Fiberglass vs Aluminum Alloy

Chapter 3


What material?  Aluminum versus fiberglass…


With versus in there you would think that we have a battle going on between the two competing materials.  We don’t.  In fact we own some fiberglass boats.  The two predominant boatbuilding materials in this size range represent different strengths and weaknesses.


In the previous chapter we made the case that for boats in this category the true deep-vee hull form is, without question, the single best hull form.  (Some may argue for catamarans.  We do not agree.)


The answer to what is the correct material is different. 


First off - Most recreational boats get used less than 100 hours per year.  Kind of a shocking fact.  Since these boats are used “recreationally” we’re also sure that they are not routinely used in adverse conditions or in adverse ways - high winds, rough seas, boardings, beachings, high-speed runs.  In plain terms they aren’t used hard.  They are gently used in moderate conditions for short periods of time.


For this sort of usage fiberglass is a great, fantastic, superb, wonderful boatbuilding material. It will last 20, 30, 40 years with maybe some oxidation and light cracking of the gelcoat. 


If you use your boat in this way we’d suggest fiberglass and further we’d like to suggest the following builders as top-notch, high quality builders of the true deep-vee offshore hull form:


SeaVee (our favorite)





(Tell ‘em Rock Salt sent you!)


On the government/military side the usage is quite different. 


For instance – the USCG has to respond to offshore emergencies in the roughest imaginable conditions.  They put hundreds and hundreds of hours on their boats annually.  What do they buy?  Marine-grade, plate, aluminum alloy.


What about the boats that serve the offshore oil industry bringing goods and supplies to the rigs?  These boats have to deliver personnel and materiel to the rigs every day, day-in and day-out.  They can’t look at the weather forecast and put it off until things “clear-up”.  They have to “dock” with the rig in rough weather.  What are the thousands of these service and supply vessels made from – Marine-grade, plate aluminum alloy.


In places where boats are used 200, 300, 500, 1000 hours a year in the most adverse conditions possible the choice is almost invariably aluminum.


So doesn’t it make sense that recreational boat owners who use their boats extremely hard would also choose aluminum?  We hope so.


Let’s take a closer look at the two materials.


Fiberglass:  First of all this commonly used term is a bit of a misnomer.  Correctly labeled it would be either FRP (fiberglass reinforced plastic) or GRP (glass reinforced plastic).  If you have a “fiberglass” boat you have, in actuality, a plastic boat.  The plastic is reinforced with a glass fiber material in much the same way that today’s concrete structures are reinforced with steel rebar.  No one calls a concrete structure a steel structure because it is reinforced with steel.  The idea to call the composite substance fiberglass came from the dim view most people have of plastic when it comes to toughness.  Would you want to spend $300,000 on a plastic boat? 


But this plastic composite material is pretty cool.  It is infinitely formable.  Just make a mold, lay down the fiberglass cloth and impregnate it with the plastic and pretty soon you have a hard, smooth, relatively waterproof item.  The shower in you bathroom is probably fiberglass – wonderful!  Now imagine dropping a lead dive weight in your shower, or a large rock or a piling.  Craaaaaaaack!


Fiberglass is what is called in material science a “brittle” material. In other words it is intact and can take a certain amount of stress and then abruptly fails.  It doesn’t yield or deform it breaks.  In the light-duty usage described above this lack of strength doesn’t really matter – you don’t need a strong material.  Your tub doesn’t get rough duty!


But for rough duty this is a big issue.  The forces a boat takes are substantial.  If you took a well made BMW SUV and subjected it to the ongoing and continual pounding that a boat takes it would fall apart like it was made of matchsticks.


As mentioned above fiberglass boats can and do survive light to moderate usage.


But some boaters are not “light” or “moderate”.  Maybe you?


