Displacement Hull Speed Calculator
The Speed a Displacement Hull Can't Easily Push Past
A displacement hull moves through the water rather than over it, and as it speeds up, it has to push aside a bow wave whose length grows with speed. Once that wave length approaches the boat's own waterline length, the hull is effectively trying to climb its own wave — a physical wall that takes disproportionately more power to push through. Hull speed is the long-standing approximation of where that wall sits.
The Formula
Hull Speed by Waterline Length
| Waterline length (ft) | Hull speed (knots) |
|---|---|
| 16 | 5.36 |
| 20 | 5.99 |
| 25 | 6.70 |
| 30 | 7.34 |
| 40 | 8.47 |
| 50 | 9.48 |
Because the formula uses a square root, doubling waterline length does not double hull speed — going from 16 ft to 50 ft (roughly 3x the length) only raises hull speed by about 77%, which is why a longer boat isn't proportionately faster under displacement conditions.
Where This Matters
- Engine sizing — pushing a displacement hull meaningfully past its hull speed requires exponentially more power for diminishing speed gains, which shapes how much engine a given hull actually needs.
- Passage-time estimates — for a sailboat or trawler-style powerboat, hull speed is a realistic practical ceiling for average cruising speed, useful for estimating trip duration.
- Comparing hull types — planing hulls and multihulls aren't bound by this same limit, so hull speed is specifically a displacement-hull consideration, not a universal boat-speed formula.
How to Use This Calculator
- Enter the Waterline Length (ft).
- Select Calculate to see the approximate displacement hull speed in knots.
Related Calculations
Turn this speed into a trip estimate with the Marine Distance Calculator, or check fuel needs at cruising speed with the Boat Fuel Consumption Calculator.