Before explaining the Under deck tonnage let us first understand the meaning of tonnage deck.
The tonnage deck is the upper deck in vessels having one deck and for the vessels having more than one deck, tonnage deck is the second deck from below. Spaces above tonnage decks are ‘tween decks (if present) and superstructures.
The underdeck tonnage is the total volume (measured in tons) of 2.83 m3 of the ship below the tonnage deck to the inside of frames, underside of deck plating and above the inner bottom. This is obtained by detailed calculations, in a manner somewhat similar to that for displacement calculations and varies in detail for individual regulations; tonnage measured according to the tonnage regulations may, therefore, be slightly different from the actual volume of the spaces below the tonnage deck calculated for, say, cargo capacity.
The overdeck tonnage is the volume to the inside of frames and deck plating of the ‘tween decks, poop, bridge, forecastle, deckhouses and erections above the tonnage deck less the exempted spaces. Spaces exempted include dry cargo space (unless in a break in the deck) and certain closed-in spaces associated with machinery, safety equipment, navigation, galleys, washrooms, water ballast and workshops. Refer to the picture below.
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