Aluminum – Material Difference

Author:
Publish date:

Aluminum and steel are tougher and more durable than fiberglass. You might also be surprised at how they compare in terms of price.

Aluminum and steel are tougher and more durable than fiberglass. You might also be surprised at how they compare in terms of price.

As I write this, I’m sitting aboard my 53-foot 1974 fiberglass motorsailer. She’s a very tough, very good boat, and I love her. But I’ve seen fiberglass boats on the reef. I’ve been under water with them and seen their guts strewn about the sand and coral: clothing, bedding, plumbing, computers, pots and pans, tools. It’s a sight you don’t forget.

Read the other story in this package: Aluminum - EagleCraft 

I once saw a steel boat sail onto a rocky ocean reef in a 40-knot nor’easter. After four days of pounding and grinding by huge ocean seas, the storm abated and she was floated off with very little damage.

 

 For decades, an old grounded and abandoned steel freighter served as a landmark when closing in on Egg Island in the northern Bahamas. She only broke up after all those years of neglect because of a direct hit by a major hurricane.

I like tough steel, but I’ve also watched the heartbreak of rust eating away the boat, dreams and income of steel-boat owners.

After years of wood, fiberglass and inflatable skiffs, I decided that I wanted a dinghy I could beat the heck out of, dive from, throw lobsters and conch into, run up on the rocks, lift with manual davits, carry loads of fuel and supplies in and out of breaking inlets, and that would also get up on plane. I helped design and build one … and she’s aluminum. She’s still my pride after 17 years of very rough rides. But I’ve seen million-dollar aluminum yachts with paint and faring popping off. I’ve seen riveted aluminum skiffs mercilessly leaking, and I’ve seen small geysers in aluminum boats where electrically induced corrosion had eaten through.

Like finding the perfect boat, deciding on the perfect hull material can present a dilemma. Disciples of steel, aluminum and fiberglass will debate forever — all with excellent points — the merits of each material. There’s only one inescapable, irrefutable conclusion: It depends.

It depends on the type of boat you want, how you want to use it, and how it is going to be built. It even depends, in some cases, on the size of the boat. All of these factors and more are important variables when choosing a hull material. I’ll take a look at some of the issues and options here, and hopefully this will help you when you’re thinking about your next boat.

Fiberglass

Fiberglass long has been the material of choice for boats of many sizes and uses, and I’ll devote a bit less space to it here than steel and aluminum for that reason, and not because it isn’t an excellent material. But often overlooked is the fact that the term “fiberglass” includes many significant variables as to materials and construction. When you see a fiberglass boat, you don’t necessarily know what you’re looking at.

There are different types of fiberglass materials — for example, woven roving, mat, chop and unidirectional fibers — and different types of resin with various characteristics. To further complicate things, two apparently identical boats using the same types of glass and resin can turn out very differently because the people building one may have inadequately wetted some of the glass with resin or infused too much. Or these two boats could turn out differently because the builders didn’t use the same layup method and schedule.

For example, mat may have been used in areas that needed more woven roving for strength. Or the ambient curing temperature for one boat may have been more conducive to proper curing than for the other, which therefore might be too brittle or porous. Any of these differences could make, what appear to be identical fiberglass boats, quite different in their soundness.

When you add to the brew of possibilities some of the more exotic materials, such as Kevlar and carbon fiber, the subject becomes even more complex. And to further complicate matters sometimes there is coring, of which there are many types and many methods of construction.

But it’s not as confusing as it seems. Current manufacturing techniques are providing much more consistency to glass hulls than in the past. Years ago, builders weren’t sure how to create the strength needed for the usage and often guessed, laying up fiberglass in great thickness and “overbuilding.” But it’s also true that less was known years ago about other issues, such as the best resins to use and the best curing conditions to establish.

With many production hulls, the sheets of fiberglass now are cut robotically according to computer design. They then are numbered or placed in bins, and workers know exactly which piece of glass is supposed to go where, and in what direction, as they lay up the hull. Resin is metered and delivered in computer-generated proportions, sometimes using special techniques to facilitate uniform impregnation, such as the creation of a vacuum to draw resin in or around all the coring material.

Using these more precise technologies and available data as to the strengths of materials and layups, and making informed assumptions as to how boats will be used, builders have been able to construct vessels “strong enough” for their intended usage, but not stronger than needed and, therefore, not more expensive. This concept has allowed many to have boats that they wouldn’t otherwise have enjoyed. You can pay for a boat that’s built to circumnavigate if that’s what you want to do, but you can also pay less for a boat built to take you fishing on a lake.

