What Is Hull Speed?
Hull speed is a classic estimate of the maximum efficient speed for a displacement boat. As a displacement hull moves, it creates bow and stern waves. At higher speeds, the boat settles into the trough between those waves, and pushing faster requires dramatically more power.
The calculator above gives a practical, quick estimate in knots, plus mph and km/h conversions. It is most useful for sailboats, trawlers, and other displacement hull designs.
Hull Speed Formula
The traditional formula is:
Hull Speed (knots) = C × √LWL(ft)
- C is usually 1.34 for many displacement hulls.
- LWL is waterline length in feet.
If you enter meters, this calculator converts to feet automatically before applying the formula.
How to Use This Hull Speed Calculator
Step 1: Enter Waterline Length
Use the true waterline length (LWL), not necessarily LOA (length overall). Manufacturer specs or survey documentation usually list LWL.
Step 2: Choose Units
Select feet or meters. No manual conversion needed.
Step 3: Adjust Coefficient (Optional)
Keep 1.34 unless you have a reason to tune it for a specific hull shape or operating profile.
Step 4: Calculate
Click the button to see estimated hull speed in knots, mph, and km/h.
Example Estimates
- LWL 16 ft: about 5.36 knots
- LWL 25 ft: about 6.70 knots
- LWL 36 ft: about 8.04 knots
These are theoretical guideposts, not hard limits.
Important Limitations
It Is Not a Universal Speed Cap
Planing hulls, semi-displacement designs, and multihulls can exceed this estimate. Surfing conditions, hull form, and modern materials also affect real-world performance.
Conditions Matter
- Sea state and wind
- Boat weight and trim
- Bottom condition (clean vs fouled)
- Propeller and engine efficiency
Why Boaters Still Use Hull Speed
Even with its limits, hull speed is still valuable for route planning, fuel budgeting, and setting realistic cruise expectations. It's simple, fast, and surprisingly useful for displacement craft.
Quick FAQ
Is hull speed the same as top speed?
No. It is an efficiency threshold estimate for displacement hull behavior.
Should I use LOA or LWL?
Use LWL whenever possible.
Can sailboats exceed hull speed?
Yes. In favorable conditions, some sailboats exceed it temporarily, especially lightweight or performance-oriented designs.