Composite panels for Bulkheads, Decks etc

Discussion in 'Boat Design' started by Mat-C, Dec 4, 2007.

  1. Mat-C
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    Mat-C Senior Member

    I have a bit of a 'how long's a piece of string' question.
    I'm hoping to build a 30 foot planing powerboat. I will probably use Duflex, or equivalent (maybe Coretech?), panels for the bulkheads, superstructure and decks.
    I know that it depends on the distance between supports, but from experience can anyone tell me what sort of thickness and weight of laminate I should be thinking about for these?
    I ask because I'm using the scantling calcs from Dave Gerr's "Elements of Boat Strength" and the laminate weights are sometimes more than double what suppliers are telling me is typical for these sorts of applications. I know that Gerr's rules are considered to be 'conservative', but whilst I don't want a boat that's unnecessarily heavy (or expensive to build), I don't want it to fall apart after a couple of years either....
     
  2. ted655
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    ted655 Senior Member

    Mr. Gerr will help you build a boat that will take you around the world!
    Those "suppliers"... well remember their main drive is to SELL. Not that they ever lie, but I bet they dance around the edge of a lie. On a list of who I'd believe? They would be about 3rd., just ahead of the paper boy.
    ALSO, remember, there are one hell of a lot of cheap *** boats out there. One reason Dave writes books.
    .
    Get the standards from top rate society's & insurers. See what other sources recomend. THEN you will know how long the string has to be.
     
  3. Willallison
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    Willallison Senior Member

    You're both correct in suggesting that scantlings drawn from Elements of Boat Strength tend to err on the conservative side: Dave suggests as much in the intro to the book.
    It can be interesting to compare the method to others, however. There was an article in a recent Professional Boatbuilder magazine, for instance, that compared the scantlings produced by Elements, ABS and 1st principles calculations.
    The 1st principles did indeed come up with a lighter laminate, but ABS and Elements were both so close that they were effectively the same.
    It's also important to consider the longevity of the boat - we always hear about the quest for saving weight - but at what cost? The resulting structures may well be strong enough in the simple sense, but how durable will they be? What's the point in a deck so lightly built that the 1st time you drop something on it, its integrity is compromised?

    Ok - rant over - in terms of your question, it is indeed one without a direct answer, but, looking at the 'standard' Duflex panels, bulkheads and superstructure can usually get by with around 600gsm either side. Deck's are often built using the same, but I prefer to see somewhat heavier laminates - at least 900gsm. These are what I would consider to be a minimum and certainly depend on the distance between supports

    I don't have any experience with Coretech panels, can't suggest anything on that front, sorry
     
  4. Mat-C
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    Mat-C Senior Member

    Thanks Will, Ted

    Let me come at this from another angle. Will, this is exactly what I was talking about: assuming the panels are strong enough in terms of spanning between supports, how heavy do the laminates need to be in order to be tough enough to withstand normal day to day use?
    I've had suppliers tell me that there's no way I'd need anything more than 600gsm faced panels for decks - but whack a test piece with a hammer and see what happens to it....
     
  5. raw
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    raw Senior Member

    There is no way in the world I would specify a cored deck laminate on a 30 footer that had an external laminate of 900g. Has nobody here seen an indented deck due to light laminates?

    1200-1500 is where I'd be looking at as a ball park in order to get the necessary impact resistance, then it comes down to stiffener spacing to determine the final laminate and core thickness/specification.


    raw

    ps sent you a pm mat
     
  6. ted655
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    ted655 Senior Member

    :confused: We don't know the internal design, or the intended purpose of the boat. How many bulkheads, how spaced? How many stringers, how spaced? Flush deck? hull shape? Beam?
    It seems a better figure to know would be laminate thickness in relation to unsupported square surface area. Also, how much of the deck is responsible for strength of entire hull design at a figure over the minimum expected stress loads.
    Is this boat a lake ski boat, an open ocean racer , or,??
     
  7. Willallison
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    Willallison Senior Member

    Raw - note I said minimum, I agree, I'd tend to want to go heavier than that too. It would also depend on what else is to be laid - teak overlay for instance.
    There are any number of boats that feature deck laminates of 900gsm or thereabouts. I was only looking at the schedule for a 55 footer yesterday that had 900gsm decks.
    You should also take into account whether you're using balsa or foam. Balsa tends to have much greater impact resistance.
     
  8. Mat-C
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    Mat-C Senior Member

    The boat, as I've described in other posts, is to be a trailerable 30ft planing powerboat, with a top speed of about 25 knots. It's intended for coastal cruising.
    In my early sketches - and that's all they are - I have bulkeads no more than about 1200mm apart.

    From what I can establish though, a panel that's strong enough to span between supports may still not be strong enough to resist impact.

