A hull is a pressure vessel of an awkward shape, and the pressure has to get from the water to the hull girder through three things in series: plating, stiffeners, and frames or bulkheads. Each one collects from the one before it. Scantlings are just the sizes of those three, and choosing them well is mostly about where you put the supports — not how thick you make the metal.
The load path
Plating spans the short way between stiffeners. Its bending stress is β·p·b²/t², where b is the short side of the bay. Both dimensions are squared, which is the important fact on this page: halving the stiffener spacing quarters the stress, and the thickness needed only falls as the square root of the pressure. Work it with the plate panel tool.
Stiffeners collect a strip of plating one spacing wide and carry it to the frames as a beam: the load per unit length is w = p·s, and for a member continuous over its supports the end moment is about w·l²/12. Required section modulus is that moment over the allowable stress. Work it with the stiffener tool, which counts the attached plating as the member’s other flange — on a shallow bar that plate is most of the section modulus.
Frames and bulkheads collect the stiffener end reactions and carry them round the section. Then the whole hull acts as one deep girder in longitudinal bending — which for a small craft is rarely what sizes anything, but it is the reason a long hull wants continuous longitudinals rather than a forest of transverse frames.
Why spacing is the cheap lever
Plate covers the entire boat, so every millimetre of thickness is paid for over the whole shell area. A stiffener is a strip: it costs its own small section, once per spacing. That asymmetry means there is an optimum spacing, and it is usually tighter than instinct suggests — closer frames and thinner plate almost always weigh less than the reverse.
Left to itself that argument runs away: on stress alone, ever-closer stiffeners with ever-thinner plate keep getting lighter. What stops it is a minimum thickness — the rule floor, the corrosion allowance, or simply the thinnest plate your shop can weld without burning through. Once the plate is on its floor, closing the spacing further adds stiffener and saves nothing, and that is where the optimum comes from.
It is not free either. Every stiffener is metres of weld, and welding is where the hours, the distortion and the fatigue details live. The curve below is weight only; the shop’s view is the other half of the decision, and it always argues for fewer, wider frames.
Try it — weight against stiffener spacing
What a rule adds that this does not
Everything above is mechanics. A classification rule — ABS or DNV for high-speed craft, or your customer’s own standard — adds the parts mechanics cannot tell you: what pressure to design for (a slam pressure from the vessel’s length, displacement, deadrise and design vertical acceleration, not a static head), minimum thicknesses regardless of stress, corrosion and abrasion allowances, and the allowable stress itself as a fraction of the welded yield. Those numbers come from the rule book, and the rule book is the deliverable.
Use the mechanics to explore the layout — spacing, span, where the bulkheads go — then run the chosen arrangement through the rule. The rule will not tell you that closer frames are lighter; it will only tell you whether what you drew passes.
A bottom panel comes out 15% over its allowable stress. Cheapest fix by weight?
Stress goes as the square of the short side, so splitting the bay takes roughly three quarters of the stress out — far more than the 15% you needed — for one strip of stiffener. Going up a plate gauge fixes it too, but you pay for that thickness over the whole panel, and on a welded aluminium hull a stronger alloy usually does not raise the welded allowable at all, which is the number the panel is designed against.