PANAMA AND SUEZ CANAL CERTIFICATE* 519 provision on a national basis for the effective training of ship's crews in boat work has, so far (1936), been established in the United Kingdom. PANAMA AND SUEZ CANAL CERTIFICATES. Under Deck Tonnage is a measure of the internal space between the top of the ceiling or double bottom in the hold and Fig 4—Steamship in Canal Lock. the under surface of the tonnage deck. The unit of measurement is a ton of 100 cubic feet. Gross Register Tonnage is a measure of the total internal volume of the ship, and is equal to the under deck tonnage plus the tonnage of all enclosed spaces above the tonnage deck.