    Raw - I got your PM - see my reply
     
  9. raw
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    raw Senior Member

    Will - I'd like to know what recognised standard that 55 footer was designed to: My guess - none.

    Mat - you area starting to get it, don't forget that a panel than may be strong enough and have enough material for a basic level of impact resistance my also suffer from excessive deflection....

    pm all sorted.
     
  10. Willallison
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    Willallison Senior Member

    Raw - see my pm...

    Well - you guys have a good weekend - I'm off on the boat for a couple of days R&R.....:D
     
  11. Willallison
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    Willallison Senior Member

    Clarification: I should point out that where I noted in a previous post that I was looking at the 'schedule' for a 55 footer that had 900gsm for its decks, that it was in fact not the complete laminate schedule. In talking with the builder however, I did confirm that this was all he was planning on using....
    As Raw has quite rightly pointed out, and as I have also noted, this is definitely on the light side from the point of view of impact protection. Unless really light weight is your goal - and you are prepared to accept the associated fragility - then heavier laminates &/or additional protection (in the form of teak overlay etc) would be my suggestion.
     
  12. Roly
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    Roly Senior Member

    Wow ,900gm? Using DG's scantling rules (on my 34' sloop deck) my requirement for 25mm deck core, calc'd out at 3.2mm, which was at least 3x 810gm triax.
    I reduced this to 2x 810gm. (1620gm).Otherwise it would be lighter to build the deck out of timber.
    One 810gm triax isn't even 1mm! If you drop an anchor on that you could
    punchure it.
     
  13. masalai
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    masalai masalai

    Has anyone thought of doing "due diligence"? By making a square (1000mm or the widest span to be covered), fastening it securely, (if the weight can bounce like on a trampoline not an effective test, just a poor trampoline) and dropping a 2kg or larger spherical weight, from increasing heights, on it till destruction? Then you will know what thickness to make or design to.
     
  14. rwatson
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    rwatson Senior Member

    The "Due diligence" test suggestion is really interesting to me.
    Years ago I read about a catamaran designer concerned with laminate strength, tested the strength of a 1 metre, 4 x 2 bit of hardwood, and discovered it toook a weight of x dropped from a height of y to break it.
    He then built some 1 metre laminate test panels, and secured them into a frame, and bounced the same weight off these panels from the same height to get a comparison of strength.
    Some years later I saw this "weight test" specified in a design book as if it were regarded as a set standard.
    What my feeble mind couldnt get around was, why did he think that a comparision with a 4" x 2" was valid?
    For a start, what timber was the 4 x 2 from (saying hardwood is no standard), and I doubt many 40ft boats would have a hardwood hull 4" thick, and did the fact that the timber was only 2" wide have an influence on the result? So were his final panels way over engineered ?

    My puzzle is similar in this due diligence suggestion - if one made a panel the width of 2 hull stations, then bounced a weight off it till it disintegrates, would it provide usefull information? Is dropping 2kg (of unspecified area) from, say 10ft high the equivalent of a 30 ft wave travelling at 15knots (a possible scenario) or is it equivalent to a shell from navy patrol boat travelling at 150knots ? (highly unlikely and usually not a major concern)

    To my mind, surely a designer would have to calculate the actual pressure in terms of something like kilonewtons, to represent say a wave of water travelling at x knots and then test the panel against this.
    Then of course he may calculate the strength required for a span of the hull to resist a 20 knot impact against a stone pier, and express that in formal physics.
    The really cautious designer might then calculate the strength required in a hull to resist a fully laden boat weighing x kilos resting on a rock 2ft square to simulate the tide going out and the boat being forced to rest on a rock.

    To my mind, if the minimum calculated force x area can be specified in *specific* formal measurements, then any destruction tests should mimic that specified force - merely testing a panel to destruction doesnt seem to equate to anything (though it would be great fun)

    Its a fascinating topic, and I am intrigued to hear different boat builders express views borne by experience. I heard one boat builder say he was unhappy with the thickness of a macgregor 26 footer hull, and yet I have never heard of any hull failures for that particular craft (except when blown off a trailer in a cyclone).

    In my very limited experience, I have never seen any tables specifically say that a hull material of a craft of x weight should be able to resist a force of y velocity on area of z.

    I wonder if there are some formulae designed for this purpose, or are the tables mostly the result of practical experience ?
     

  15. Roly
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    Roly Senior Member

    In my mind, (as one of the unwashed rabble) the forces involved in a seaway are many, and difficult to resolve. Cyclic loading means that fatigue
    has to be considered in the laminate.
    I can only consider what has been successful in the past and carefully modify that for my own particular need. If I go the math way I am sure I will end up way conservative owing to safety factors.
     
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