Marc,
Please take another look at my original email. Notice that I said that
the tensile strength in fiberglass comes "MOSTLY" from the fibers. I
even put "mostly" in italics. Notice that I also said that the
compressive strength comes "MOSTLY" from the resin (also in italics). I
did not say all or always. I did not say that fibers in aircraft wings
do not help with compression.
You can disagree all you want, but this is why composite tanks resisting
internal pressure can be very thin and composite tanks resisting
external pressure (like submarines) are usually very thick. Also
remember that vacuum tanks only have to resist less than 15 PSI. I
specifically used italics because I was speaking in very general terms.
Greg Cottrell
--- On *Wed, 6/2/10, greg cottrell /<jgcottrell2002@yahoo.com>/* wrote:
From: greg cottrell <jgcottrell2002@yahoo.com>
Subject: Re: [PSUBS-MAILIST] Fiberglass hulls
To: personal_submersibles@psubs.org
Date: Wednesday, June 2, 2010, 7:07 PM
In a nutshell, the biggest difference between fiberglass and steel
are the strengths they posses in resisting compressive and tensile
loads. Both have greater tensile than compressive, but the ratio is
much different- while steel's compressive is almost as much as it's
tensile, fiberglass has great tensile and poor compressive. There's
an old saying "you can't push on a string". Tensile strength in
fiberglass comes /mostly/ from the fibers- compressive strength in
fiberglass /mostly/ comes from the resin.
Since sub design is all about resisting compression, steel is still
the winner. As a side note, there were some experimental hard suits
made using carbon fiber instead of glass (in tensile, carbon fiber
is much stronger per pound) Surprisingly, the carbon fiber suits
turned out to be weaker than the glass and the idea was canned.
That doesn't mean that great subs can't be built using glass- the
British LR series subs were all GRP and certified by Lloyd's.
Hawke's first Deepflight was too. But the engineering is critical
and way beyond us "little guys"
Greg
--- On *Wed, 6/2/10, Alan James /<alanjames@xtra.co.nz>/* wrote:
From: Alan James <alanjames@xtra.co.nz>
Subject: Re: [PSUBS-MAILIST] Fiberglass hulls
To: personal_submersibles@psubs.org
Date: Wednesday, June 2, 2010, 5:23 PM
Hi & thanks Alec & Greg,
I've vacillated between fiberglass & steel for nearly 2 years
now & have a few good books
on the subject. It's very complex unlike steel, that's why when
I get the polystyrene full
scale model as close as possible to what I want, I'm going to
draw up the design &
book in some time with a composites engineer. I've had a brief
talk with a design engineer who's
worked on Americas cup yachts. His firm has a large program that
can analyze for pressure
& design a lay up plan also. Maybe I'll have to draw in big
safety margins. I'm prepared
to abandon the idea of composites if it's looking too costly or
not viable in other ways.
& no I'm not going fiberglass to make it lighter. It's going to
end up heavier as I have to go
thicker than with metal. Bottom line also is it will be tested
unmanned to twice the operating depth.
I'm trying to make this really small wich has been my design
ambition from day 1. This involves an
ergonomically designed hull that transitions from spheres to
cones to cylinders & involves varying
thicknesses of material, wich would be really difficult to
achieve with metal.
Regards Alan
----- Original Message -----
*From:* Smyth, Alec
<http://us.mc1117.mail.yahoo.com/mc/compose?to=Alec.Smyth@compuware.com>
*To:* personal_submersibles@psubs.org
<http://us.mc1117.mail.yahoo.com/mc/compose?to=personal_submersibles@psubs.org>
*Sent:* Thursday, June 03, 2010 5:56 AM
*Subject:* RE: [PSUBS-MAILIST] Fiberglass hulls
Alan,
I really hate to throw cold water on someone's dream boat,
but please reconsider this path. There is room for lots of
innovation yet in sub design, but there are a few
fundamentals that one really cannot escape with any
confidence. What worries me is not so much the calculated
strength of the hull, but the degree of confidence in that
number. Use of non-standard geometries or materials
introduces uncertainty in both calculations and fabrication.
Consider just the fabrication aspect. In a conventional
cylinder, if the roundness is off by just a tiny percentage
the depth rating falls off out of all proportion to the
fabrication variance. Greg recently mentioned something very
similar having to do with acrylic windows that were not
correctly annealed -- their performance went from many
hundreds of feet to 50 feet. Composites can be strong, for
sure, but my understanding is that their strength varies
tremendously with the skill of the person using them. There
have been composite hulls, such as those made by Hawkes, but
they were not hand laid.
Composites are wonderful for things that want to fly through
the air or float on the surface, but a sub is the only type
of vehicle I can think of where weight is actually an
advantage. You'll still have plenty of opportunity to use
them on non-structural components like fairings and soft
tanks. But if you make a light pressure hull, you'll end up
with lots of lead on board to make it sink.
Thanks,
Alec
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*From:* owner-personal_submersibles@psubs.org
[mailto:owner-personal_submersibles@psubs.org] *On Behalf Of
*Alan James
*Sent:* Tuesday, June 01, 2010 6:34 PM
*To:* personal_submersibles@psubs.org
*Subject:* Re: [PSUBS-MAILIST] Fiberglass hulls
Hi, Frank , Glenn & Vance.
My sub will be about 25" in diameter in a semi spherical
shape around my upper body
& transitions down to about 14" around my legs. The hulls
about 6 ft long with external battery
pods out the back. So it's not much more than a kayak. My
program shows that the shell
only needs to be .5" thick at the most on the 25" dimension
to get a crush depth of over 1000ft.
Some reinforcement will need to be 2" thick. I only need to
go to 250 ft to match my dome.
So for my design in epoxy & fiberglass ( not wound carbon
fiber) it doesn't seem too prohibitively expensive.
I agree it would be stupid to use it for a big sub, but how
many kayaks are made out of steel?
Alan
----- Original Message -----
*From:* ShellyDalg@aol.com
<http://us.mc1117.mail.yahoo.com/mc/compose?to=ShellyDalg@aol.com>
*To:* personal_submersibles@psubs.org
<http://us.mc1117.mail.yahoo.com/mc/compose?to=personal_submersibles@psubs.org>
*Sent:* Wednesday, June 02, 2010 4:50 AM
*Subject:* Re: [PSUBS-MAILIST] Fiberglass hulls
I like some of the benefits of a fiberglass hull like no
rust and the ability to make complex shapes. As for the
cost though, It's not cheap. The fiberglass fairings and
dive planes I made cost almost a thousand dollars just
in material. They are a minimum of 3/8 inch thick with
some small areas up to 1-1/2 inch. If a guy was to be
making a 5 inch thick pressure hull from fiberglass, I
can't imagine what the cost will be. ( LOTS $$$ )
Now I know there are places to get FRP materials cheaper
than where I went but the cost savings isn't all that
much. It's not only the resin and cloth. There's all
kinds of stuff needed. It's the little things that add
up. Brushes, buckets, acetone, tools, tape, glue,
paints, mold materials, dyes, mold release, sand paper,
rasps, and that list just goes on and on. And another
note.....Fiberglass dust will EAT an electric motor like
a grinder, sander, saw, or whatever tool you've got.
Keep the dust out of the motor with masking tape and be
careful not to overheat the tool.
Frank D.
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