Before you start
Gather the appropriate supplies and review the specific equipment manual.
- Length at waterline
- Boat specifications
- Speed log
Step-by-step instructions
Identify the correct waterline length
Use LWL rather than overall length for the common displacement hull-speed estimate.
Understand the rule of thumb
The traditional estimate is about 1.34 times the square root of LWL in feet, in knots.
Recognize design differences
Wide sterns, light hulls, surfing and planing designs can behave differently.
Account for conditions
Wind, waves, loading, fouling and current affect GPS speed and performance.
Compare with actual boats
Use polar data, owner logs and practical sea trials for realistic passage planning.
Safety note
Hull speed is not a mechanical speed limiter or a guarantee of performance.
Key takeaway
A useful estimate is only one data point when comparing boats.
Keep this job on your maintenance schedule
Save the topic as a maintenance task, choose a due date or engine-hour target, and record future service. Free and stored on your device.
Add to my maintenance plannerThis guide is for general education. Follow your equipment manufacturer’s instructions, current navigation guidance and applicable marine safety standards. When uncertain, use a qualified marine professional.