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The K-250 submarine is usually configured to be a one person personal submarine that is rated to go down to 250 fsw. The sub was designed by retired US Navy WWII submarine captain George Kittredge. It's a basic solid submarine that is usually privately owned.
Kittredge used to build these subs during the 70's and 80's, and there are still a number of them still in use today. They generally weigh 2200 pounds when fully outfitted. Their hull is a quarter of an inch thick, and usually made out of A-36 mild steel, or ASME-516 Grade 70 high carbon steel. They have twin keels, that also act as skids so the sub doesn't need to be placed in a cradle. The K-250 has a 150 pound drop weight, that can be released to make the sub float to the surface if all the other systems have failed. The drop weight is made up of a metal tray and a number of lead bricks. If released, having the lead broken up into smaller bricks makes them easier to recover by a SCUBA diver if they are not too deep.
The plans call for the sub to have three 12 volt car batteries, and two SCUBA tanks, used primarily to blow the water out of the front and rear soft ballast tanks. These external soft ballast tanks are usually made out of a fiberglass composite or steel. Some K-250's have a single action grabber arm in the front.