Some — including me — worry about trying to outguess Mother Nature. Even though the typical user is going to have his or her boat at a dock most of the time and only go out locally on weekends, there’s nothing reliably typical about what weather and water can dish up. My Gulfstar liveaboard/cruising motorsailer is 32 years old. I’m sitting in it, and my feet are dry as I write this. They weren’t so smug about outguessing Mother Nature when she was built, so she’s vastly overbuilt compared with many boats today. I like this.

My four displacement-hull cruising boats have been fiberglass for two reasons. First, most boats their size (27, 41, 47 and 53 feet) in the past were production boats and, thus, were more affordable. Fiberglass is well-suited to production boatbuilding. Once the mold and other tooling are built, that high expense can be amortized over many boats. With fiberglass some of the processes desired in metal boats can be skipped. When the boat is popped from the mold, the exterior coating (gelcoat) is a done deal. There is no faring or painting. Also, once construction formulas are established, the labor involved can be very repetitive and, as a result, less costly.

The other reason my displacement cruisers have been glass is that fiberglass is relatively easy to maintain. Fiberglass boats do suffer various types of deterioration, such as possible water absorption and blisters, but not rust. Aside from that, if you don’t maintain a fiberglass boat well, it simply won’t look as good.

But one-off fiberglass boats present a different picture. “We’ve found consistently that a one-off boat that is fiberglass costs about 15 percent on average more than aluminum or wood/epoxy,” says Dave Gerr director of Westlawn Institute of Marine Technology. Molds, tooling and programs aren’t amortized over a huge number of boats, and that alone adds substantially to the cost.

That perhaps is the reason why many have looked to aluminum and steel for non-production boats. However, these materials now are competing more and more with even production fiberglass boats. I won’t make many comparisons between aluminum and steel versus fiberglass boats because it would be difficult to compare apples with apples given the multitude of construction variables in the latter. But there’s much to be said about steel and aluminum.

Steel

The following paragraph from Eric Sorensen’s book “Sorensen’s Guide to Powerboats,” (International Marine), on some of the properties and characteristics of steel explains why many people choose that material for larger displacement boats.

“Steel weighs 490 pounds per cubic foot … and melts at 2,796 F. It has a yield strength (the force at which it starts to permanently deform or stretch) of 36,000 psi and an ultimate strength (the force required to make it separate or fracture) of 60,000 psi, so it has a relatively large ‘plastic’ region of 24,000 psi, or 40 percent. In other words, steel is quite ductile. Its large ‘plastic’ range means that it dents easily in relation to its ultimate strength, but this ability to stretch also allows it to absorb more impact energy before rupturing. Steel is relatively immune to fatigue resulting from the millions of stress cycles a vessel is exposed to over time. Steel welds are almost as strong as the members they join together. … Steel is very hard and, therefore, highly resistant to abrasion.”

I’d like a steel cruising boat because I live and work aboard and I run thousands of miles a year in many different environments, including the ocean. I can’t forget the scenes I described above. I want something that will survive if I hit a reef or a container bobbing on the surface. At sea at night, I slow down my heavy fiberglass boat to give it that extra chance should we hit something. I’ve seen what happens even to thick fiberglass hulls when they do. (I’d worry less if my boat had watertight compartments, as some of the better fiberglass builders are doing today.) Now a steel boat might be more feasible, even for me.

As noted earlier, fiberglass lends itself to production boatbuilding, while in the past most steel and aluminum boats were essentially one-offs. But with modern computer design and computer-cut plating, steel and aluminum boats can be built more repetitively than before. Therefore, they compare more favorably in terms of cost with boats built of fiberglass.

In the past I’ve watched welders laboriously cutting plates, making them fit, tacking them in place, welding them, and then spending many hours sandblasting to white metal, cleaning and coating to prevent rust. Whatever the coating, it’s critical that the hull surface be pristine when it’s applied and that the coating permeate every nook and cranny of the hull. That’s a tall order in terms of labor, but read on. Even the coating process for steel boats can be done more efficiently. (I’ll discuss some points that are common to both steel and aluminum in this section to avoid duplication, and facilitate comparison of the two.)

