Aluminum vs. Fiberglass Hull

I have read that 1/4 " of the correct grade ( 5086 ? ) aluminum is at least 10 times stronger than 1/4 '' of fiberglass ? or does it depend on the makeup / material used ? I am a little gun shy of the newer fiberglass hulls . Pros and Cons ? Thanks

 

You don't compare strength by thickness, unless this dimension is a limiting factor in the design.

I don't think fiberglass will ever work successfully as a hull, there's no future in boats built with composites. It just doesn't work.

 

You can't just generalise - it all depends on so many things.

You could start off by comparing the Youngs Modulus of the materials - in the table shown in this link,
Young's Modulus - Tensile and Yield Strength for common Materials https://www.engineeringtoolbox.com/young-modulus-d_417.html
the modulus for aluminium is about 70, that for steel is about 200, and for a 'glass reinforced polyester matrix' they say it is 17.

Based on modulus, you could say that steel is 3 times stronger than aluminium - but steel is also almost 3 times heavier (ie denser).
If the value of 17 for GRP is true, then you could say that aluminium is 4 times stronger than fibreglass - a far cry from the claim of 10 times that you mention (do you have a link for this claim?).

You mention that you are a 'little gun shy' of the newer fibreglass hulls - what are the aspects of these hulls that you are uncomfortable with?

 

Pleasure boats are about feeling good, you choose something that makes you happy. If the "newer fiberglass hulls" (whatever they are) don't do it for you, it's OK, plenty of options out there. For example "older fiberglass hulls", carbon fibre hulls, kevlar hulls, basalt hulls, ferrocement hulls, HDPE hulls, wooden hulls (of all possible descriptions), steel hulls, aluminium hulls, titanium hulls, and probably something I forgot or never knew it existed. Your money, your choice, there is no "better", only "makes me sleep good at night".

As for "newer fiberglass hulls" I'm going to go out on a limb and tell you they are like "newer automobiles", better and worse at the same time. We now know much more and can optimize the boats much better, altough some of the directions we go while doing this is not to everyone's liking. Why that happens is simple, market pressure, the customer wants it like that. For example both newer boats and cars can be hard to repair (welding is not allowed anymore on many newer cars) simply beacuse our economy has moved to a place where buying new with insurance money is preferred.

 

Comparing fiberglass hulls to aluminum hulls is an apples and oranges exercise. Which is better? first you have to ask, better for what? Since most recreational boats are made of fiberglass your argument of the superiority of aluminum is a bit suspect. But when people ask me which is better, I say, which is better for you? Buy what you want. If you want aluminum there are literally hundreds of models available in aluminum. Or buy that which is most suitable for whatever use you will have for the boat. What boat you should buy is mostly determined by what you are going to do with it. If a fiberglass boat meets your needs then buy a fiberglass boat. Fiberglass lasts almost forever. The industry is still trying to figure out how long it will last and how to recycle it. Yes, today fiberglass boats are built a lot lighter than they were in the 60's and 70's, but designers have developed new techniques to provide stronger and better hulls with less weight and less fiberglass. My 1972 18 foot Sea Ray weighs over 300 lb more that the same size boat today. But I'd be willing to bet the new equivalent boat is just as strong and will last just as long as mine.

As for aluminum. They are probably much more rugged. Here in the Pacific Northwest people do a lot of river running and fishing in rocky areas. Aluminum is far better for that than fiberglass. Go to any hot fishing spot off the Coast or in the rivers and sounds and most of the boats you see are aluminum. Again it's a matter of use.

 

On thing rarely spoken about plate alloy boats, water noise that can be annoying, like marbles rolling around in a barrel.

 

4 times stronger still sounds better . Long story , but my boat was damaged due to blocking . Thanks for the opinions .

 

Re your damaged boat, is she fibreglass?
What sort of damages did she sustain?

 

The damage to your boat had little to do with the materials, it was how it was built or handled. It didn't fail because it was fiberglass.

Thin aluminum would fail if blocked incorrectly too.

Design and build the hull correctly, and most materials can survive quite well.

