Marine Applications of Aluminum Alloys: Part Two

Major European-built liners have used aluminum extensively in superstructures and equipment, ranging from 1000 to 2000 tons per ship. The "Oriana", "Canberra", and "France" employed welded construction, using sheet, plate and extrusions of aluminum-magnesium and aluminum-magnesium-manganese alloys. Since appearance is important in this class of ship, the structures are painted; aluminum allows at least 50% longer time until repainting is required.

Commercial Ships

Passenger vessels utilize large quantities of aluminum in superstructures and equipment. The 4.5 million pounds (2000 metric tons) in the "SS United States", built in 1952, resulted in an 8000-ton decrease in displacement. The lighter topside weight permitted a beam reduction, saving hull weight and allowing reduced power capacity, while still providing the high service speeds necessary on modern ocean liners. Riveted 6061-T6 plate was employed for the deckhouse structure, superstructure decks, and bulk- heads. The remainder of the aluminum was utilized in furniture, equipment, ventilation ducts, ladders, stair treads, and railings.

Major European-built liners have used aluminum extensively in superstructures and equipment, ranging from 1000 to 2000 tons per ship. The "Oriana", "Canberra", and "France" employed welded construction, using sheet, plate and extrusions of aluminum-magnesium and aluminum-magnesium-manganese alloys. Since appearance is important in this class of ship, the structures are painted; aluminum allows at least 50% longer time until repainting is required.

Dry cargo ships have been affected by new design trends that emphasize a need for lower topside weight. Heavier cargo handling gear and related machinery, and more narrow, hydrodynamically contoured, high-speed hulls have increased stability problems. Thus, weight saving is required to permit more efficient hull designs. Approximately 100 tons of welded 5086 plate and shapes 0.25 to 0.75 in. thick was used in the midship structures in each of ten ships built recently. Aluminum structures normally weigh only 40% as much as steel structures, using construction details similar to steel practice (welded plate and stiffeners bracketed at the decks). In some instances, additional weight is saved by application of special extrusions.

Bulk carriers normally can take direct advantage of any weight saving by carrying additional cargo. Six Canadian ore carriers utilized 150 to 250 tons aluminum in deckhouse structures, hatch covers, and equipment such as railings and lifeboats. Aluminum booms of riveted 2014-T6, 6061-T6, or 6070-T6 extruded or rolled shapes on self-unloading ore carriers enable operators to achieve economies in converting older ships to update their usefulness. In a typical installation, a 190-ft steel boom on the "J. R. Sensibar" was replaced with a 256-ft aluminum boom, increasing the dockside unloading area available to the ship without having to alter the existing boom support structure.

Passenger ferries use aluminum superstructures to maintain safe stability while carrying more passengers. In 1939, New York City’s Staten Island ferries initiated this application of aluminum with three riveted superstructures, involving a total of 100 tons of 6061-T6 plates and shapes. This resulted in a reduction in fuel consumption of 220 gal per round trip for each ferry and an annual operating cost that was $107,000 less than for a similar ferry with an all-steel superstructure. Latter group of three ships, built in 1964, used 100 tons of welded 5086 plates and extrusions for the topside structure. Smaller ferries in operation off the Pacific Coast and in the St. Lawrence River have hulls of welded 5086-H32 plate, 3/16 or 1/4 in. thick.

Barges. Aluminum barges, carrying various chemical products, have been operated on the American inland waterway system since 1960. A 5200-bbl barge is 97 ft long, 35 ft wide and operates at an 8.5-ft draft. A 9000-bbl barge is 100 ft long, 50 ft wide, and carries 1200 short tons at an 8.5-ft draft. The aluminum barges have 7/16 in. -thick welded 5086-H34 hull plating; the cargo tanks are 5/16-in. 5052-H34 plate. Approximately 120 short tons of aluminum were used in constructing the 100-ft barge, resulting in a weight saving of 150 tons compared to a similar steel barge. This permitted 15% more cargo to be carried, but the corrosion resistance of the aluminum alloys employed in tanks, piping, and hull was the major factor in selection of aluminum for these chemical barges.

These barges have also been used to move such commodities as lubricating oils, fuels, nitrogen fertilizers, petroleum solvents, and glycols. Inspections by the United States Coast Guard, the American Bureau of Shipping, and the owners have shown virtually no corrosion of the aluminum structure or structural damage from handling in river tows.

