...And now, as Monty Python say, for something completely different...Put on the comic book or sci fi writer hat. Make the double hulled sub, but FREEZE the water between the hulls. Throw away both the inner and outer hulls, otherwise known as molds. Now dress as a commando, approach an enemy shore on a suicide mission, and conceal your sub after landing by letting it melt. Or carry it with you and drink it. Hey, you wouldn't even need viewports.:)Alec