Yesterday I received a call from a prospective owner – a charter captain from South Florida who regularly goes 110 miles offshore.  All weather, hundreds of hours per year, serious fishing.  Perfect Rock Salt owner.  He understood immediately the value of an all-welded aluminum boat.  No fiberglass to crack, no t-top screws to come loose, no gelcoat to mar, no wood to rot.  Beat it up, hose it off and put it away!  Welded plate aluminum alloy.


Aluminum.  If you’re an east coast mariner the word aluminum generally has a negative connotation when it comes to boats.  You think of jonboats and pontoon boats and riveted sheet metal boats like Grumman and Lowe.  These boats are fine light-duty lake boats but no one, no one would consider them saltwater boats much less serious saltwater boats.   But that is not what we are discussing or what we sell.


Our boats are not big, thick Lunds.  They are small versions of the materials and technology that have served the yacht, offshore oil, military and high speed ferry markets.


Some differences to make the point:


Thickness:  Riveted sheet metal boats are usually 1/10th of an inch thick or so.  Our hulls are made from ¼” sides and bottom, ½” transom and 5/8” keels.  Serious, serious metal.  Beat it with a sledgehammer thick.  Run it up on the rocks thick.


Alloy:  Riveted sheet metal boats are made from 5052 alloy.  The least expensive and weakest of the “marine-grade” aluminum alloys.  No serious builder uses the stuff to make hulls.  We use either 5086 or 5083 which are 30%-40% stronger for a given thickness than 5052.  Multiply that greater strength by the fact that we are 2 1//2 times thicker and we are not anything like them.


Rivets:  Great for airplanes bad for boats.  Simply put it is a terrible idea to have hundreds and hundreds of holes in a boat below the waterline. 


So unless you have direct experience with aluminum yachts, offshore oil service and supply vessels, small high-speed military craft or high-speed ferries you probably have questions about aluminum in saltwater.

From my experience speaking with literally thousands of prospective owners there are three main questions:


1)     Will the aluminum corrode in saltwater?

2)     Does the aluminum get hot in the sun?

3)     Is lightening a bigger worry?


I’ll answer all three of these questions in the next section in full but first allow me to describe the material and technology of welded aluminum, plate alloy boats.


Pure aluminum is never used in any meaningful way in industry.  From aircraft to the space shuttle (80% aluminum by weight) to automobiles when someone says something is made from “aluminum” what they are really saying is that it is made from a predominately aluminum alloy.  Various other metals are combined with the aluminum (chromium, copper, magnesium, nickel, etc) and then the resulting “alloy” is either heat treated or work hardened.  The resulting material has specific and testable properties.  It is a completely “know” material.


The alloys are divided into 6 “series – 1XXX, 2XXx, 3XXX and so on with the first number representing a family of alloys.


The 5XXX or 5000 series of alloys is commonly referred to as the “marine-grade” aluminum alloys.  The defining mixture for this series is that no metals that are “less noble” like copper are involved in the mix.  If a copper/aluminum alloy was submerged in water you’d have the corrosion problems that those unfamiliar with marine-grade alloys are initially afraid of.


Of the 5XXX series the three most commonly used are 5052, 5086 and 5083 with 5058 and 5083 being the stronger, more expensive and more corrosion resistant than the cheap, relatively weak 5052 used in riveted boats.


So we use .250” 5086 or 5083 for our hulls and we join the plates using both MIG (metal inert gas) and TIG (tungsten inert gas) welding methods.  The resulting hull is impervious to saltwater, incredibly tough, fireproof and essentially “one piece” as the welding makes two plates one.


Besides the inherent differences between a plastic composite and a metal alloy is that fiberglass quality and properties are completely and utterly dependent upon the technician or technicians who put it together.  In other words a fiberglass boat that is built by “Frank” who is a careful, diligent and experienced worker can be completely different than a boat built by “George” who is hungover, lazy and uncaring.  Fiberglass boat companies (even the best quality) cannot certify the material their product is built of.  They cannot tell you the strength, completeness about their material. 


Aluminum alloy plate is made and tested.  The materials properties are always the same, its characteristics are known.  This is a significant and important difference.