Marine Applications of Aluminum Alloys: Part One

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Introduction: The Advantages of Aluminum in Marine Applications

Aluminum is extensively 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 primary incentive for employing aluminum is its significant weight reduction compared to steel. With weldable aluminum alloys having strengths approaching or comparable to mild steel, equal-strength structures can be designed with weight savings of 55 to 67%. However, due to aluminum's lower modulus of elasticity and to conform to normal deflection limitations, the actual weight reduction is typically somewhat lower, but still substantial.

The principal advantages of weight saving in many marine vessels include increased payload, expanded capacity for equipment, and decreased power requirements. In other vessel types, the chief benefit is improved weight distribution, enhancing stability and facilitating efficient hull design. Although aluminum typically results in higher initial costs, these premiums are justified over the vessel's lifetime through benefits of reduced weight and lower maintenance expenses.

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

Aluminum Alloy Selection for Marine Environments

The 5xxx series alloys dominate commercial marine applications, offering weld yield strengths of 100 to 200 MPa. These aluminum-magnesium alloys maintain good weld ductility without requiring post-weld heat treatment and can be fabricated with standard shipyard techniques and equipment. Weldable aluminum-magnesium-zinc alloys are also gaining attention in this field.

Corrosion resistance is another major factor influencing the selection of 5xxx series alloys for marine applications. Tensile strength reductions in 10-year sea-water corrosion tests of 1.62mm (0.064-in.)-thick bare sheet specimens show only 2 to 5% decrease. The 6xxx series alloys, widely used for pleasure boats, demonstrate a 5 to 7% decrease in similar tests.

Alclad aluminum alloys are rarely required in marine vessel construction. They are used in specific applications such as piping, where maximum protection against excessive pitting is needed. Alclad 2xxx and 7xxx series alloys are selected when tensile strengths of 70,000 to 80,000 psi (482.6-551.2 MPa) are required—considerably higher than what's available in the 5xxx series. These high-strength alloys are employed where welding isn't required and where their superior strengths offer advantages. Due to their lower resistance to seawater corrosion, protective measures such as cladding, painting, or cathodic protection must be implemented for satisfactory service life in marine environments.

Aluminum in Pleasure Boats

The use of small boats has grown rapidly since 1945, with early aluminum applications primarily in canoes and small fishing boats, where aluminum is now the dominant material.

Small Craft Construction and Advantages

Runabouts and small outboard cruisers up to 20 ft (6 m) long are generally constructed of either aluminum or plastic. While styling has recently become emphasized in aluminum boats, the material offers several practical advantages:

• Reduced weight lowers construction costs, requires less power, and enhances portability
• Improved damage resistance, with accidents that would cause fractures in wood or plastic often resulting in merely repairable dents in aluminum
• Less damage compared to steel boats of similar thickness, as aluminum's lower modulus of elasticity provides greater energy absorption capacity
• Lower maintenance requirements, particularly valuable for rental services and private craft

The practical minimum thickness for repair welding aluminum parts is considered to be 0.090 inches. While lighter gauges common in small-boat construction can be welded, perforated sections are typically repaired by riveting sheet patches in place, with dents hammered smooth similar to automobile body repair techniques.

Most aluminum boats are sold painted for either fresh-water or salt-water service. In fresh water, aluminum boats commonly remain unpainted for 10 years or more, whereas wood boats require annual caulking and painting. For salt-water service, typical maintenance includes annual touch-ups with complete repainting every three to four years.

Materials Selection for Small Craft

Small boats are fabricated from various aluminum sheet alloys, primarily from the 5xxx and 6xxx series, which offer an optimal combination of strength, cost, fabrication ease, and corrosion resistance. For hulls requiring no stretch forming, 5052-H32, 5052-H34, or 6061-T6 is typically used. Where stretch forming is employed, 6061-T4 sheet (which may be subsequently artificially aged to the T6 temper) is utilized. Structural and decorative sections like keels, chines, gunwales, and spray rails typically use 6061 or 6063 extrusions.

Recommended rivets include 2117, 6053, or 6061 alloys. Generally, rivets should have similar mechanical properties to the sheets being joined, neither much harder nor appreciably softer.

