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| Note: Shelter Deck and Full Scantling Types. Full scantling was the default type, with maximum hull strength (scantlings) and the entire hull below the top continuous deck (the upper deck) enclosed and watertight. The shelter deck type was developed to exploit a regulation that allowed the exclusion from gross and net tonnage calculations of any space on or above the upper deck that was sheltered but not enclosed, giving shipowners considerable savings on canal tolls, etc. In a three-deck "open" shelter deck ship the top continuous deck (the shelter deck) covered a 'tween deck space that had "tonnage openings" in its internal bulkheads and in its after overhead deck to make it "not enclosed," and the deck below it (the upper deck) became the top of the enclosed and watertight portion of the hull. Lowering the top watertight deck (also called the freeboard deck) one level however increased the required freeboard of the ship by the same amount, reducing its authorized draft and its ability to carry heavy weights. Increasing the freeboard also reduced the need for maximum strength in the ship's scantlings. A variant of the "open" shelter deck type was the "closed shelter deck" or "scantling draft ship", which had the reduced scantlings associated with a shelter deck but in which the tonnage openings in the shelter deck were permanently closed, increasing registered tonnages while also allowing an increase in the authorized draft. For more on three-deck shelter deck ships see the page on EFC Design 1079, for an example with measurements of the difference between open and closed three-deck shelter deck ships see the page on EFC Design 1057-B, and for more on two-deck shelter deck ships see the page on EFC Design 1037. Shelter deck ships were best suited to carry relatively light, bulky cargo for which the maximum possible draft was not required, while full scantling ships were better suited for heavy, dense cargoes. The larger U.S. EFC freighter designs (except for Design 1133) were of the shelter deck type while the smaller ones were of the full scantling type. Note that two large EFC tanker designs also had shelter decks. |