Re: [PSUBS-MAILIST] Fiberglass hulls

[Marc de Piolenc <piolenc@archivale.com>]

On 6/4/2010 9:10 AM, Alan James wrote:
> Thanks for your support Marc.
> I'm printing that email out because it was a good summary of what I'd
> read on the subject.
> Additionally the weave is important because where the fibers cross each
> other they bend
> & create a point of weakness.

That's not the right way to look at it - there are no "points of 
weakness." What any crinkle in the fibers signifies is that you have to 
apply correction factors - available in the literature - to the strength 
and stiffness of the resulting laminate. Similar correction factors have 
been used for nearly a century to predict fabric strength from yarn 
strength. Fabric being the thoroughly standardized material that it is, 
those correction factors are highly reliable and can be used for design. 
Many designers - especially amateurs - make the mistake of specifying 
unidirectional tapes because of the very high strength and stiffness 
achieved under ideal conditions, not realizing that minor errors in 
workmanship carry a high penalty with uni that would not even be 
noticeable with fabric. Better by far to specify fabric and accept the 
lower allowables.

> Fiberglass is the " F " word on the Psub site. Mainly because safety is
> a big part of what the
> group is all about. Ironically you need to know how really bad
> fiberglass is & what the pitfalls
> are before you should consider using it. I've learnt enough to know I
> don't know enough &
> that's why I want to talk to a composites expert; & if not too
> prohibitively expensive, get my
> design analyzed & a lay up designed. With some great programs out there
> it should make fiberglass
> safer & more viable.

There's not much to say to this. If a Psubs member or anyone else 
doesn't want to use FRP, he's entitled to his preference, but in purely 
objective terms composites make more sense for small submersibles than 
metals, for the reasons that I have already given. As for learning the 
pitfalls of a material, that is indisputable advice, but it applies to 
ALL materials, not just composites. I have a strong feeling that people 
who assume that metals are the only way to go are seeing them through 
rose-colored glasses, simply because they are the established technology 
and there doesn't seem to be any need for research.

Best,
Marc



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