Why Aluminum Boats Are Superior

My opinion, by Travis Brandt. Founder Ravenark Boats.  August 24, 2021

As an aluminum boat manufacturer, I am frequently asked why aluminum boats are superior to fiberglass boats, a surprisingly difficult question to answer quickly.

The real answer is summarized nicely by Chester Holten a member of the Society of Naval Architects and Marine Engineers when he states, “Marine aluminum alloys are used in boat hulls and structures because of their corrosion resistance, strength, weldability, light weight, and ease of fabrication. Years of successful service of a wide variety of vessels have proven aluminum’s suitability. However, in order to take maximum advantage of the metal’s inherent resistance to corrosion, consideration should be given to the details of a vessel’s design, fabrication, and operation in order to optimize its performance.”   Holtyn, Chester H.. “Corrosion Protection Guidelines for Aluminum Hulls.” Mar Technol SNAME N 22 (1985): 155–163. doi: https://doi.org/10.5957/mt1.1985.22.2.155

Some of these attributes are explained in this Soundings Article by Eric Sorensen (ref: https://www.soundingsonline.com/features/aluminum-boats-are-built-for-life)

Some of us boat builders keep seeking game-changing advances that are inspirationally new.

“If at first the idea is not absurd, there is no hope for it.” ~Albert Einstein

The boat hull being the physical barrier keeping the boat floating, requires resistance to the harsh environments of rocks, docks, sun, snow, and organics (mold, barnacles, etc.) Which of these is the most important?  That depends on the operation underway.

Corrosion resistance, marine grade aluminum (5000 series) is known for its corrosion resistance.

Strength, for the thin aluminum alloy, the ability to carry G-forces from the impact of waves and forces acting on the boat.

Weldability, refers to manufacture and repair down line, aluminum boats have a more straightforward manufacturing process not requiring the tooling infrastructure of fiberglass boats.

Light weight, compared to synthetic (infused) boats, Aluminum boats weigh about the same, but aluminum weighs less than traditional fiberglass boats.

Ease of fabrication, creating a fiberglass boat, whether infused or traditional requires a significant amount of work to create a prototype, then a plug, then a mold, then another prototype.  Once that tooling is created, it’s very difficult to make changes to that hull. Aluminum is MUCH easier because of the mechanical design, supply chain, and manufacturing process.

My top three reasons to use aluminum are:

1) Longevity. Aluminum boats when correctly designed and manufactured will last decades longer than their fiberglass counterparts.  And by ‘last decades longer’ I mean endure at a high level of performance, not merely a dock queen or trailer queen.

2) Transparency in the design and structure, which translates into outfitting and periodic inspections. The aluminum hull, and structure, if property designed and manufactured is visible and repairable.

3) Resistance to salt water elements for many decades.  Aluminum does not need to be kept up in the same sense that fiberglass boats require attention.  If the aluminum boat is designed, manufactured, and appropriately rigged, the boat hull will be 100% viable in 30 years, with very little question.  A fiberglass or synthetic boat has a tendency to degrade, and any amount of inspecting makes it difficult to know for sure how far any delamination or deterioration has manifested.  Sometimes surveyors or engineers will take core samples of the boat by cutting a 1″ or 2″ circular hole (with a hole-saw drill) in the boat to check the lamination.  There’s no need for this in Aluminum boats, thicknesses can be measured physically and easily by other methods.

(Reference on composites in general, not specific to boats: https://www.compositesworld.com/articles/vinyl-esters-make-tough-parts-for-highly-corrosive-applications)

In hybrid synthetics there’s a bit of an illusion because each requires a chemical barrier (resin/vinyl ester/etc) to seal in their mechanical properties in, so the core doesn’t become water-logged with salt water, take on the waters weight, and grow micro-mold which destroys the material by expanding in the cracks.  The easy to puncture micro-thin barrier is a few millimeters thick (like a piece of paper on the cheap boats to a thick canvas on higher quality boats), but even if that exterior layer is synthesized to be a hard shell of a surface resistant to bug scratches and resistant to UV light, it is still much softer than aluminum when met with physically sharp and hard objects.  This fact is self evident in the marketplace because aluminum boats are everywhere, anywhere North of warm.

