Hull Speed Calculator

Calculator

Tip: For long, slender designs, this formula is still an approximation. Validate with resistance modeling when performance is critical.

Example Data Table

LWL (ft)	C	Hull Speed (kn)	km/h	mph
20	1.34	5.993	11.098	6.896
28	1.34	7.091	13.132	8.16
35	1.34	7.928	14.682	9.123
45	1.34	8.989	16.648	10.344

These are illustrative values using the common 1.34 coefficient.

Formula Used

For displacement hulls, a widely used approximation estimates the practical speed limit at which wave-making resistance rises sharply:

V_h (kn) = C × √(LWL_ft)

V_h is hull speed in knots.
LWL_ft is length at waterline in feet.
C is the coefficient (commonly 1.34).

This calculator also converts results to km/h and mph, and can apply a planning reduction.

How to Use This Calculator

Measure or estimate your boat’s LWL (not overall length).
Select the unit (feet or meters) and enter the value.
Keep coefficient at 1.34 for typical displacement hulls.
Optionally apply a safety reduction for conservative planning.
Press calculate to view results above the form.
Download CSV or PDF to attach in design notes.

Operational context for displacement craft

Hull speed is a practical planning boundary for displacement vessels where wave-making resistance dominates. As the hull approaches a Froude number near 0.40, the transverse wave system grows and the required power rises rapidly. Use the calculator to set realistic targets for cruising efficiency rather than peak sprint figures, especially for long passages and fuel budgeting.

Why waterline length matters most

Waterline length, not overall length, governs the dominant wave length that the hull must climb. Increasing LWL by 10% raises predicted hull speed by about 4.9% because the relationship follows a square root. This is why modest trim changes, loading, or appendage immersion can shift achievable speed without any change to rated engine power.

Selecting a coefficient for your design

The coefficient C captures hull form effects and empirical conventions. Conventional displacement monohulls often use 1.34. Slender hulls, efficient canoe bodies, or performance-oriented shapes may be evaluated with higher values during early sizing, while heavy cruising hulls may warrant a conservative value and a safety reduction. Treat C as a tuning parameter validated by trials or resistance curves.

Interpreting the safety reduction

The safety reduction provides a margin for sea state, fouling, propulsive losses, and operational constraints. A 10% reduction is common for preliminary design notes and voyage plans. Use a larger reduction when you expect head seas, frequent maneuvering, or significant hotel loads that reduce shaft power available for propulsion.

Unit conversions for reporting and compliance

Many engineering documents mix knots for maritime operation, km/h for regional standards, and mph for legacy reporting. This calculator outputs all three simultaneously to reduce transcription errors. When preparing specifications, keep units consistent across propulsion curves, sea trial logs, and performance guarantees, and document the assumed LWL and loading condition.

Using the curve to compare scenarios

The Plotly curve visualizes how hull speed scales with waterline length around your input, helping you compare lengthening options, trim changes, or design variants. The marker shows the current case, while the two lines show base and safety-adjusted predictions. Use the curve to communicate tradeoffs with stakeholders before committing to detailed CFD or towing-tank work.

Additional comparative points support clear review decisions for teams using baseline logs,

FAQs

1) Is hull speed a hard physical limit?

No. It is a practical threshold where wave-making resistance grows quickly for displacement hulls. With enough power, some boats exceed it, but efficiency typically drops and handling can change.

2) Which length should I enter: LOA or LWL?

Enter LWL. The wave pattern depends on the effective waterline, which shifts with trim and loading. LOA can overstate speed potential, especially with long overhangs.

3) What coefficient should I use?

Start with 1.34 for conventional displacement monohulls. Adjust only if you have comparable sea-trial data, resistance estimates, or a validated design rule for your hull form.

4) Why add a safety reduction?

It builds planning margin for fouling, wind, waves, propulsive losses, and operational constraints. A modest reduction helps convert an ideal estimate into a more usable engineering target.

5) Does this apply to planing boats?

Not well. Planing craft transition to a different resistance regime and can exceed displacement hull speed efficiently at higher power-to-weight ratios. Use planing resistance methods for those designs.

6) Can I use meters and still get correct results?

Yes. The calculator converts meters to feet internally, then applies the same formula. Outputs are shown in knots, km/h, and mph to match common reporting needs.