Hull Speed — 1.34 × √LWL
1.34 × √LWL. 30 ft → 7.3 kt, 42 ft → 8.7 kt. Fr ~0.4 limit. Displacement hull wave-making resistance.
Why This Stat Matters
Why: 1.34√LWL. Fr 0.4 limit. 30 ft ≈ 7.3 kt. Planing hulls exceed it.
How: Enter LWL (feet or meters). Get hull speed, Froude number.
- ●1.34 × √LWL
- ●30 ft → 7.3 kt
- ●Fr ~0.4 limit
📋 Quick Examples — Click to Load
📊 LWL vs Hull Speed
Speed by waterline length
📈 Hull Speed Curve
1.34√LWL
🍩 Froude Zones
Displacement vs planing
📊 Key Values
Constants and examples
For educational and informational purposes only. Verify with a qualified professional.
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30 ft LWL → 7.3 kt
1.34 × √LWL formula
Fr ~0.4 displacement limit
Hull speed = 1.34 × √(LWL in feet). Displacement hulls are limited by wave-making resistance. Froude number Fr = V / √(g×L); at Fr ≈ 0.4 the "hump" occurs. 30 ft LWL → 7.2 knots. 42 ft → 8.7 knots. Planing hulls exceed this limit.
Sources: Marchaj Sailing Theory & Practice, USCG, Westlawn Institute.
Key Takeaways
- • Hull speed = 1.34 × √(LWL feet). Longer boat = higher displacement limit.
- • Froude number 0.4 is the wave-making resistance hump for displacement hulls.
- • 30 ft LWL → 7.2 knots. 42 ft → 8.7 knots.
- • Planing hulls exceed hull speed; displacement hulls are limited.
Did You Know?
How Does Hull Speed Work?
The Formula
Hull speed = 1.34 × √(LWL). The 1.34 comes from √(g/2π) in imperial units. Wave length ∝ boat length; speed ∝ √wave length.
Froude Number
Fr = V / √(g×L). At Fr ≈ 0.4, the stern wave catches the bow—wave-making resistance peaks. Displacement hulls can't efficiently push past.
Planing vs Displacement
Displacement hulls (sailboats, trawlers) push through water. Planing hulls (powerboats) rise and skim; they exceed hull speed with sufficient power.
Expert Tips
LWL vs Hull Speed
| LWL (ft) | Hull Speed (kt) | Froude |
|---|---|---|
| 20 | 6.0 | ~0.4 |
| 30 | 7.3 | ~0.4 |
| 42 | 8.7 | ~0.4 |
| 50 | 9.5 | ~0.4 |
Frequently Asked Questions
What is hull speed?
Hull speed = 1.34 × √(LWL in feet). It's the theoretical maximum speed of a displacement hull before wave-making resistance dominates. A 30 ft LWL boat has hull speed ~7.3 knots. Planing hulls exceed it.
What is the Froude number?
Froude number Fr = V / √(g×L). At Fr ≈ 0.4, displacement hulls hit the "hump"—wave resistance spikes. 1.34√LWL corresponds to Fr ~0.4. Above that, planing or semi-displacement needed.
Why are displacement hulls limited?
Wave-making resistance increases sharply as the boat approaches hull speed. The stern wave catches up, creating a "wall" of water. Pushing harder wastes fuel; speed barely increases.
What speed does a 30 ft boat make?
30 ft LWL: hull speed ≈ 7.3 knots. 42 ft: ≈ 8.7 knots. The 1.34 constant comes from √(g/2π) in metric, converted. Longer = faster displacement limit.
Can boats exceed hull speed?
Planing hulls (powerboats, some sailboats) rise on the water and exceed hull speed. Displacement hulls (trawlers, most sailboats) are limited. Semi-displacement hulls can push past with more power.
What is LWL?
LWL = Length Waterline—the length of the hull at the waterline. Shorter than LOA (overall length). Use LWL for hull speed; it's the length that "makes" the wave.
Key Statistics
Official Data Sources
⚠️ Disclaimer: This calculator is for educational purposes only. Actual hull speed varies by hull shape, load, and conditions. Consult naval architects for design decisions. Not professional marine advice.