RE: [PSUBS-MAILIST] payload

["kory mosteller" <hardcoremosteller@hotmail.com>]

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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