Frank, Well, the density doesn't change much with salinity, unless you decide to dive the same sub sometimes in fresh water and sometimes in sea water. The difference is about 3%. So on a 4,500 pound sub, that works out to about 135 pounds! :) Alec ________________________________ From: owner-personal_submersibles@psubs.org on behalf of ShellyDalg@aol.com Sent: Mon 12/4/2006 9:19 PM To: personal_submersibles@psubs.org Subject: Re: [PSUBS-MAILIST] RE: payload Kory. I have always thought trim tanks were to adjust trim, not compensate for varying loads. Water density changes with temperature and salinity, but not very much. It is possible to run into a salty current or water layer and trim can be added or reduced to compensate. Sometimes subs have forward and aft trim tanks to change the angle of the sub while still maintaining neutral buoyancy. Adding lead ballast before a dive, depending on load, is easier than having a separate, large system with associated piping and tanks which requires maintenance, cost, and monitoring. With a few dives, you know how much added weight your sub needs to reach neutral, and then you adjust with weights from there for added passengers, cargo, new equipment, or whatever. Trim tanks need to be at one atmosphere, whether full or empty. Ballast tanks are ambient. If your trim tank is half full and open to the sea, ( like a ballast tank ) the air will compress when you get deeper, and you'll start sinking fast. Large, one atmosphere tanks for trim will require heavy walled tanks, fill valves, drain valves, pressure monitoring, and take up room and add weight because they're thick steel. And you have to flush them out after every dive so they don't corrode. Better to have small tanks for trim and carry some lead. Just my opinion....Frank D. The contents of this e-mail are intended for the named addressee only. It contains information that may be confidential. Unless you are the named addressee or an authorized designee, you may not copy or use it, or disclose it to anyone else. If you received it in error please notify us immediately and then destroy it.
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