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 -----
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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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 -----
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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