It doesn't matter where the hard tank is, you are changing weight without changing displacement. Vance
wow- it seems that there is a lot of confusion about VBTs and payload. here's my stab at the whole thing.
I think what it comes down to is whether your hard VBT is internal or external. if it's internal, such as with a kitterage design, then you are adding or subtracting water weight to achieve neutral boyancy, without really changing the volume of the passenger compartment or the overall external volume of the sub. External hard VBTs would be essentially changing the overall volume of the sub in the same manner, adding or subtracting water from sealed containers. If your sub is a 6' tube, 4 ' diameter, with hemisphere endcaps, your looking at 109 cu. ft., or about 6758lbs f/w. An internal VBT of simular design, 1' long, and 1' in diameter with hemisphere endcaps is 1.3 cu. ft. Filling this adds or subtracts 80lbs of water wieght to the sub, withought ever changing the subs internal or external volume, only changing its contents (air or water).
Taking that same sub and externally mounting the hard VBT would work like so- the VBT being full of air would make the subs total displacement 6838, or 110.3 cu. ft. Full of water, it's like it's not there at all.
Earlier, i had suggested using larger external hard VBTs (say about 5 cu. ft or 300lbs f/w) to compensate for payload changes or the difference between f/w and s/w. (2lbs per cu. ft.) If you weighted your sub to be nuetrally boyant with you as the lone occupant and your VBTs 4/5ths full (4 cu. ft. or 248lbs f/w) this would leave you with 1cu. ft. or 80lbs f/w to use as normal trim during your dive. with the addition of a passenger, you would simply empty thier weight worth of trim (up to 248lbs).
one advantage i can see of this system over lead is that there will be water at the dive sight and you won't have to add lead to your trailer weight or in the back seat of the pickup.
I hope this helps someone else, i really just wanted to write it all out to get the ideas straight in my own head.. but once it's on paper, why not share so it may help someone else right?
kory in so. cal
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