A larger aluminum barge, 195 by 50 ft, was built in 1963. All-welded 5083 plate 5/16 to 9/16 in. thick and structural shapes were used to fabricate this barge. It carries acetic anhydride, although classed for a range of other chemical products also. The rated capacity is 2264 short tons at an 8.5-ft draft. This capacity is approximately 14% more than a stainless-clad tank barge of the same size; hull weight of the aluminum barge is only 200 short tons, compared to 486 tons for steel construction.

Hydrofoils. These high-speed craft are used for commuter and excursion service. Although their use in Europe has been an economic reality for decades, the availability of competing forms of transportation has restricted application in the US. Early European applications dictated minimum hull weight to utilize practical power sources. This led to the exclusive use of a 6061-type alloy and riveted construction for the hull, cabin, and bulkheads. Craft built recently in the United States have employed welded construction with the 5xxx series alloys.

The 90-ton, 105-ft-long hydrofoil "H.S. Denison" was built in 1961 for the Maritime Administration. A combination of riveting and welding was used in fabricating the hull, cabin, and bulkheads of 5456 sheet, plate, and extrusions. Alloy 7079-T6 forgings formed part of the steel foil structure. Piping systems were of aluminum or plastics, following aircraft practice.

Smaller hydrofoil craft have also been constructed of aluminum. Two 45-ft-long craft of welded 5456 were built, with 10 more planned for similar construction. Welded 5086 sheet, plate, and extrusions were used to build the 34-ft "Albatross", which can carry 24 passengers at speeds up to 40 mph.

Aluminum is not only the accepted material for hydrofoil hull structure, but is used also in small foil systems in the form of alloy 356-T6 castings and 6061-T6 extrusions. However, in large craft, such as the "Denison" and the Navy’s PCB and AGEH (discussed in the next section), high-strength steels are employed for the foil structures. Although many questions remain concerning the relative importance of the various factors in material selection, it has been established that the strength and stiffness provided by the high-yield-strength steels (150,000 to 200,000 psi) are necessary in the large craft.


Naval Vessels

Destroyers. About 6000 tons of aluminum per year, more than any other type of marine application, is used on destroyers. During the 1930’s, extensive application of aluminum in destroyers was developed, and design practices were refined. During World War II, construction reverted to steel because of the shortage of aluminum. Following the war, with growing emphasis on electronic equipment, deckhouse structure weight became a critical factor, and aluminum was reinstated to combat this problem.

Alloy development in the past decade has resulted in an almost standard application of 5456 plate and extrusions in the welded deckhouse structures of destroyers. Quantities now used range from 100 to 350 tons per ship, depending on the type of destroyer. The weight savings in the aluminum deckhouse structure, normally about 40 to 45%, are utilized to maintain sufficient ship stability while employing the narrow hull necessary for high service speeds. Additional equipment installations also are permitted topside.

Over half the aluminum used is in the deckhouse structure, the remainder being employed in a variety of equipment applications. These include lockers, desks, chairs, ’bunks, doors, windows, ladders, gratings, and galley equipment: A wide range of the more corrosion-resistant wrought and cast alloys is found in these items, including 5052, 5086, 6061 and 356.

Aircraft Carriers. Aluminum applications totaled over 1750 tons on the carrier "Enterprise" (CVA-65) completed in 1961. The largest single item (8) was the four deck-edge elevators. The first such elevator platforms, employing welded 6061-T6 members in a tubular-truss structure, had been installed on the carrier "Shangri-La" (CVA-38). Later, alloy 5154-H36 was used for the welded elevators on CVA-61. Alloy and welding developments led to application of alloy 5456 plate and extrusions in the elevators on the "Enterprise". These were designed with an open grillwork structure; the deep girders were fabricated from 3/4 to 2 in. thick. The 52 by 85-ft structures weighted 105 tons each, 35 tons less than similar steel units. Reduced inertia, during operation between the flight and hangar decks, permitted reductions in operating machinery.

The "America" (CVA-66), completed in 1965, employs elevators of stiffened-plate configuration, using high-strength steels; they weighed 115 tons each. A similar design but employing alloy 5456 resulted in aluminum being specified for the elevators on carrier CVA-67, now under construction, at a weight reduction of 15 tons each.

Other uses included the items of equipment mentioned for destroyers, along with radar masts, superstructure, cooling panels in the flight deck, and comparable structures.


June, 2004
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