[PSUBS-MAILIST] Starting my sub

I guess it's my turn , I never thought this day would come a friend of mine was cutting up some scrap iron from his fab yard and he was just about ready to lay waste to a beautiful 4' x 12' tank in perfect condition and of course I asked if I could have it. Well I have the tank now but I have a problem all of this seemed so easy when it was dream, now that its all real I am stuck on stupid, So can someone point me in the right direction until I get over the shock that I have started to build my own sub. from what I understand you take the dia 4' squared x .7854 x L L=12' = 150.7968 CF x 62.4 lbs = 9409.72032 llbs of water displacement -- 2.500 lbs this is how much the tank weigh = 7088lbs and this would be the weight that I would have to meet in order for the sub to sink ?
and how do you calculate-how deep you can go and stiffeners needed.
Thanks.
Deric from
Lancaster,CA