Sent: Wednesday, February 24, 2010 7:32
PM
Subject: Re: [PSUBS-MAILIST] New Guy
from down under needs help
Hi Les,
Welcome aboard, I live across the ditch in NZ,
so we are on a similar time zone.
I'm building a very small 1 person fiber glass
submarine out of epoxy, fiberglass & steel.
There's no weight advantage to be had in going
fiberglass or carbon fiber. You in fact make a heavier boat.
Although carbon fiber
is stronger for a given weight than steel, you need to go
something like 4x the thickness
to achieve the same strength. So if you have
two subs, one of steel & one of carbon fiber, with an identical
internal space;
the one of carbon fiber will be larger,
displacing more water & require more weight to sink it. You would have
the expense of
carbon fiber & extra lead as well. Another
way of looking at the weight issue is, if you have a 200 liter drum it's got
to weigh
200 kilos to sink, wether its made of lead
steel or polystyrene. The only way to get the weight down is to make it
smaller.
Carbon fiber is good in tension but I don't
believe its any better than epoxy / fiberglass in compression.
Round forms are the most pressure resistant
& just about all subs stick to cylindrical, spherical & conical
shapes wich put
mainly compressive forces on the material.
Being finer fibers its harder to get a resin to fiber bond with carbon
fiber.
Steel has uniform strength in all directions
but fiberglasses don't. If you pull them in the direction of the fibers
they're strong, but if you
pull them diagonally to the fiber
direction they're not. You need to lay up in a way to compensate
for this.
There was a good article on why not to use
fiberglass in the psubs FAQ section but I couldn't find it.
I'm using fiberglass because I'm going
very small, it suites my design / construction technique & I'm prepared
to over compensate with thickness.
I'm aiming for the 250-300ft
range.
The size of the dome you mentioned would mean
the diameter of your sub would require you to go quite thick with the carbon
fiber.
To get to 300ft with a dome 5ft in diameter
you'd need about a 45mm thick cast dome & I would hate to even
guess at what it would cost you,
Someone else might wade in with a guess.
Getting the Stachiw book on acrylic is a must. I held off buying it but am
glad I did. 1000 pages
all about acrylic. If you join psubs the
discount on the book that you'll get pretty well pays for your
membership.
Here is a link to a free pressure program; you
have to download the sequel server first. You can also download the manual.
It analyzes cylinders
cones, hemispheres & spheres under external
pressure & gives crush depths, displacement etc. It doesn't show where
ribs should go.
A great tool for getting a rough idea of what
thickness material is needed for someone designing there own
sub.
How many people is this for?
Regards Alan
----- Original Message -----
Sent: Wednesday, February 24, 2010
7:58 PM
Subject: [PSUBS-MAILIST] New Guy from
down under needs help
HI,
My name is Les, in Australia, I joined psubs
almost a year ago,after much much reading, and learning, an
experience I am sure that you are all aware, I do have a couple of
questions that some-one may be able to assist me with.
My aim is a specialized maximum depth 300feet
1atm sub, of which weight is most critical, therefore I am
considering the exotics such as carbon fibre and
Kevlar .
1. Any experienced knowledge on
these two materials re this pressure 150psi that anyone can give me
would be most appreciated.
2. Any information regarding acrylic
bubbles or more precise half bubble or dome 4 feet (1200mm) to 6 feet
(1800mm)diameter for this depth and pressure, would also be
of assistance.
Looking forward to learning more
Cheers
Les
