Q. How can a single set of plans be built in aluminum (alloy) or steel when the materials do not weight the same? Doesn’t the weight and strength differences of the material prevent this?
A. The weight of a boat is determined by its waterline. Regardless of what material you use to build a boat, when fully loaded the boat must sit on its waterline. The weight of water that it displaces is exactly equal to the weight of the boat.
We know that when built to an equivalent strength (scantlings) the alloy hull and machinery empty will weigh 2/3 the equivalent in steel. Also, we know that the ratio of ballast to displacement is a critical item in boat performance and safety.
Ballast Displacement Ratio:
- 25% – minimum
- 33% – performance
- 50% – racing
From this we can solve the weight budget for an offshore cruising sailboat. For simplicity the budget is calculated as 12ths of the displacement, which is unchanged regardless of material.
Weight Budget Steel – in 12ths of the displacement
- 3 – ballast (25% of 12)
- 6 – hull and machinery
- 3 – cargo
Weight Budget Alloy – in 12ths of the displacement
- 4 – ballast
- 4 – hull and machinery (2/3 of 6)
- 4 – cargo
Ballast Displacement Ratio
- Steel Empty – 33%
- Steel Loaded – 25%
- Alloy Empty – 50%
- Alloy Loaded – 33%
As we can see, the loaded steel boat has a 25% ballast displacement ratio (BDR). This is at the low end of what we want. Especially offshore. To compensate for this, when loading a steel yacht for cruising it is important to keep weight as low as possible to compensate. Pack your tinned goods under the floors. A metal boat should have a dry bilge. Unloaded the steel yacht has a respectable 33% BDR and will show performance equal to a loaded alloy yacht.
Unloaded, the alloy yacht with a 50% BDR will perform well racing. When loaded for cruising, with a 33% BDR the alloy yacht will still give good performance. Downwind the alloy yacht can be loaded high for comfort. Upwind it can be loaded low for performance. Notice the alloy yacht has greater cargo capacity than the steel yacht, which translates into increased stores for cruising.
Keep in mind, these numbers reflect a budget. Careful attention to weight when building will give improved performance and cargo capacity. Building heavier than required reduces performance and cargo capacity. Also, the larger the yacht the easier it is to maintain the weight budget. A small yacht has a thinner hull and deck than a large yacht because the forces of the water act over a smaller surface.
However, the weight of a person does not decrease as a yacht gets smaller. This limits how thin you can make the hull, especially the deck. As hull size goes much below 40 feet in steel, the deck weight becomes a significant “weight aloft”, reducing performance below expectations. Alloy is not such a problem because of reduced weight.