 

There is a big difference between 1/4" of CSM and Poly resin vs 1/4" of epoxy and Uni or Satin weave roving. Even if one used Uni, it is essential to lay the fabric so that the fibers run straight and are taut. Loosely laid cloth that has been distorted and having wavy fibers will be much weaker and have a considerably greater deflection for a given load.
Aluminum can be a great material, but only if the weld prep is clean and uncontaminated and the welding carried out precisely. Its very easy to contaminate the surfaces and for wind to disrupt the shielding gas. It has quite a lot of shrinkage on solidification so that translates into a lot of stop-starts and each one is potential for oxidation, contamination and pinholes.
On a steel hull, blasting and painting the structure can be as expensive or more than the cost of the hull plating itself. The material itself is a bit more resistant to process problems, but that is highly dependent on the alloy used. The higher the yield point of the alloy and especially the higher the carbon content, the more critical the processing will be and the more susceptible one will be to hydrogen embrittlement.
All material choices are compromises and which factors are important to you will depend on the intended purpose and the location where you use it.

 

Yes , Fiberglass . Two cracks about 12 '' long . The story is on BoatDesign Nov.2020 '' Hull Damage Question '' Thanks

 

Oh, now I remember, how has that played out ? Clearly GRP has been a huge, huge success in boat building, I can't imagine what recreational boating would look like without it, except it be much the poorer. So get that prejudice in perspective. Sure, it has limitations, and for certain special applications, it is best avoided, but as a general rule, no, it has far more advantages than disadvantages.

 

Referring to the table that Bajansailor linked above

Within its elastic region

Youngs Modulus of Elasticity is not a measure of strength but a measure of stiffness. Ie when a certain load, lbs, is applied to a cross section of 1 square inch it creates a stress, lb/(in x in), it will stretch and a convenient way to quantify this is in inches per inch, inches/inches

When plotted on a graph, the slope of this graph is rise/run or lbs/(inch x inch) , pounds per square inch, the stress divided by the strain, or inches per inch, in/in.
The slope of the graph then is pounds per square inch. The slope is the Youngs Modulus of Elasticity, The higher the number the more resistance that the material has to elastically stretch, strain for a given stress

You can have similar Modulus of Elasticity Values, but very different Yield limits and Ultimate strength limits.
When you apply a load, with a corresponding stress and the material stretches/strains, if it returns to its original dimension the material is operating within its elastic range.
When you apply a load, and the material does not return to its original dimension, then it has yielded, and reached its Yield stress.
For many materials, aluminum, steel, the material will not break but continue to yield, and as part of its stretching process becomes a bit stronger, then eventually breaks, the highest value that the material
reaches before breaking is the Ultimate Tensile strength.. ( quite a few generalizations here)

So from the table, and just unitless for comparison,

Material Modulus, E Ultimate Stress Yield Stress

Stainless Steel, 180 860 502

Steel, Structure 200 400 250

So while the Modulus is quite close, the yield and Ultimate stresses are approximately double.

The task of an engineer/naval architect is to ensure that loads are determined that keeps the stress below Yield by a factor of safety. The factor of safety for some of a boats elements might be 1/3 of maximum slamming stresses as an example. ( I am guessing at the Factor of Safety being 3, reading through the technocrats posts on this forum, it appears that there are many country standards for these FS values)( and as an aside, most designers would work with Scantling Tables to determine hull stiffeners rather than doing the actual calculations to ensure conformity etc)

Depending on the element, the goal is to keep the stresses below the factor of safety determined stress but then find that even at that stress level, the deflection may become the design limit.
Ie the element/component is at 1/3 of yield but say the foredeck on the boat flexes to an uncomfortable level. Which may require a thicker panel, more support, a change in cross section throughout the stiffeners.

When working with composites other factors come into play, shear, shear flow etc.

 

That certainly was a long running saga (8 pages) with your Ranger Tug - here is a link to the thread :
Hull Damage Question https://www.boatdesign.net/threads/hull-damage-question.64732/

It has been a few months since your last post about the Ranger - have you managed to get her repaired?
Although I get the impression that nothing has yet been resolved (?) re how you are now looking at aluminium for constructing a hull.

 

Aluminum if designed right is stronger and stiffer than a fiberglass.

To build with aluminum, you need a shear, drill, a riveter, perhaps an aluminum welder.

With fiberglass you need a scissor and a paint brush.