Rob, This is true of steel subs also. The only ones that are not very strongly buoyant without ballast are the seriously deep divers in the Alvin league. The problem with composites is that the most important material attribute of all for a submarine is the confidence you have in it's performance as calculated theoretically. There is nothing inherently wrong with composites, but they do have an awful lot more unknowns than steel when it comes to the reliability of your math. In steel you would use a very tightly controlled grade, and what you have to worry about as the constructor is the quality of the welds and the accuracy of your geometry (e.g. that a cylinder is not an obloid). Both are things that you can readily evaluate. But evaluating the strength of a manually laid fibre hull is another matter entirely. There's all the different kinds of weave, air bubbles between layers, the temperature on the day you did the work, etc., etc. Even if you decided you would build a number of hulls and test some to destruction, there's no guarantee the hulls would be the same. rgds, Alec -----Original Message----- From: owner-personal_submersibles@psubs.org on behalf of Robert Ossian Sent: Tue 12/27/2005 12:33 PM To: personal_submersibles@psubs.org Cc: Subject: Re: [PSUBS-MAILIST] Hull material Very good points, but but I am looking to make a sub that is by design very postively bouyant so that a manually released drop weight will guarrantee a safe surfacing in case every other saftey measure fails. Plus, my dry compartment is (to put it kindly) tiny. Thanks for your input though - This is exactly the kind of input that makes PSUBS such a great source of info. /Rob --- Paul Kreemer <paulkreemer@gmail.com> wrote: > Robert, just one comment regarding the weights. It > sounds like you're > considering the weight of aluminum as a negative > feature when compared to > the weight of composites. But really, in many sub > designs some additional > weight is required to get the boat neutrally > buoyant. Another way to put > this is: any weight saved in building a design in > composites will need to be > added back in as ballast. > > A 50cf dry compartment will have a buoyancy of > 3200lbs (50cf * 64lb/cf). > When ready to dive with crew and all aboard the boat > will need to weigh more > than 3200lbs to actually sink! A lot of sub designs > are more like strong, > heavy balloons than aircraft. That might be a > little stretch of the analogy > but it gives a general idea. :-) > > take care- > Paul > > > On 12/27/05, Robert Ossian <rob_neptune@yahoo.com> > wrote: > > > > I am in the very early process of designing a sub > that > > will have a maximum depth of about 200-250 feet > and > > have been wondering if there are any experienced > > opinions about using composite materials (enhanced > > fiberglass) as a hull material? The obvious > benefits > > are that it could be custom moulded to my > > designs/shapes and of course rust-proof. > > > > I have thought about using Aluminum, but the > inherent > > weight and the fact that Aluminum would be > > (relatively) more expensive, I am looking to > explore > > alternative materials (any suggestions are > welcome). > > > > Any thoughts? > > > > > > > > > > __________________________________________ > > Yahoo! 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