Hi Dean and Hugh. The pontoon boat I was contemplating at about 16'
long would be for fair weather and close to shore sub operations. The sub
would merely "tie up" to it.
Your concept of a large pontoon boat sounds more like a floating "dock"
and is a great idea. The tender for the Lake Diver was big enough to act like
a barge and bring the sub "up on deck" which is also a good idea.
I'm not sure how large a boat should be to carry a sub in the open ocean
but I'm guessing at least 80 feet.
The size is really dependant on how heavy the sub is, and where to locate
it on deck so it balances with the boat.
One problem with bringing a small sub up into the middle of a pontoon
boat like you explained would be securing it so the two would travel in tandem
when swells, waves, or wakes start "rockin' the boat". The sub could damage
the pontoons if it was rockin' around between them.
Lake Superior gets pretty rough even on nice days.
I like the concept of floating pontoons next to the sub. It will
take a pretty massive frame to hold it together with an opening in the back
big enough to motor the sub in there. Maybe you could have a rigid frame and
bring the sub "up" into the middle. Or have a removable frame member at the
back that would slide in or drop down and re-connect the pontoons in the back
to re-establish a rigid frame.
Hugh, your sub at 6 tons will take a pretty big boat to haul up the sub.
There are some "floating docks" available and they can get huge. Maybe buying
a used barge and outfitting it for sub operations would be possible.
There was a few for sale in San Fran Bay and some were used for house
boats. The local harbor in Santa Cruz has one they use in channel dredging and
it carries a lot of weight.
Frank D.