Larger Pleasure Craft Construction

Larger inboard boats ranging from 20 to 125 ft (6 to 38 m) in length also benefit from aluminum construction. These vessels typically employ welded construction for hulls, interior structures, and cabins. The most popular hull alloy is 5086-H32, in thicknesses ranging from 0.45 to 12.7 mm. Bulkheads, fuel tanks, and cabins usually use the same alloy as the hull, though 5052 or 6061 are sometimes utilized. Structural members, whether special extrusions or standard structural shapes, are typically 6061-T6 or 5086-H112 in all-welded construction.

Sailing Craft

Sailing vessels follow patterns similar to power craft, with smaller boats using riveted construction of 5052 or 6061 alloy, and larger custom yachts employing all-welded construction in 5086. The light weight of aluminum hulls allows designers considerable flexibility in balancing sail area and ballast-displacement ratio. A comparative study of 30-ft Naval Academy yawls showed that plastic hulls were 10% heavier and wood hulls 37% heavier than aluminum. The standard plastic yawls have a ballast-displacement ratio of 0.433, compared to 0.390 for older wood designs, while aluminum construction would permit a ratio of approximately 0.47.

Commercial Small Craft Applications

Personnel and Work Boats

The expansion of the offshore oil industry since World War II has accelerated personnel and work boat construction. Initially dominated by steel, aluminum didn't enter this sector until the mid-1950s when the first aluminum personnel boat entered service. This all-welded 6061-T6 boat quickly demonstrated the advantages of reduced hull weight, achieving higher speeds with the same horsepower. As time saved in transporting personnel to offshore rigs offers significant wage savings, the industry rapidly adopted aluminum craft.

Crew boats typically feature hard-chine, planing-hull designs with developable surfaces, resulting in efficient hulls that are economical to fabricate. Builders benefit from the lighter material weights during construction, requiring fewer workers and less hoisting equipment. One personnel boat builder has demonstrated that aluminum hulls require 33% less fabrication time compared to steel.

Most aluminum personnel boats are fabricated using 5456-H321 sheet and plate (0.188 to 0.375 inches thick) and 5456-H111 or 6061-T6 extruded shapes. Alloy 5086 is also widely used for hull plating. Cabins typically use 5052 sheet (0.125 to 0.25 inches thick) and 6061 extrusions.

Fishing Vessels

The weight-saving and corrosion-resistance benefits of aluminum have proven to exceed initial investment premiums in fishing vessels. Following brief service testing in 1957 of two 36-ft welded aluminum purse seine boats built with 0.25-inch-thick 5052-H32 plate and 6061-T6 shapes, entire fleets converted to aluminum. Initially valued for weight savings that allowed additional net-handling gear while maintaining stability, four years of service in rugged conditions convinced owners that aluminum boats also offered lower maintenance costs.

A 57-ft Alaskan fishing boat constructed with 0.25-inch-thick 5086-H32 plate achieved a loaded speed of 18 knots—more than double that of conventional boats. This increased speed translated to $5,000-$10,000 in additional annual earnings by allowing more time at fishing grounds. The unpainted craft also saved an estimated $2,000 annually in maintenance costs, quickly justifying the 15% initial cost premium.

Similar economic advantages were reported for 70 gill-netters operating in the Pacific Northwest with welded 5086-H32 0.25-inch hull plating. These 36-ft craft earned 30% more in normal seasons than the standard wood-hull gill-netters they replaced.

Equipment and Specialized Applications

Aluminum is frequently used for equipment aboard fishing vessels. Aluminum fishrooms, common in European vessels, are beginning to appear in U.S. vessels. Extruded or roll-formed aluminum hold sections in 6061-T6 or 6063-T6 create fishroom systems that are non-absorptive, sanitary, and easily reconfigurable by crews. Fish spoilage is reduced while carrying capacity increases, as aluminum sections weigh less than one-third of wet-wood fish or pen boards (portable dividers for compartmentalizing holds).

Refrigerated fish tanks using unprotected 5052 or 6061 sheet and plate (5.0 to 1.5 mm thick) have proven more sanitary and less expensive to maintain than coated or treated steel tanks. With brine as the typical coolant, steel tanks require organic or metallic coatings to achieve useful service life, with coating maintenance representing an ongoing expense.

Government survey boats with aluminum hulls typically carry more surveying equipment than conventional steel boats, though some utilize the weight savings to expand shallow-draft operations. Both state and federal agencies operate these vessels, which are fabricated by personnel boat builders using similar alloys and construction techniques.