Teresa Flowers, vice president of Custom Steel Boats (www.customsteelboats.com ) in Merritt, N.C., says the steel plates for its boats are computer cut, sandblasted and primed at the factory before shipping. There, they are coated with a product called Nippe Ceramo, which she says is “weldable.” She adds that the welding “burn back” is considerably less than with some coatings, so the coating is easier to protect. She says the company still must spend time on rust-protective measures, but the precoating and cutting saves many hours. (There are various rust-protective measures that can be used.)

Fair Metal Boats (www.fairmetalboats.com ) of Ann Arbor, Mich., uses precut plates almost in kit form, according to company president Paul Mooney. Mooney says Fair Metal’s aluminum plates are cut by waterjet and its steel plates by plasma. He says waterjet blasting shoots water that includes an abrasive material at 60,000 psi. It’s cleaner and more accurate than plasma cutting and is better for aluminum, says Mooney, because you don’t end up with a heat-affected zone that you then have to clean before welding.

“It costs more,” he says, “but it’s worth it in the long run.”

Mooney says the price of a Fair Metal steel boat includes, after sandblasting to white metal, a 3- to 4-mil inorganic zinc primer as a first coat, which provides cathodic protection similar to hot zinc spraying. Three or more coats of epoxy “high build” then are applied to a thickness of around 20 mils, followed by Awlgrip above the waterline.

Mooney says that with Fair Metal’s standard designs, the costs are around 10 to 15 percent more than a comparable midrange production fiberglass boat, and that the price is about the same for steel and aluminum.

Flowers says if an owner sticks with a plan that Custom Steel Boats previously has built and makes no changes, the cost is competitive with similar production fiberglass boats.

(Westlawn’s Gerr says that, as a rule of thumb, a one-off steel boat is normally about 5 percent less expensive for the same basic one-off aluminum boat.) Mooney notes that metal building is still more labor intensive than production fiberglass, but that most metal-boat builders are relatively small compared to production fiberglass builders, and thus have lower overhead.

You can further reduce the cost of a steel or aluminum boat by curbing your desire for those classic curved lines that some feel are so important. Sorensen notes in his book that both aluminum and steel come in flat plates. (Some small aluminum skiffs are built of thin riveted sheets, but that’s not what I’m discussing here.) The plates can be “bent or twisted into simple or developable curves [around a single axis] with relative ease,” he writes. “Try to bend a plate into a compound curve [around two axes], however, and you will find the going much harder.”

If you can accept a metal hull with relatively few curves and those around a single axis, the costs are going to be lower. There are vessels circling the world that don’t have the “yachtie” curves and, in my opinion, they are just as beautiful. However, this raises performance concerns for some people. For example, they might believe a more rounded stern will handle following seas better, or perhaps a more flared bow would handle oncoming seas better. But differences in the overall picture, if any, may be negligible for you.

While you seldom see it mentioned in brochures, another cost saver with metal vessels is the choice of not faring. Without faring, weld seams and individual plates are visible, the hull looks more like a freighter, and it may have more drag in the water. But the application of faring material is time-consuming, and if moisture is introduced under the faring material — as when you ding it — corrosion will develop. Even with aluminum this can loosen (blister) surrounding faring, even though the hull integrity may not be impaired. With steel, this could blister the faring material and impair hull integrity by allowing rust.

The flip side to this is that with properly applied and maintained faring there is less likelihood of water ever reaching the metal. Most people prefer faring, but if you can curb your penchant for having the prettiest boat on the block, you can save more money. The hull still must have a good rust preventative coating or paint.

Aluminum

Aluminum, according to Sorensen, is the “prime metal” for yachts and small boats. “Aluminum has a melting point of 1,220 F. It weighs 170 pounds per cubic foot,” he writes. “Although aluminum’s strength varies depending on the alloy selected, commonly used marine-grade 5083 H-32 aluminum plate has a yield strength of 34,000 psi and an ultimate strength of 45,000 psi, with a plastic region of 11,000 psi, or 25 percent. Since it is more brittle than steel, aluminum will start to yield at about the same loading as steel, but it fails before denting as deeply as steel and is more susceptible to fatigue. Aluminum welds are about 60 percent as strong as the members they join.”

Although aluminum won’t survive as well as steel on a reef or when it collides with a container, it’s still typically better than fiberglass. Steve Dashew notes in his “Offshore Cruising Encyclopedia” (Beowulf Inc.): “If you find yourself in the unhappy circumstance of being stranded on a reef, aluminum will survive severe, prolonged pounding under the same conditions in which a fiberglass yacht would be destroyed, unless it was built very heavily.”

