If the builder converts 3/16 steel plate to 1/4 alloy that’s only an increase of 1.3 times which will be a little weaker than steel but not alarmingly so.
Talking common boatbuilding materials. Alloy needs to be 1.4 times thicker that steel to have the same resistance to buckling. Stiffness scales linearly to Youngs modulus and by the cube of the material thickness. Young modulus or stiffness (desigated E in GPa) of most boat building alloys is around 70 and steel is 200. so for the same thickness alloy is only 35% as stiff (for 30% of the weight). If we take the cube root of (1 / 0.35) we get 1.42 as the required thickness increase. Or one half the weight of the equivalent structural material for the hull for 1.4 times the volume of material.
Alloy makes sense for lightweight boats and can be further lightened and made stronger by adding framing since deflection also goes by the cube of the span. So halving the span reduces deflection by a factor of 8. That’s why with alloy you juggle the skin and framing to find a good balance of strength and weight.
But novel alloy structures also need a careful analysis to ensure that not only material stress is within fatigue and buckling limits, but weld stresses are below the fatigue stress allowances. Alloy should also be professionally welded. It’s the worst material for the home builder to attempt to build in and it’s very easy to produce nice looking welds which have no penetration and can fail easily.
But I think you’d be daft to build an alloy Brent boat, you’d never recoup the material costs in resale. You’d buy a used Brentboat for less than the alloy cost.
It would be much more sensible to opt for a better designer and build compliant to ISO or some class scantlings. At least they catch the design pitfalls which can be numerous. With alloy the devil really is in the detail. You would also have a much better cost to strength ratio.
You find that most common materials have quite similar stiffness to density figures or what’s called ‘Specific Modulus’ Lighter materials have the benefit of being able to increase stiffness with thickness at a much greater rate than they increase total mass. But lot of boats don’t benefit much from a lighter hull construction particularly if they are a heavier type of vessel by design. Alloy does corrode less but a hot zinc or aluminium sprayed steel hull (internal deck and even topsides) comes close in durability if you can afford it.
Span has nothing to do with what direction the framing runs in . Otherwise ,one would have to believe, as it appears Mike does, that if you take a flat plate with stiffeners running vertically, and turn it 90 degrees, so they run horizontally, the plate immediately loses all strength, and you have two options to regain that strength . Either turn it back upright, when it will immediately regain it's lost strength ,or cut the stiffeners off and re weld them back on, running vertically.
Sea captain and cruise ship designer Emanuel ( at sealegacy.com) said they have calculations for running stiffeners either way.
With the Van de Stadt 34 , frames are structurally irrelevant, as the chines, far stronger than frames, are close enough together to give far more strength than any transverse frames could ever give. They are merely there to appease those who are incapable of thinking of structural strength in three dimensions. Span in that case, is the distance between chines. Longitudinal curve takes the place of transverse framing, when it comes to strength and stiffness, doing a far better job of it, without the hard spot, point loading, and increased suceptibility to holing of transverse framing and thinner plate. Ditto longitudinal stringers. The plate support of transverse framing only goes a couple of inches either side of them. Chines are far stronger, in that they maintain the curve of the topsides , adding far more strength that the merely local stiffening a couple of inches either side of transverse frames .
Thinner plate with more framing has far greater chance of being holed on a shipping container, or a sharp rock, than thicker plate with less framing. It is also far more expensive and labour intensive to build, and far less forgiving when it comes to corrosion. .It is also much harder to eliminate distortion in.
Many make the common mistake of confusing resale price with resale value. Resale price is what you can get for a boat. Resale value is the difference between what you can get for a boat, and what she cost you in the first place . The higher the cost of building the boat, the narrower the gap between the cost of building and the resale price. My first steel boat sold for over 4 times what she cost me to build. The first 31 I built, the owner sold for three times what she cost him to build. Both buyers felt they got a great deal . It is not uncommon for some to spend an extra $40 K to increase the resale price by $20K, a net loss of $20K.( negative resale value)
The gap between building cost and resale price of an aluminium boat and a steel one would usually put the aluminium one at a distinct disadvantage. While the resale price of aluminium would be greater, the gap between price and cost ( resale value) would be far narrower than with steel. Aluminium could well work out to a net loss ( negative resale value.)