Hi Dean. This is Frank. Although I'm not an expert ( still building my first one ) I have a few thoughts about your questions.
Ballast tanks make you ''come up'', and adding more capacity puts your hatch higher out of the water. The drawbacks are, the larger the tanks, the more air it takes to fill them, so increasing the size means you must increase available high pressure air or settle for fewer ''tank blows''
An extra scuba tank or two doesn't take up that much room, so I'm going with that.
The shape and location are very important. In the case of a ''K'' boat, you'd probably make them longer to add the extra capacity. Most of the guys put a bottom on the tank to keep the air inside during a roll or pitch.
It's important to keep the tanks as high as possible so your ''center of buoyancy'' stays high, while your ''center of gravity'' stays low.
The shape of the tanks affects the speed and stability of the boat. George planned the shape and location very well. The air exits the tanks at the high point, next to the hull.
The one thing I don't like about the design is that the tanks are welded to the hull, making it difficult to grind and paint in the corners where the two meet.
I have opted for a bolt-on set of tanks so I can remove them, re-paint the hull, and bolt them back on.
I'm using fiberglass fairings to smooth out the shape.
I think metal tanks are safer, in case you hit something. A fiberglass tank could crack or break if you impact hard enough, where a metal tank will probably just dent.
Take a look at my frappr pictures
and you'll see how much larger my tanks are relative to hull size. I hope to get the top hatch out of the water about 28 inches. I built test models with the tanks and stability is good. Steel is lighter in water than in air, so once you have the steel at the surface, it takes a lot of displacement to raise it out of the water. With the ''K'' boats,you only need to get the tower above the surface.
I'm going to need a splash guard to keep waves from washing into the sub when the hatch is open. I'm working on an inflatable design for that. (not done yet, but getting close)
Got any new pictures? Love to see them. Frank D.
I am using the 350's MBTs and I am expanding them, both length and depth. My original thought was to use the
MBT's for base line at complete flood and use VBT hard tanks to fine tune.
I have discovered that hard VBT's are not so easy to get much positive bouyancy. I wanted to get around
200lbs worth, for a passenger etc but it is going to be tough.
Now I am leaning towards making them larger and leaving some air in the MBTs for some adjustment.
The larger air volume has been addressed I am using a 250 cu ft ( I think the biggys anyway) sized compressed air tank
A SCUBA will be inside as an emergency to tap in to the system
Are you using a VBT system and if so how much capacity?
The planned K350 vbt is only about 68ls (my lunch could weigh that much)
Now that the frapper is back I am not. I am in Michigan for about another week and the pics are
home in Minnesota on my puter O well promise to get them out when I return
Basically I have stretched the 350 to 20' and an outer hull about 4' diameter, bow for some surface handling, rudders
and dive planes. Everthing is outside and should be a fairly slippery beast and real quick on its feet.
Battery capacity will be 24 - 85amp deepcycle for a lot of endurance
Dean