Boat Theoretical Hull Speed & Displacement Power Calculator
Calculate theoretical maximum hull speed (knots & mph) for displacement hulls from Waterline Length (LWL), Froude number (Fn), and required shaft engine horsepower (SHP) based on displacement tonnage.
Vessel Dimensions & Displacement
Length at the waterline (9.75 m)
Total weight (6.25 Long Tons)
Standard displacement limit: 1.34 (Cruising: 1.0–1.2)
Theoretical Hull Speed
Fn = 0.40 (Bow Wave Trap)
7.58 Knots
Equivalent to 8.72 mph (14.04 km/h)
Economical Cruising Speed (S/L = 1.10)
6.22 Knots
Saves ~50% fuel vs maximum hull speed
Required Shaft Horsepower (SHP)
28.5 SHP
Recommend 35–40 HP auxiliary diesel
Naval Architecture Hydrodynamic Metrics
Displacement-to-Length Ratio (D/L):
191 (Moderate / Medium Cruiser)
Froude Number (Fn):
0.40 (Theoretical displacement limit)
Bow Wave Wavelength:
32.0 ft (Equal to LWL at hull speed)
Frequently Asked Questions
Can a displacement boat ever exceed its theoretical hull speed?
A true heavy displacement hull cannot significantly exceed $1.34 imes sqrt{LWL}$ without surfing down large ocean swells. However, ultra-narrow multihull hulls (catamarans with beam-to-length > 1:12) produce minimal bow waves and easily exceed traditional hull speeds.
Why do passagemaking trawlers cruise at S/L = 1.1 instead of 1.34?
The power curve between S/L 1.1 and 1.34 is extremely steep. Dropping from 7.6 knots to 6.2 knots cuts required horsepower and fuel burn in half, doubling the vessel's cruising range.