Hull Speed and Speed-Length Ratio
Hull speed is not really a ratio. It is a speed, computed from one measurement: waterline length. But it is one of the most useful numbers to understand because it sets the practical ceiling for ordinary displacement sailing.
You'll see it in broker brochures as the boat's "theoretical maximum," and you'll hear it whenever sailors debate passage speeds. The short version is this: as a displacement hull moves faster, its bow and stern waves get longer. When the wavelength matches the boat's waterline length, the boat is sitting in its own trough. Going faster takes either much more power or a different mode of movement: planing, surfing, or a very long, narrow multihull hullform.
Formula
- LWL — Waterline length in feet
- 1.34 — A constant derived from the physics of gravity waves and unit conversions, not from any empirical fit
Restated as a ratio:
When SLR = 1.34, the boat has reached hull speed. SLR > 1.34 means the boat is in semi-planing or planing mode (or is otherwise breaking displacement-mode physics).
Where the 1.34 comes from
The 1.34 constant comes from wave physics, not from a rule of thumb. The length of a free-running ocean wave is governed by gravity:
where V is wave speed (in ft/s) and g is the acceleration due to gravity (32.174 ft/s2). These are called gravity waves because gravity sets the wavelength.
Rearranging:
That quantity, V / √gL, is the Froude Number — a dimensionless ratio invented by British naval architect William Froude in the 1870s. Froude discovered that ship resistance, when properly scaled, depends on this number — which is why model testing in towing tanks works.
Through tank testing, Froude showed that resistance rises sharply when a boat's waterline length matches the length of its own bow wave. At that point, the boat is trying to climb over a wave as long as itself. That is hull speed.
To convert Froude Number back to the more practical "knots and feet" form, you multiply by the speed unit conversion (6076 ft per nautical mile, 3600 seconds per hour):
The constant 1.34 is not tuned to a particular boat. It falls out of physics and unit conversion, which is why it has stayed useful across decades — as long as you remember what kind of hull it applies to.
What hull speed means physically
At hull speed, the boat is trapped in the trough between its own bow and stern waves, one full wavelength apart. To go faster, the boat must either:
- Generate a longer wave — which requires enormous extra power. Adding a few horsepower to a 30-foot cruiser already near hull speed buys very little extra speed.
- Climb up onto its own bow wave — i.e., plane. This requires a flat, light hull (typically D/L < 100) and dynamic lift from the hull shape. Most cruising boats can't do it.
- Be long and narrow enough to barely make waves at all — multihulls and ultralight monohulls with high L/B. They don't have a meaningful "hull speed" barrier because their wave-making drag never rises sharply.
Beyond hull speed: the three regimes
Speed-length ratio defines three operating modes for any boat:
| SLR | Mode | What's happening |
|---|---|---|
| < 1.34 | Displacement | Boat is moving water out of the way as it goes. Resistance follows the well-understood Froude relationships. |
| 1.34 – 2.5 | Semi-displacement / semi-planing | Boat is trying to lift over its bow wave. Power requirements rise sharply. Many fast cruisers and motorsailers live here. |
| > 2.0 – 2.5 | Planing | Dynamic lift raises the hull onto the water's surface. Resistance drops dramatically, but only if the hull is shaped for it. |
Hull speed is not a brick wall. It is a steep rise in the resistance curve. With enough power, almost any displacement boat can be pushed past it, but the efficiency collapses. Around the barrier, going from 6 knots to 7 knots may require dramatically more thrust.
Caveats and exceptions
Long overhangs cheat hull speed. Older designs with sweeping bow and stern overhangs immerse those overhangs when heeled under sail, extending their dynamic waterline length. A CCA-era cruiser with a static LWL of 28 ft might sail with a dynamic LWL of 31 ft, raising its effective hull speed from 7.1 to 7.5 knots. Modern plumb-bow boats don't have this trick available.
Multihulls don't obey it in the same way. Long, narrow individual hulls make much smaller waves. A racing trimaran with an L/B of 16:1 does not see the same wave-making barrier a beamy monohull does. Performance cats and trimarans routinely sail at SLRs of 2 to 4.
Surfing exceeds it. A boat can briefly exceed hull speed by being pushed forward by a passing wave. Surf speeds of 10–14 knots are common in 40-footers on big downwind passages — but the average over the ride is still set by hull speed.
Hull speed isn't average speed. Don't expect a boat to cruise at hull speed — that's the theoretical maximum in displacement mode, not the realistic passage-making average. A typical sailing-yacht passage averages 60–75% of hull speed when conditions cooperate.
Reading the number as a buyer
You do not need to internalize Froude numbers to use hull speed. If a listing gives you hull speed — or you calculate it from LWL below — treat it as a ceiling for ordinary displacement-mode sailing, then plan your passages below that number.
What hull speed predicts (and what it doesn't):
- It's a ceiling, not an average. A boat with hull speed 7.5 kn may touch 7.5 kn in good conditions, but it might average 4.5–5.5 kn over a multi-day passage. A practical cruising estimate is often 60–75% of hull speed, less for slow days or short-handed sailing.
- It only applies in displacement mode. A planing boat (D/L well under 100) can blow through hull speed entirely. A surfing boat can briefly hit double hull speed down a wave face. A multihull with very high L/B simply doesn't see the barrier in any practical way.
- It scales with the square root of LWL. That's why a 40-ft LWL boat (hull speed 8.5 kn) is only ~25% faster than a 25-ft LWL boat (hull speed 6.7 kn), despite being 60% longer.
How to use it for buying decisions:
- Plan passages with the average, not the ceiling. A 35-ft LWL boat (hull speed 7.9 kn) averaging 70% = 5.5 kn × 24 hours = ~130 nm/day. Pick your routes accordingly.
- Compare on LWL, not LOA. Two 38-foot boats can have very different LWLs (one with traditional overhangs at 28 ft, one with a plumb bow at 36 ft). The plumb-bow boat has a meaningfully higher ceiling.
- Spot exaggerated claims. Any cruising monohull claiming to "regularly cruise at 9 knots" with a 25-ft LWL is exaggerating, planing, or briefly surfing. Hull speed at 25 ft LWL is 6.7 kn.
- For older designs, factor in dynamic LWL. A CCA-era cruiser with a 28-ft static LWL might sail with 31 ft of waterline under heel — that's 7.1 → 7.5 kn of effective hull speed.
A quick example. A Catalina 22 (LWL 19.3 ft) and a Beneteau Oceanis 46.1 (LWL 43.4 ft) live in different speed worlds because of waterline length alone: 5.9 kn vs 8.8 kn theoretical hull speed. Over a 24-hour passage at 65% of hull speed, the Catalina covers about 90 nm; the Oceanis covers about 138 nm.