Naturally, if your boat scrapes a dock, or if you accidentally let the tide recede out and you scrape bottom with your fiberglass-anything boat, a great deal of damage can be done in a matter of a second or two which will destroy the boat, or at the very least require an expensive patch, but the boat will never be the same again.

Repairing synthetics in the field is tricky because the surrounding material has aged, and the preparation, application, and blending of the new material nearly always leaves permanent scars and propensity for future delamination.  Surprisingly, these immediate repair scars are actually much more visible on non painted aluminum, but within months the natural oxidation blends and the disparity disappears.

For the boat, the superior outer shell is clearly an aluminum alloy developed for both corrosion resistance to salt water and the elements, incredible strength, weight considerations (not too heavy, not too light), and most of all resistance to impacts, scratches, and repair work years down the line.

The best below the waterline material in the marine industry is generally accepted to be the aluminum alloy 5086H116. (There is some argument for 5083, but the differences are not huge, and most builders will use 5083 if a customer really has a strong preference for it. I prefer 5086H116 because it is well received and known material.)



Slamming Loads

The strength of a boat is often a discussion about the hull rigidity in rough water, in reference to ‘slamming loads’.   According to David K Brown’s 2010 book The Grand Fleet, Warship Design and Development 1906-1922: Slamming is a large flat area of a hull hitting the surface of the water with force all at once. It can involve many different sections of a hull depending on its shape and is taken into consideration when designing ships.  (ref: https://www.metstrade.com/news/construction-and-material/the-effect-of-slamming-on-composite-boats/)  Along with slamming loads is ‘whipping loads’ and ‘Acceleration and Motion-Induced loads’  These forces acting on the hull, as the boat is going through the water, induce violent G-forces into the boat and the occupants, these forces can delaminate fiberglass boats and can equally harm aluminum boats.  A proper design is key, and adherence to that design during manufacture is imperitive.

Reference to Slamming Loads in ABS (American Bureau of Shipping) can be found here: https://ww2.eagle.org/content/dam/eagle/rules-and-guides/current/design_and_analysis/177_slamming/slamming-guide-july20.pdf

In slamming loads and the other forces and loads, the difference between aluminum and fiberglass boats becomes interesting, especially considering the hydrodynamic design of the hull; how a particular hull shape and weight penetrates the water, how it scoots along the water when planing, how it interacts with waves, and how it turns.  The shape of the hull (hull-form), weight and balance, chine structure, strakes, etc all play a huge role in the forces imposed into the boat.

Foam infusions actually have incredible numbers compared to aluminum, they can withstand greater forces than aluminum for the same weight, but not necessarily the same thickness. However this one area alone is not enough to convince aluminum manufacturers to start building out of synthetics.  Here’s why:

When a boat slams into water, a shock wave flows through the boat and its occupants.  The job of the naval architect is to mitigate these shock forces by calculating the spacing, thickness, and locations of the load carrying structure.  Several Naval Architectural societies publish categories along a progressive continuum and provide numbers and formulas for to aid the architects and builders towards these desired categories that are oriented towards a specific intended mission for the boat. These categories generally can be referred to as ‘Classes’

In the most extreme of environments, the ability of aluminum to carry these loads and forces far exceeds terminal velocity, e.g. anything a hurricane or typhoon can throw at it.  It may be true that some synthetic internal boat structures, on paper, outperform an aluminum boats structure, but that reality is so far fetched into hypothetical forces that without laboratory testing gear, or maybe explosion testing with military grade explosives, the delta between aluminum and fiberglass structures is not practical to discuss.  This means that practically speaking the aluminum and synthetic structures are essentially equal when it comes to slamming loads, presuming the equivalent and appropriate engineering, leaving the other factors of repair, delamination, scratches and punctures, etc. as the categories that matter in real life.  The conclusion for nearly 50% of the boat market is that aluminum is superior.

A final note on the graphics above, were made to represent the interaction of the concepts as they relate to each other and the whole boat.

All things considered, aluminum is the superior material for the hull, structure, cabin, etc of boats.

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