Long geeky answer
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north sea water salinity 35g/litre
caspian sea salinity 12.5(ish) g/Litre
Say roughly, density of caspian seawater is therefore 1012g/litre instead of say 1035g/Litre.
You need that buoyancy law – archimedes. Upthrust force is due to the "weight" of the displaced fluid.
w= m*g
w=v* g * p (mass of water = volume * density)
Say for a super spartan (example workclass)- volume of the buoyancy is roughly 2.5m*1.5m*0.6 (ok – ignoring routed-room for thruster cut outs, and also the equipment loadout ontop of that!)= 2.625m^3 – this would be equivalent to 26kN of upthrust roughly (ABSOLUTE MAXIMUM-BIT OVERESTIMATED!!).
Balanced by weight of approx 22,500N – so in seawater the rov should roughly be neutral. Hmm.
The difference in seawaters would be equivalent to a loss of upthrust of the order of (1.035-1.013)*2.625*1013*10=574N (57kg of push equivalent!)
If using syntactic foam – advantic, density= 0.35g/cm3 = 350kg/m^3, hence 1m^3 of syntactic gives aroun 6630N of upthrust.
then you need a volume of around 0.088m^3 to get the desired upthrust of 574N.
Say 2.5cm thick, 1.5m wide x 2.5m long
Short answer
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Stick another slab of buoyancy on it and trim your verts to apply downwards force!!!
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