Gross registered tonnage was the total internal capacity below the uppermost continuous deck (the ‘tonnage’ deck for most ships) plus all permanent enclosed spaces above this deck, less exempted spaces. Examples of exempted spaces were double bottom tanks, if used only for water ballast, and crew ablution rooms above the upper deck.
Net registered tonnage was the gross tonnage less deducted spaces. Examples of deducted spaces included machinery spaces, crew accommodation, chart room and radio room. Note that the expressions ‘upper continuous deck’ and ‘permanent means of closing’ gave the naval architect certain scope. If the shipowner built a ‘shelter’ deck over his uppermost continuous deck and left a small ‘tonnage’ opening he had greatly increased his cargo-carrying capacity but not the ship’s gross tonnage. Such a ship was known as a shelter deck ship. The uppermost continuous deck was the deck from which the freeboard was measured in both cases. Now, the shelter deck ship was not as structurally sound as a vessel of equal carrying capacity, where the top deck was the uppermost continuous deck. So in the interests of safety the pre-1982 British Rules allowed ships to have two gross tonnages and two net tonnages. A ship that had been so measured indicated it by the tonnage mark. If any part of the tonnage mark was submerged, the vessel used the larger gross tonnage. If it was not covered, it used the smaller. This allowed the shipowner to have all the advantages of the shelter deck ship with the safest possible construction. It also allowed him to carry a greater deadweight of cargo so the ship was more versatile.
Dry Bulk Shipping News - Shipbroker Notes