This is what could happen to a boat at sea with large partially full tanks. Small, full tanks actually help to stabilize a boat in wave action. Barrels were usually made of wood staves and depending on the liquid carried in them came in various standard sizes. Sometimes termed deadweight carrying capacity, is the difference between the light and loaded displacements of a ship or barge. The Deadweight tonnage comprises the cargo, stores, ballast, fresh water, fuel oil, passengers, crew and their effects.
Gross tonnage is a measure of the internal capacity of a ship, tug or barge.
It is a cubic or space measurement of all areas of a vessel with some allowances or deductions for exempt spaces such as living quarters. Net tonnage is a measure of the internal capacity of a ship, tug or barge's cargo space volume only. Lightweight tonnage is the weight of the ship itself. The hull, machinery and all its fittings. Fuel, stores and crew are not included.