Marine Applications of Aluminum Alloys: Part One

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Aluminum is used in hulls, deckhouses, and hatch covers of commercial ships, as well as in equipment items, such as ladders, railings, gratings, windows, and doors.
The major incentive for employing aluminum is its weight saving compared to steel. Because it is common practice to use weldable aluminum alloys having strengths approaching or comparable to mild steel, equal-strength structures can be designed to a weight saving of 55 to 67%. However, to compensate for the lower modulus of elasticity of aluminum and to conform to normal deflection limitations, a somewhat lower, but substantial, reduction in weight is usually obtained.

Aluminum is used in hulls, deckhouses, and hatch covers of commercial ships, as well as in equipment items, such as ladders, railings, gratings, windows, and doors. The major incentive for employing aluminum is its weight saving compared to steel. Because it is common practice to use weldable aluminum alloys having strengths approaching or comparable to mild steel, equal-strength structures can be designed to a weight saving of 55 to 67%. However, to compensate for the lower modulus of elasticity of aluminum and to conform to normal deflection limitations, a somewhat lower, but substantial, reduction in weight is usually obtained.

The principal advantages of weight saving in many types of marine vessels are to increase payload, to expand capacity for equipment, and to decrease the power required. With other types of vessels, the chief benefit is to permit better distribution of the weight, improving stability and facilitating efficient hull design. The use of aluminum normally results in initial cost premiums that are justified over the life of the application by the benefits of lightweight and low maintenance cost.

The weight saving achieved depends on the approach to design, which varies with different applications. Where known or rule loadings exist for specific structures, normal design principles are applied along with consideration of the mechanical properties of specific alloys.

Alloys. The 5xxx series alloys used for the majority of commercial marine applications have weld yield strengths of 100 to 200 MPa. These aluminum-magnesium alloys retain good weld ductility without postweld heat treatment, and they can be fabricated with normal shipyard techniques and equipment. The weldable aluminum-magnesium-zinc alloys are also receiving attention in this field. The corrosion resistance of the 5xxx series alloys is another major factor in the selection of aluminum for marine applications. Tensile strength reductions in 10-year sea-water corrosion tests of 1.62mm (0.064-in.)-thick bare sheet specimens are only 2 to 5%. The 6xxx series alloys, widely used for pleasure boats, show a 5 to 7% decrease in similar tests.

Alclad aluminum alloys are seldom required in construction of marine vessels. They are used, however, in a few applications, such as piping, for maximum assurance against excessive depth of pitting. Also, alclad 2xxx and 7xxx series alloys are selected where tensile strengths of 70,000 to 80,000 psi (482.6-551.2 MPa) are required, considerably higher than now available in the 5xxx series alloys.

The high-strength alloys are employed where welding is not required, and where their higher strengths can be used to advantage. Because of their lower resistance to corrosion by sea water, protective measures such as cladding, painting, or cathodic protection must be used for satisfactory life in marine service.

Pleasure Boats

The use of small boats has expanded rapidly since 1945. Early applications of aluminum were mainly in canoes and small fishing boats, in which aluminum is now the dominant material.

Small Craft. Runabouts and small outboard cruisers up to 20 ft (6 m) long generally are constructed either of aluminum or plastic. Styling often is more important than engineering superiority in these consumer products. However, only recently have builders emphasized styling in aluminum boats.

Light weight is advantageous, in that it reduces construction costs, allows a boat to be driven with less power, and provides portability. Furthermore, many accidents that would have caused fractures and extensive damage in wood or plastic boats result only in repairable dents with aluminum. Also, aluminum boats are damaged less than steel boats in service experience to date (even though aluminum plating and steel plating are usually of the same thickness), because the lower modulus of elasticity of aluminum results in a larger capacity to absorb energy. For both aluminum sheet and steel sheet, thickness is selected mostly on the basis of weldability, for which the requirements are the same for the two metals.

The practical minimum thickness of aluminum parts for repair welding is considered to be 0.090 in. Although lighter gages (common in small-boat construction) can be welded, usually they are repaired at perforated locations by riveting sheet patches in place. Dents are hammered smooth, as in automobile body repair.

Low maintenance reduces the cost of operating a rental boat service, and it is also an advantage with private pleasure craft, which are carefully maintained for appearance. Most aluminum boats are sold painted for fresh-water or salt-water service. For operation in fresh water, aluminum boats are commonly left unpainted for 10 years or more, whereas wood boats require annual caulking and painting. For salt-water service, the typical practice with aluminum is an annual touch-up plus repainting every three or four years.

Aluminum is used for boat trim and accessories, regardless of hull material. Anodized bright trim, either as extrusions or roll-formed sheet products, is widely used in boats of all types for rub rails, dash panels, and other parts.

Small boats are fabricated from a wide range of aluminum sheet alloys, mainly in the 5xxx and 6xxx series. These have an optimum combination of strength, cost, ease of fabrication, and corrosion resistance. Generally, 5052-H32, 5052-H34, or 6061-T6 is used for small hulls that need no stretch forming. Where stretch forming is employed, 6061-T4 sheet, which may be subsequently artificially aged to the T6 temper, is utilized. Extrusions of 6061 or 6063 are used for structural and decorative sections, such as keels, chines, gunwales, and spray rails.

