What is Shelter Deck?

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.