Don’t kill the messenger. It was Archimedes (someone from ancient Greece) who said it, not me.
@Archimedes of Syracuse wrote:
Any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object.
In the case of sweet water one liter equals one kilo. So, a fully submerged 25 liter barrel has a buoyancy (lift) of 25 kg in sweet water. If it holds a 1 kg object the total buoyancy (lift) is 24 kg. The two barrels I carry have a total volume of 50 liters, well above the 40 liters the rules require. In other words I can store op to 10 kg of goods in them.
In seawater things get better because sea water is heavier. Have you noticed how your boat floats higher in sea water?
Why anyone would want to store tins of beer in a waterproof barrel in the aft buoyancy tank is beyond me. Beer is much better stored in the bilge where it stays cool and where you can grab it while sailing and where the weight is in a much better place for boat balance. On my boat booze not allowed, except for medicinal purposes in which case it should be kept within easy reach, because one never knows…..