Okay,
two photos.
this is a custom built 1/8" acrylic addition to the original tech rack. designed to allow for the addition of the tank, without all the leftovers for the lead acid batteries. on the top, from rear to front: three servos, rear dive planes, rudder, front dive planes. two 400 size graupner motors with 3:1 planetary gearboxes, motor speed controller, ballast tank. in front of the ballast tank is space where the ballast tank controller and receiver were located. on the bottom, from rear to front: 5 cell nimh battery, ballast tank, and room for 3 more 5 cell nimh batteries. the battery packs were each 3000mah, giving me a total of 6v 12000mah for the battery. speed controller fed the receiver. for the specifics, the way it was setup, the rear of the ballast tank is 280mm from the inner edge of the rear endcap. the plastic tube that you may see some of in the photos for the piston rod to go through ends 65mm from the inner edge of the rear endcap (the 1/2" white "line" running between the motors). in all honesty, i think the ballast tank could be moved a little further forward, based on the amount of foam i had to put in the stern free flooding section. maybe 20mm further forward.
the big white strips you see underneath the ballast tank are lead weights
the brass tube running along side the tank is an 8mm ID tube to feed the ballast tank. it is in two pieces. one is soldered to ballast tank, one is bonded to endcap. the 1"+ piece of shrink tubing links the two together where the ballast tank motor is located.