Underdeck tonnage
Underdeck tonnage is the measurement, in tons, of the space below a ship's tonnage deck and above its double bottom. In ships with two decks, the tonnage deck is the upper deck; though it is generally the second deck from above in ships with more than two decks. Underdeck tonnage is applied using Simpson's [rules (ship stability)#Simpson's 1st rule|Simpson's first rule of ship stability]. It is measured in gross cubical capacity.