Bottom deflection in design

Discussion in 'Powerboats' started by waikikin, Jul 25, 2007.

  1. waikikin
    Joined: Jan 2006
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    waikikin Senior Member

    I've been doing some cosmetic repairs(unrelated to question) on a flybridge style boat around 38' & capable of around 30 knots of composite contruction- solid glass to bottom panels & have noticed & been made aware of by owner concerns in regards to deflection mid panel from frame/ stringer of the hull laminate , stringer spacing(deep engine beds timber encapsulated in laminate) about 2'9" & frame spacing up to 6/8', the question is how much mid panel deflection would be considered acceptable, how much to be concerned about & potentially dangerous to continue opperation, would it be 1/16, 1/8, 1/4, 3/8, 1/2, 5/8" ? I havn't been on the vessel whilst running but as indicated by pulled screws in some joinerwork expect that "movements in the upper end of this scale" could be expected, from a moral & safety perspective I advised the owner that this was really the knowledge territory of surveyors & NA's & I will consult a freind NA in regards to this but need peace of mind & coarse of action if the defection exceeds reasonable parameters as the safety aspect is of much importance. Also I am not the builder of this vessel, just a humble shipwright asked an opinion beyond my experience base as I've mostly worked on sailing vessels of cored structure that seem very stiff compared to this. Thanks in advance & regards from Jeff.
     
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  2. tom28571
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    tom28571 Senior Member

    Hopefully a qualified surveyer will post. In my experience any repeated deflection is ruinous to fiberglass panels. I have seen hulls of small boats become very soft after being improperly fitted to a trailer for too long. I know that FG panels experience micro-cracking of interior fibers every time they are flexed. At some point it's beam strength is compromised. Avoiding such boats is the safest policy.

    Running a powerboat at 30 knots is very rough service. Slamming loads on the forward part of the hull that meets the waves requires a large safety factor in layup schedules well beyond that necessary for slower boats. If you look at David Pascoe's website, you will see a rogues gallery of seriously damaged boats caused by inadequate attention to fiberglass scantlings.

    Wood is much more adapted to flexing. The tree has a natural acommodation to flexing and is thus more tolerant to such treatment.
     
  3. waikikin
    Joined: Jan 2006
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    waikikin Senior Member

    Thanks for the input Tom, I'll check out the Dave Pascoe site, also I was thinking to mount some cone shaped mounds of modeling clay to the hull skin with a small coin or washer atop to inhibit sticking in way of fixed joinery or strait edge mounted diagonally across frame/stringer intersections to kind of "witness" the movement so that it(the gap) can then be measured to better indicate the amount of flex at the panel centre. I'm also thinking that if adjacent panels are exhibiting similar movements either side of say a bulkhead thats fairly lightly tabbed in, the bulkhead being a fixed "hard point" kind of focuses the flex times 2? & would be the likely site for some micro cracking? & at a later date macro(if thats the word) cracking(busted)?. Regards from Jeff.
     
  4. raw
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    raw Senior Member

    Under AS4132.3 for single skin (solid) laminates we can design to a deflection limitation of 1/50 of the lesser unsupported span when we apply a design pressure load.

    ie if your panel is say 2000x600 then the maximum deflection allowable would be 12mm. Note I say, a design load pressure which is a static pressure designed to account for dynamic loadings. I would not expect to see a 12mm deflection in the case above as many panels are strength limited, and so the actual deflection will be far less at the point at which the maximum stresss reaches a predefined limit.

    Don't believe anyone who tells you that the structures don't move. I have seen a perfectly adequate structure on a large sportfish that exibited enough movement during slamming, to crack floor tiles that were glued directly to the sole. Even though the tiles cracked, the flowcoat on the members did not exhibit any signs of hairline cracking anywhere. Remounting of the tiles on a "floating" floor fixed that problem. That said, if you have movements in your structures that cause continual cracking in joinery and won't let you open cabin doors, you might have a problem.

    raw
     

  5. waikikin
    Joined: Jan 2006
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    waikikin Senior Member

    Raw, thats reassuring info that the deflection may be within reasonable limits of the AS4132.3 as I beleive its fairly conservative, I've found my copy of it but without the first AS4132.1 that defines the static pressures applicable to the vessel & bottom area & I imagine the pressure, p for pascals goes up with the design speed of the boat? Interestingly & I suppose obvious is the "Note:The design principles cover vessels opperated in a seamanlike manner under adverse weather and sea conditions" in sect' 3 design which may indicate the cause of the pulled fastenings I've seen. I'll stay in contact with the owner & make him aware of what I've learned but will still advise to get opinion from a Qualified NA-surveyor for his(& my) peace of mind as personally I don't feel confident or qualified to "run the numbers" on something like this & will stick to construction & repair work to schedules provided by those qualified to do so. Regards & all the best from Jeff.
     
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