Judging from the fiberglass boats I’ve seen on reefs over the years, including some of very heavy build, I’d say that even a very heavily built glass boat will break up quickly if there is wave action — and there almost always is.

Chris Barry is a senior naval architect with the Boat Engineering Branch of the Coast Guard. A licensed naval architect and marine engineer for 30 years, he’s also chairman of the Society of Naval Architects and Marine Engineers’ Small Craft Technical Research Committee and has worked in design offices and shipyards in California, Washington, the United Kingdom, and the Washington, D.C., area.

In 1999 he wrote a paper titled “Aluminum for Planing Yachts.” (I should note that the report quoted below was done solely in his private capacity and doesn’t necessarily reflect the opinions or policies of the Coast Guard or any other branch of the government.)

In the paper, Barry made the following observation regarding the strengths of aluminum and fiberglass. “On the balance, an aluminum boat will be lighter for the same allowable strength as one built out of conventional fiberglass, or stronger for the same weight. … Because of the ductility of aluminum, the energy absorbed prior to actual failure of aluminum is almost 11 times that of fiberglass. As a practical example, since impact energy is proportional to the weight of the struck object, an aluminum hull could survive hitting an object 10 times as heavy as one that would hole a fiberglass one.”

And there’s another strength factor, one that involves the constant flexing and impact that any boat suffers at sea. Given enough time, most material will begin to break down from fatigue. Barry writes that “fatigue is the tendency of a material to lose strength due to repeated load cycles. … A stress of 30 percent of the ultimate (not the yield, but the higher ultimate stress) can be applied to aluminum for more than 10 million cycles, whereas the high-performance laminate can carry less than 20 percent of its ultimate for the same number of cycles. Only steel is better for large numbers of load cycles, but it is much heavier.”

Although aluminum isn’t as strong or as abrasion-resistant as steel, its light weight is a selling point for many. “An aluminum vessel’s scantlings need to be about 50 percent beefier to achieve the ultimate strength of a similar vessel built of steel, but the aluminum vessel will still be about 30 percent lighter,” says Sorensen.

A lighter vessel means the ability to use a smaller engine (which means lower fuel consumption), the ability to carry more fuel and/or water, and the ability to go faster. And if you’re considering a boat smaller than 35 to 40 feet, most experts deem it impractical to build a steel pleasure boat of that size because of the weight.

Barry points to another reason people choose aluminum: It won’t rust, he says. “The product of aluminum corrosion is aluminum oxide, which is corundum, in bulk form the gemstones ruby and sapphire, the second-hardest naturally occurring substance,” he says. “When aluminum corrodes, it forms a thin, tight, strongly adherent, transparent coat of aluminum oxide. Any damage to this coat is promptly healed by formation of another coat.”

Strength, stability and other features of aluminum plating can vary to some extent with the grade of the alloy. Barry says that aluminum alloy 5086 is the most commonly used for boatbuilding. “It has trace amounts of iron, silicon, copper, chromium, manganese, zinc and titanium in addition to aluminum and about 4 percent magnesium.”

Many builders use different grades for different applications, according to Dashew. “For hull and deck plating, 5083 and 5086 alloys are used. They’re ductile, easily welded, and extremely corrosion resistant. They have the further advantage of achieving their strength without heat-treating, meaning that when welded they maintain most of their mechanical properties. Of the two, 5083 is the stiffer, favored for decks and topsides, while 5086 conforms more easily to compound curves on the underside of the hull. … 5000 series alloys are more corrosion resistant than 6000 series alloys. While classifications societies allow the use of 6000 series throughout the boat, practice indicates that only 5000 series should be used below the waterline.”

As I discussed earlier, although there is much more consistency in the production of fiberglass boats in modern plants, the term “fiberglass” can still refer to many different variations of material and build. With aluminum (and steel) you may have a much better idea of what you’re getting. As Barry notes: “Aluminum is made under highly controlled conditions in a mill, and each batch is actually chemically and structurally tested and certified.” There also are various controlled grades of steel.

Aluminum has one significant bugaboo: Many avoid it because it is seriously susceptible to corrosion from electricity. This can happen slowly when current is created between other metals and aluminum (which is low on the scale of nobility), or much more rapidly when stray current from poor wiring or defective electrical equipment is introduced to the hull.

