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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.
http://www.deepsea.com/tools.html
How many people is this for?
Regards Alan
 
 
 ----- Original Message -----
From: Les & Anna
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