John! I saw someone else’s sub recently that was using a battery box on a carriage/wheels/track that moves forwards and backwards on the subs centre axis to give trim! an elegant engineering solution to a simple problem and no need for complicated trim tanks/extra plumbing/controls etc Everything is kept internal that way too! KISS = Keep It Simple Stupid… a computer term but applicable to anything really! Stew From: owner-personal_submersibles@psubs.org [mailto:owner-personal_submersibles@psubs.org] On Behalf Of Firebolt As far as the ballast tanks go, which Is the most popular? I am thinking of the keel tank would work for me but then the batteries.... I plan to have some sort of motorized "weight trim" so the tank would have to be split for it to work. john |