Rivets of 2117, 6053, or 6061 are recommended. Generally, a rivet should be neither much harder nor appreciably softer than the sheet to be joined, and it should have similar mechanical properties.

Larger inboard boats, 20 to 125 ft (6 to 38 m) long, are fabricated of aluminum alloys for reasons similar to those for small craft. Normally, these boats employ welded construction for hull, interior structure, and cabins, The most popular alloy for hulls is 5086-H32, in thicknesses of 0.45 to 12.7 mm. Bulkheads, fuel tanks, and cabins are usually of the same alloy as the hull, although 5052 or 6061 can be utilized. Structural members, either in special extrusions or standard structural shapes, can be of 6061-T6 or 5086-H112 in all-welded construction.

Sailing craft follow a pattern similar to that for power craft; the smaller boats using riveted construction of 5052 or 6061 alloy, and larger custom yachts using all-welded construction in 5086. The light weight of aluminum hulls in sailing craft allows the designer wide latitude in providing balance between sail area and ballast-displacement ratio. A study of hull weights for wood, glass-reinforced plastic, and aluminum in a 30-ft Naval Academy yawl showed that plastic was 10% and wood 37% heavier than aluminum. The currently standard plastic yawls have a ballast-displacement ratio of 0.433, compared to 0.390 for the older wood design. Aluminum construction would permit a ratio of approximately 0.47.

Commercial Small Craft

Personnel and work boat construction has accelerated with the expansion of the offshore oil industry since World War II. Initially, steel was established as the standard construction material for these craft, and it was not until the middle 1950’s that the first aluminum personnel boat went into service. This all-welded 6061-T6 boat quickly demonstrated the advantages of lighter hull weight, resulting in higher speed for the same horsepower. As time saved in transportation of personnel to the rigs offers significant wage savings, the industry rapidly adopted these craft.

Crew boats are normally of hard-chine, planing-hull type, using developable surfaces in the hull form. This results in an efficient hull that is economical to fabricate. The builder also benefits from the lighter weight of the material being lifted into place, since fewer workers and pieces of hoisting equipment are required. One builder of personnel boats has shown that a 50-ft aluminum hull requires a fabrication time 33% less that for steel.

The majority of aluminum personnel boats are fabricated of 5456-H321 sheet and plate 0.188 to 0.375 in. thick, and 5456-Hlll or 6061-T6 extruded shapes. Alloy 5086 is also widely used for hull plating. Cabins are normally of 5052 sheet 0.125 to 0.25 in. thick, and 6061 extrusions.

Fishing Vessels. In these craft, the weight-saving and corrosion-resistance economies of aluminum have proven to exceed the initial investment premium. Following a short service test in 1957 of two 36-ft welded aluminum purse seine boats built of 0.25-in.-thick 5052-H32 plate and 6061-T6 shapes, an entire fleet was changed to aluminum. The original advantage attributed to aluminum was the weight saving, which was used to compensate for additional net-handling gear while retaining adequate stability. After four years of abuse of the boats in this rugged service, the owners were convinced that aluminum boats also had lower maintenance costs.

A 57-ft Alaskan fishing boat constructed of 0.25-in.-thick 5086-H32 plate attained a loaded speed of 18 knots, more than double that of conventional boats. This resulted in extra $5000 to $10,000 earnings per year, since more time could be spent at the fishing grounds. The craft is unpainted; maintenance savings are estimated at $2000 per year. The increased earnings and the maintenance savings quickly justified the 15% initial cost premium.

Similar savings were reported for 70 gill-netters with welded 5086-H32 0.25-in. hull plating, operating in the Pacific Northwest. These 36-ft craft earn 30% more in a normal season than the standard wood-hull gill-netters they replaced.

Equipment aboard fishing vessels is often aluminum. The aluminum fishroom, common in Europe, is used in some vessels in the United States. Extruded or roll-formed aluminum hold sections in 6061-T6 or 6063-T6 result in fishroom systems that are nonabsorptive, sanitary, and easily rearranged by the crew. Fish spoilage is reduced and more fish can be carried, as the aluminum sections are less than one third the weight of wet-wood fish or pen boards (portable boards for dividing the hold into small compartments).

Refrigerated fish tanks of unprotected 5052 or 6061 sheet and plate 5.0 to1.5 mm thick have proven more sanitary and less expensive to maintain than coated or treated steel tanks. With brine as the normal coolant, steel tanks require the protection of organic or metallic coatings to achieve useful service life; in addition, coating maintenance is a continuous problem.

Government survey boats with aluminum hulls normally carry more surveying equipment than the conventional steel boats, although some use the weight saving to expand shallow-draft operations. Both state and federal government agencies operate these aluminum boats. The survey boats are fabricated by the builders of personnel boats, and they are made with similar alloys and construction practice.

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