Hi Vance. Thanks for the experiences. I always figured most guys use that method of ''riding the bubble'' sometimes, even though conventional wisdom says no. I'm sure all pilots adjust to neutral, as a starting point, and make slight adjustments as they're going. That's why you've got a depth gage. ABS prefers a slightly positive approach, with the motors holding you down. My guess is, not many guys do that. Never having driven one submerged, I can appreciate all the input from experienced guys, as I'm sure all the guys do. I have a question though.
If a ballast tank is blown dry, and the boat is on the surface, there will be pressure in the tank equal to the pressure realized at the lowest point on the tank where it's open to the sea. If the tank is large, and it's bottom opening is say three feet under water, then the internal pressure would equal that depth relative to the top of the tank that is at the surface and therefore at surface atmospheric pressure. Is that right ?
I'll be using sheet metal for the tanks, bolted to the frame, so they'll be removable for maintenance tasks. At about 100 gallons each,( maybe a little more ) I'll need baffles/stiffeners to prevent minor collisions from crushing/denting them. The tanks are sort of triangular so the bottom, although small, will be about 30 inches below the surface. Here's a rough sketch showing the general shape of the rear tank.
Seems like I have to add pressure into the tank enough to force the water out the bottom, and keep it from getting back inside, so there is a small amount of pressure inside when surfaced. Of course, there's always going to be SOME water in the tank. My thought was with a triangular shape, viewed both from the end and the side ( kind of like an upside down pyramid ) I should be able to get a pretty steep pitch or roll without losing the bubble. With the base of the pyramid at the top, it gives me a good (high) center of buoyancy. I calculate the weight above water when surfaced at 740 lbs. ( exposed hull, hatch, top fairings, and minor stuff ) and with 200 gallons or so of ballast displacement, will get the hatch far enough out of the water. I'm finalizing a design for an inflatable splash shield around the hatch with the shape to deflect any small wash away from the open hole. Got any thoughts on this ? Frank D.