Steel or aluminium: the right material depends on speed.

Aluminium is lighter than steel, but not three times lighter, as is often said. In reality, the saving is around 35 to 45% on a large vessel. What really matters: weight does not cost the same at every speed.

The speeds below are orders of magnitude, not thresholds. Everything depends on size, hull form and load.

Displacement hull, about 10 to 15 knots

The hull stays immersed and pushes the water aside. It never lifts. Weight is almost painless: 20% more displacement means about 12% more fuel. It even brings advantages: seakeeping, less roll, more tank capacity. This is the regime that favours steel, and the only one where composite, aluminium and steel all remain valid.

Semi-displacement hull, about 16 to 22 knots

The hull is flatter aft. The boat lifts a little without really planing. Weight has a price: 20% more displacement means about 25% more fuel, or 2 to 3 knots lost. This is the trade-off zone: steel is defensible at the lower end of the range, aluminium wins as soon as speed matters.

Planing hull, beyond 22 to 25 knots

V-shaped bottom, wide transom. The water lifts the boat, which glides instead of pushing through. Every kilo is paid for in full. Here it is aluminium or composite: steel is absent from this segment in yachting.