There are relatively easy and well-recognized ways to prevent either type, such as ample zincs and proper wiring and a carefully isolated electrical system. Aluminum boats can be equipped with meters to enable an owner to know if either type of electrical activity is present. Therefore, while this should always be a concern with an aluminum boat, it shouldn’t stop you from choosing this material. (If you’re thinking about a simple skiff with limited electrical systems and, better still, which you keep on a trailer, there’s even less concern.)

Aluminum also provides a cost-saving option that steel doesn’t offer: You don’t have to coat or paint it (except as is necessary below the waterline) because of its naturally forming protective coating. We’re seeing more and more unpainted aluminum boats around, and they’re appearance is, in my admittedly pragmatic opinion, quite fine.

“Some builders say that once you add it all up, a painted aluminum boat costs slightly more to build than steel, whereas an unpainted aluminum vessel may actually cost less,” according to Sorensen.

However, others feel that painting is more important. Gerr says he thinks that, over the long term, it’s better to paint an aluminum boat. I’ve had my one-off 12-foot aluminum tender since 1989, and I’ve never painted it and don’t intend to. It gets many admiring comments from serious boaters and worried glances from proud owners of gleaming yachts when I pull alongside them at the dock.

Smaller boats

You might think that I’m only talking about larger boats here. However, this discussion also is relevant to smaller boats and skiffs, except, as previously noted, steel boats smaller than 35 to 40 feet. But fiberglass, aluminum and wood (particularly cold-molded laminates) are used often for smaller boats, and the choices when you buy can be just as exciting as with larger boats.

Most of us are familiar with the plethora of fiberglass skiffs, runabouts, pocket cruisers and sailboats smaller than 40 feet. They are — as they should be — well-accepted. But we often have a tendency to dismiss smaller aluminum boats because we’re used to seeing them in the form of what some call “tin boats” — small riveted skiffs of relatively thin aluminum that’s more like sheet metal than plates. They’re relatively inexpensive, and many of us have experienced problems with them when the seams start leaking after a few years of banging waves or beaches. I don’t mean to imply that you should exclude riveted aluminum boats from your consideration if that suits your usage. Depending upon your preference, this may be the boat for you. For example, Lund Boat Company (www.lundboats.com) of New York Mills, Minn., says its boats have double rivets, and that it uses I-beam construction for superior strength.

But there are an increasing number of welded aluminum boats smaller than 35 feet that are intended for harder use. An important consideration if you’re thinking about smaller aluminum boats is plate thickness. If the aluminum plate is too thin, it will have a tendency to distort as it’s welded. If it’s too thick, the boat may be too heavy. Gerr says 3/16 inch is about as thin as you can weld with ordinary MIG (metal inert gas) welding equipment, so many welded aluminum boats smaller than 25 feet are overbuilt. But modern MIG welding equipment is making it easier to deal with this problem.

Black Lab Marine (www.blacklabmarine.com ) of Yarmouth, Maine, offers several models of welded aluminum boats from 20 to 26 feet. (See the February 2006 New England edition of Soundings.) “With computer designing and computer-aided plasma cutting we are able to reduce the incredible number of man hours necessary to cut the pieces before welding,” says company president Jay Perrotta. “Our plates show up on a pallet of 10, precisely cut and ready to be hand-welded. Plasma cutting is like computer-controlled laser cutting, but it utilizes an ionized gas and goes through plate like butter.”

Perrotta says Black Lab uses the 5086 alloy for the entire boat. “With either 3/16-inch or 1/4-inch construction, these boats are two to three times as thick as most aluminum boats you may be familiar with.” He says the boats are comparable in price to similar size medium- to high-quality fiberglass boats.

Black Lab’s hulls are made to order by Pacific Skiffs of Marysville, Wash., which highlights another telling point: There’s more aluminum-boat building in the Pacific Northwest, with its often rough and tough boating conditions, than on the East Coast. The long-term success of this material on the West Coast is another endorsement for your consideration.

Choose your weapon

Choosing a hull material is obviously a complex and important process. But the good news is that you have a choice. We’ve only scratched the surface (easy to do with fiberglass, hard enough to do with aluminum, much harder to do with steel), but you now know that it’s worth your while to investigate when you buy your next boat. I don’t have to worry about any of this because I’m a writer and I’ll never be able to afford another boat. If I could, I’d seriously consider a steel or perhaps aluminum hull. But my type of boating may be very different from yours. Make an informed decision and get a boat that suits you for what you want to do on the water.