The USS Intrepid (CV-11) - Hull Construction
Hull Construction: One of the most noteworthy things about Intrepid's construction is its economical use of material, both in terms of reducing complications and in saving weight. Apart from the curves required in the outer bottom pating to produce the hull form and the camber of the weather decks, practically all structures are kept straight and flat. None of the decks have any sheer and even the torpedo bulkheads, although they follow the line of the hull, are made up in straight sections run as near parallel to the skin plating as possible. In addition the types of steel section employed were kept to a minimum, which obviously assisted the steel manufacturers to maintain adequate supplies. Apart from flat plates only 'I' bar and angle bar (in various sizes) was employed, other sections such as 'T' and '1' ('I' bar with one leg removed) being produced by machining the 'I' bars to suit, thus cutting an 'I' bar down the centre produced two 'T' sections. In addition the design employed a high level of welding - providing a saving in weight and time and improving water-tightness of joints. The outer bottom (or skin) plating except for the extreme bow and stern, the connections between the decks and the skin plating, the connections between the torpedo bulkheads and the 4th deck and the keel connections were of riveted construction but practically all the remainder of the hull structure was welded. (The butts of the outer bottom plating below and beyond the armor belt were also welded.) On a more detailed level some weight was also saved by machining lighting holes in deck beams and vertical girders, etc.
The hull proper (from the keel to the main deck) was constructed principally on the longitudinal system amidships and transversely at the ends, the main strength members being the keel, the main deck plating and the outer bottom plating adjacent to the keel (garboard strake) and the main deck (sheer strake). The keel, longitudinal, stringers, torpedo bulkheads and the main to 4th decks were continuous with the other structural members worked between them. Below the main deck the transverse bulkheads between the holding bulkheads were continuous with, at the ends, the platform decks worked between them. The emphasis on longitudinal strength resulted from the great length of the hull in relation to its depth (15:1) together with the high superstructure load - the entire hangar and flight deck. Transverse strength amidships particularly above the 4th deck relied heavily on the rigidity of the actual side and deck plating as the main deck beams were also longitudinal Transverse supports were few - normally the principal transverse bulkheads with one or two vertical frames between them supporting the edges of a similar number of transverse beams under each deck.
The transverse frames or floors in the ship's bottom and torpedo protection compartments were spaced exactly 4' apart being numbered from 0 at the forward perpendicular to 205 at the after perpendicular, each perpendicular being the junction of the ship's stem/stern with the waterline. (In the British and many other navies the after perpendicular is taken as the centre of the rudder post.) Those in the triple bottom were of three types - water-tight (fitted under water-tight bulkheads and at the boundaries of fuel tanks, etc.), solid non-tight, and open. The latter two types were placed alternately...except where local loading required the substitution of the solid type. Solid floors were also fitted at all stations adjacent to the keel and holding bulkheads for additional support, while the keel was still further supported by docking brackets spaced at half frame intervals )such intermediate positions were given 1/2 frame numbers, i.e. 133 1/2, etc.). At the end of the ships, local stresses required a much higher degree of transverse strength and the frames were closely spaced at 1/2 frame intervals.
The keel - a large 'I' girder built from flat plates with riders welded to top and bottom - and the longitudinals extended from the outer bottom to the third bottom, the inner bottom plates being fitted between them. The keel was water-tight but the longitudinals were non-tight except at the boundaries of the tanks in the inner bottom.
The underwater protection system was designed to withstand a warhead of 500 lbs TNT and although this may not have been adequate to prevent the penetration of the holding bulkhead by many Japanese torpedoes it would, at the very least, have restricted the area of damage. The two outer compartments of the system were liquid loaded with oil fuel (replace by sea water as the fuel was consumed) while the two inner spaces were void. Alternate compartments from outboard were framed in order to stiffen the bulkheads without risk of the impact of a torpedo explosion being transmitted directly to the holding bulkhead via the transverse structure, although this was of course a risk at the water-tight bulkheads which of necessity had to be continuous. The holding bulkheads themselves and the main transverse water-tight bulkheads, were supported by vertical girders and horizontal 'T' stiffeners. A similar arrangement was adopted in the triple bottom as protection against ground mines and other under-the-bottom explosions, but this would have been of little value as defense against such detonations is close to impossible to achieve - although steps can be taken to minimise the effects of internal shock damage.
Armour protection was provided by Class 'B' armor and STS (Special Treatment Steel), both being nickel chrome steel alloys heat-treated to a uniform (homogenous) hardness and toughness. Class 'B' was used for thick sections - the armor belt and bulkheads - and constituted deadweight but The STS, manufactured only for thin sections, was, while retaining ballistic qualities close to those of full armor plate, an excellent structural material. It was therefore possible to save weight by making much of this plating both part of the structural strength of the ship and part of its armor protection. Thus the main deck, constructed of two thicknesses of 1 1/4" STS was both the upper strength member of the hull and its defense against bombs (being capable of keeping out general purpose bombs up to 1000 lbs in weight). It would not of course have been able to stop a heavy armor-piercing bomb but it was considered that the fuse would probably be set off by the flight deck causing the bomb to detonate either before penetrating the main deck or very shortly after, in which case there was still the 4th deck, protected by 1 1/2" STS, to stop the splinters from entering the ship's vitals. However, the ships were still vulnerable to AP bombs, either with long delay fuses or through penetration of the hangar or hull side - it certainly was not a problem in the war against Japan but an Essex may well have suffered badly at the hands of the Luftwaffe whose heavy AP bombs came close to sinking the better protected Illustrious in 1941. STS was also used for combined hull strength and protection on the ship's side where the skin plating varied from 1 1/8" (sheer strake) to 3/4" behind the belt; below this and fore and aft the hull was of standard medium steel. The structural qualities of STA can be judged from the fact that it was also used for structures not related to the protection of the ship, such as the keel.
The remaining principal protection was the 1 1/2" STS 4th deck, which contributed only slightly to hull strength as it was close to the ship's neutral axis, and the 4" Class 'B' belt and bulkheads. This was intended to provide protection against 6" gunfire in case of surface attack - principally a night or low visibility attack by enemy cruisers. Other protection consisted of an armored box around the steering rear as protection against both shells and bombs, 5/8" STS platform decks over the magazines fore and aft as an additional splinter barrier, and STS splinter protection to wiring, ammunition and access trunks, boiler uptakes, armament positions, radio and radar offices, and the principal command positions in the bridge. There was also a 5/8" longitudinal splinter bulkhead between the 4th and main decks on each side (a continuation of the holding bulkhead below) closed by similar transverse bulkheads at the ends of the ditadel. In all the protective system relied substantially on reducing he extent and effect of internal damage, which, combined with a high degree of water-tight subdivision and the US Navy's excellent damage control organization, made it a great deal more comprehensive than would at first seem apparent.
The flight deck was supported by heavy vertical girder foundations, built above either bulkheads or side frames in teh hull structure. These supported deep transverse girders or bents which in turn supported a few deep girders and several smaller 12" 'I' girders laid longitudinally under the flight deck. At the sides the spaces between the foundations were filled in with light plating except in those areas where roller steel curtains - for hangar ventilation - were fitted. The flight deck itself was of thin 0.2" steel plate with 3" thick wood planking laid transversely across it.