Estimate the wetted surface area of a hull based on its dimensions
Fine-lined racing hull
Balanced cruising hull
Full-bodied motor yacht
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Download NowWetted surface area is the total skin of the hull that touches the water, and at slow and displacement speeds it is where most of your drag is born. Friction between the moving water and that wetted skin consumes power, so a smaller wetted area means a faster, more efficient boat for the same effort.
Designers judge it relative to weight rather than in isolation, because a big boat naturally has more wetted skin. The wetted-area-to-displacement ratio is the fair comparison, and a lower figure flags an easily-driven hull while a high one warns of a boat that will feel sluggish in light air.
Enter length, beam, draft and displacement, and the calculator estimates the wetted surface in square feet and expresses it against displacement so you can benchmark your form against other designs.
WSA (ft²) = L × (B + 2T) × √(0.4 + 0.04 × (B ÷ T)²)| Hull Type | WSA / ∇⅔ | Driving Effort |
|---|---|---|
| Light-weight racer | Low | Very easy in light air |
| Fine cruiser | Moderate | Balanced |
| Beamyer cruiser | Higher | Needs more sail or power |
| Full workboat | High | Sluggish, but capacious |
Indicative wetted-area-to-displacement ratios by hull type.
Formula verified against standard naval-architecture references · Updated October 2026
Water friction over the wetted skin is the dominant drag component at low and displacement speeds, so the larger the wetted area, the more power or sail you burn just sliding through the water. Minimising wetted surface for a required displacement is the key to an easily driven hull that performs well in light air and sips fuel at cruise. At high, planing speeds, wave and form drag grow to dominate instead.
There is no single number, because it scales with size and purpose, but a lower ratio generally means an easier-driving hull. Light-air sailors and racers prize low figures, accepting less form stability and interior room, while beamy cruisers trade a higher ratio for volume and stiffness. Use the ratio to compare your design against boats of a similar type and length rather than against a universal target.
Fine the sections, reduce draft and appendage size, and keep the waterline beam modest. The catch is that each of those tends to reduce form stability or interior volume, so you recover by deepening the keel or adding ballast, which adds area again. The real design skill is balancing wetted area against stability and accommodation for the boat's intended use.
Less so than a displacement hull. A planing boat rides high with much of its weight carried on dynamic lift over a reduced wetted patch, so at speed wetted-area drag matters less than it does for a hull that ploughs. However, wetted area still hurts a planing boat on the plane, when coming up onto it and when idling, so it is not irrelevant to a cruiser that spends time at slow speeds.
This tool estimates the hull's own wetted skin from the principal dimensions, giving a first-order figure. Keels, rudders, struts and centreboards each add wetted area that the box formula cannot see, and together they can be significant. For a complete picture, add the planform area of every appendage to the hull figure before comparing designs.
It is a reasonable approximation for conventional displacement and semi-displacement forms, typically within ten to fifteen percent. True wetted area is found by integrating the hull surface from offsets or a fair lofted model, so for fine performance work confirm against that. As a design-comparison and light-air sanity check, the formula is well suited to early decisions.