Sailboat Hull Speed Calculator (Displacement-Length and Sail Area Ratios)
The speed a displacement sailboat struggles to exceed, from its waterline length, plus the two classic ratios that tell a light, fast boat from a heavy, slow one.
Results
The dashed line marks the value you entered.
What this tool does
A boat moving through water makes a wave about as long as itself, and a wave's speed is set by its length, so adding speed means climbing its own bow wave: the rule of thumb is 1.34 times the square root of the waterline length in feet, in knots. The displacement-length ratio compares weight with length (below about 150 is light, above about 300 heavy) and the sail area-displacement ratio how much canvas each tonne carries (around 16 to 20 is typical of a cruiser). Planing boats and long light multihulls beat the hull speed; heavy cruisers rarely reach it.
Formula
hull speed = 1.34 × √LWL(ft) knots · D/L = (displacement in long tons) ÷ (0.01 × LWL ft)³ · SA/D = sail area (ft²) ÷ (displacement in ft³)^(2/3), with 64 lb per ft³
Variables
| Symbol | Meaning | Unit |
|---|---|---|
lw | Waterline length | m |
dk | Displacement (weight of the boat) | kg |
sa | Sail area (mainsail plus 100 % foretriangle jib) | m² |
HK | Hull speed | kn |
HM | Hull speed in km/h | km/h |
DL | Displacement to length ratio | — |
SD | Sail area to displacement ratio | — |
Worked example
- Waterline length8.5 m
- Displacement (weight of the boat)4500 kg
- Sail area (mainsail plus 100 % foretriangle jib)45 m²
- Hull speed7.08 kn
- Hull speed in km/h13.1 km/h
- Displacement to length ratio204
- Sail area to displacement ratio16.8
Limitations
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
- The formula assumes ideal conditions: no friction losses, no air resistance and no efficiency losses unless you enter them.