - drill epoxy plugs for fasteners , screw down with mastic.

    I REALLY don`t want to start the SS fasteners under the water line discussion again , but I would think SS screws in fresh water should be good. S/ Bronze for salt water.
     
    Last edited: Jan 24, 2012
  7. PAR
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    PAR Yacht Designer/Builder

    Most of these plastic can't be glued, so you screw them in place. You don't need to fully penetrate the hull shell, just hold them up in a bedding compound. It's important to drill over size holes in the HDPE as well as the counter sinks. This stuff moves quite a bit with temperature changes, so you want sloppy fastener holes to let the material move as needed.

    Stainless will be fine. The runners are sacrificial and the fasteners can be as well. Bronze is the best choice, but costly. Just replace which ever screws you have to, each time you pull a runner.
     
  8. Silver Raven
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    Silver Raven Senior Member

    G'day bloke. & Howdy PAR. 'boat-fan' - you still interested in this programe?

    PAR - with great respect - as your 'logo etc' says 'Boat design etc & builder', which is very impresssive. I don't have any of those 'letters' of qualification behind my name. My boat & yacht building along with air-craft mfgr design & building company simply got-on with building FRP items that worked for international customers over a few years.

    I have some problem with your ply vs FRP - weight/costing - personal opinion.

    Here in Australia - it simply doesn't stack-up either for weight vs stiffness nor cost per sq mtr. What with our temp & humidity - ply might be considered at best a medium-term method of building any marine structure without extremely time consuming & expensive high quality workmanship & cost.

    'boat-fan' is the proposed "houseboat - pontoon" for use up in the 'Snowy'?

    I would suggest - if you wish the project to last 'north of 30 years' then FRP might come into its own, for sure.

    PAR - a discussion - in another place - in these forums is/will be going into the advantages of - pre-stressed - pre-surface tension - back to back laminated panels which may then be treated much like plywood but that is lighter, very stiff, water-resistant & highly surface finished - from the very beginning. You might care to re-think you - 'statement of fact' due to a wider set of posibilities. With respect & regards; do have a fab 2012, ciao, james
     
  9. boat fan
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    boat fan Senior Member

    Hello Silver Raven.


    Can you elaborate a little about this please ?
    (Keeping in mind that we are talking about amateur home built , no expensive tooling etc..)

    Seems like the fiber content alone , ( of a solid laminate panel ) would be quite expensive.

    I guess if we built one boat each , it could make it somewhat more cost effective to buy materials , and maybe build a simple split / take apart mold.

    I have scanned Google , and to my surprise , have not found much at all on FRP pontoon building.

    Can you recommend a good book or illustrated text ?

    I don`t really see why glass / ply / glass epoxy laminate would not last.?

    Do you think solid laminate is realistic , cost wise ? Alternatives ?

    Tom , ( not myself) was thinking of taking a boat there , yes!
     
  10. PAR
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    PAR Yacht Designer/Builder

    There's no "opinion" used in my previous comments James, simply well established physical properties. Of course, we're talking about solid laminates compared to plywood. Pound for pound, you simply will not get all three; strength, stiffness and cost considerations that can compete with plywood, in the flat panel arrangement, being considered for the shapes employed in the Mark Twain pontoon design.

    There's no mystery here, just simple physical and economic reality. Lastly, I'm in a sub tropical climate, which I suspect rivals yours most of the time and it doesn't affect material choices very much. I have a molded plywood boat that I built in 1988 here. It hasn't a drop of paint on it, just varnish. It hasn't any 'glass sheathing either and it looks about as good as it did when I built her. I recently refinished her, scraping much of the original finish off, but she's still an Ashcroft molded (two layers of 1/8") boat with Douglas fir and oak stiffeners. The weather hasn't been a factor in her care, other then to keep UV from baking her to death and taking her in doors to refinish because of ambient temperatures, which would have to be done anyway, if you want a good result.
     
  11. boat fan
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    boat fan Senior Member

    I would think that your Florida climate would closely resemble that of our Southern Queensland ( namely Gold Coast ) region.Can be high humidity.

    I think a pontoon type hull may be a lot more vulnerable , being fully decked.

    Here , in Southern Australia temps can be very high ( 41 C day before yesterday , that`s 105.8 F for our American friends ! 37 C or 98.6 F today )
    Luckily our humidity at those temps is usually low.

    Installing good ventilation ports are key in keeping mold , fungus and rot at bay.If the hulls have inbuilt baffles / bulkheads , then installing opening inspection ports at each section is a good idea.
    Large hatches even better.

    Bottom line to all this is ,I have yet to see actual hard evidence that supports the notion that a back yard one- off builder can build a pontoon hull in glass for the same cost and effort than in plywood.

    Personally , I have always thought plywood / glass / epoxy. Still hard to beat as you say.
    Tom will have to decide for himself.
     
  12. PAR
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    PAR Yacht Designer/Builder

    Yep our weather is similar, except in the hot moths, humidity is also high, unbearably so for the northerners that come down.

    Bingo . . . pound for pound, modulus of elasticity for same, etc., dollar for dollar, plywood is decidedly the way for the home builder.
     
  13. whitepointer23

    whitepointer23 Previous Member

    plywood/epoxy no good in australia, i have never heard that before. tell us how glass is better than epoxy encapsulated ply.
     
  14. Silver Raven
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    Silver Raven Senior Member

    G'day all. Coolest day here in 5 weeks, cause we got another 200ml of rain this morning. Current temp @ 1400 hrs - 32* @ 94% humidity.

    Enjoy whatever way you choose to build your dream, 'boat-guy'.

    To the rest to you - you're all to smart for me - only been sucessfully building boats, yachts & structural laminates in FRP for 45 + years.

    Problem is you all klnow so much you can't learn anything new. Your 'shoot the messanger-boy' attitude - 'sucks' & is a sign of your expert knowledge. Hope you enjoy the way you do everything to perfectly. You are all so bloody confrontational & such experts - I'm sure you'll do just great. I had thought y'all might like to - think outside the square - a tad bit. & consider a back-to-back pre surface tensioned laminate that would be totally surface finished when the laminate was finished, was toatally water-proof & quite - cost effective. All of which I thought I'd already said.

    Enjoy the exhaulted plain you live on guys. Ciao, james
     

  15. gypsy28
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    gypsy28 Senior Member

    Hi James, I enjoy learning new techniques and materials however, to be honest I find your posts usually confrontational and "hey look at me I know lots"

    How about sharing with us what you know on glass laminates and not just saying its better than crap Plywood (which I personally love working with :p)

    Thanks, DAVE
     
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