The Weight Question, And Why It Decides Less Than Expected
Aluminium is denser than a glass laminate, but it is used in thin plate rather than thick section, so the comparison is not a comparison of densities.
Plate aluminium usually comes out lighter than a solid single-skin laminate of equivalent strength. A well-engineered sandwich laminate usually comes out lighter than plate aluminium. Structural weight is a function of scantlings, stiffener spacing, laminate architecture and core selection, and all of those are decided after the material is chosen.
Weight is an output of the engineering. It is not a property of the material name.
Geometry, And What The Process Allows
This is the difference that shapes the boat.
A fiberglass hull takes its shape from a mould. Compound curvature, spray rails, chines, steps, radii and integrated details cost tooling once and cost nothing per unit afterwards. The hull form is close to unconstrained.
Aluminium plate wants developable surfaces. Complex curvature is possible but has to be formed, which means labour and tooling on every hull. In practice that pushes aluminium hulls toward simpler, more angular geometry unless there is a strong reason to pay for the alternative.
The constraint reaches back into the naval architecture. A hull form that depends on subtle warp, fine spray rails or carefully shaped steps is far easier to realise in a mould. The material decision is a hull decision, not a downstream one.
Volume Decides More Than Either
The two cost structures are shaped differently.
Fiberglass front-loads. A plug and a mould are paid for before the first hull exists, and that investment is recovered across the series. Per-unit labour afterwards is comparatively low.
Aluminium has little to amortise. Cut files and jigs are inexpensive set against a mould. But the hours recur, and the fiftieth hull takes much the same labour as the first.
One-offs and short runs sit to the left of the crossover, Serial production sits to the right. The material decision is made by the production plan as much as by the naval architecture.
There is a crossover. Where it falls depends on the size of the boat, how complex the geometry is, and local labour rates. The useful question is not which material is cheaper. It is how many you are building.
Behaviour In Service
Damage tolerance differs in kind. Aluminium is ductile: it deforms under impact, absorbs energy, and often stays watertight after a strike that would crack a laminate. A laminate is stiffer and resists well up to a threshold, then fails locally through cracking, delamination or core disbond.
Repair follows from that. Aluminium is repaired by welding, which needs a matched alloy and a qualified welder but can be done in modest facilities. Laminate repair needs cure conditions, temperature control and time, though the materials are widely available.
Degradation differs in kind as well. Aluminium asks for galvanic discipline: bonding, anode management, isolation from dissimilar metals, and attention to crevices and stray current. A laminate asks for coating integrity, protection from UV, and control of moisture ingress into cores. Fatigue at welds is a real design driver for aluminium in high-speed craft and has to be engineered for rather than assumed away.
Neither material is more durable than the other. They demand different disciplines, and each fails when its own discipline is not applied.
The Considerations That Get Left Out
Aluminium conducts heat and sound. Insulation and condensation management have to be designed in, and they carry weight and cost that belong in the comparison. A cored laminate does much of that work inherently.
At the end of service life aluminium retains recoverable value and separates readily. Thermoset composite does neither, and regulation in this area is moving.
The yard matters too. Welding qualification and weld inspection is a different capability from laminate quality control, void content and cure management. A material that suits the design but not the builder is a poor choice.
How The Decision Is Actually Made
Series volume, hull form, operating profile, expected damage, where the boat will be maintained, and who is building it.
Weight belongs on that list. It is rarely at the top of it.