Calculator Inputs
Example Data Table
| Boat Type | LWL | Coefficient | Target Froude | Estimated Semi Speed | Froude Speed |
|---|---|---|---|---|---|
| Small Cruiser | 28 ft | 1.65 | 0.52 | 8.73 knots | 9.23 knots |
| Coastal Motor Sailor | 36 ft | 1.75 | 0.55 | 10.50 knots | 11.64 knots |
| Long Range Cruiser | 48 ft | 1.95 | 0.62 | 13.51 knots | 15.34 knots |
Formula Used
Classic hull speed: V = 1.34 × √LWL(ft)
Semi displacement coefficient speed: V = C × √LWL(ft)
Froude target speed: V = Fn × √(g × LWL(m))
Speed length ratio: SLR = Speed(knots) ÷ √LWL(ft)
Actual Froude number: Fn = Speed(m/s) ÷ √(g × LWL(m))
Displacement length ratio: DLR = Long tons ÷ (0.01 × LWL(ft))³
The calculator uses g = 9.80665 m/s² and converts metric speed to knots.
How to Use This Calculator
- Enter the boat waterline length.
- Add beam and displacement for ratio checks.
- Choose a semi displacement coefficient.
- Enter a target Froude number for physics-based speed.
- Add a trial speed to see its operating range.
- Set a safety margin for conservative planning.
- Press the calculate button.
- Download the result as CSV or PDF when needed.
Understanding Semi Displacement Speed
A semi displacement hull sits between two boat families. It is not a pure displacement hull. It is not a full planing hull either. At low speed, the boat moves water aside. At higher speed, lift begins to support part of the weight. This mixed behavior makes speed prediction more flexible.
Why Waterline Length Matters
Waterline length controls wave formation. A longer hull creates a longer natural wave. That wave can support higher speed before drag rises sharply. The common hull speed rule uses the square root of length in feet. The usual coefficient is 1.34 for many displacement craft. Semi displacement boats often use higher coefficients, such as 1.5 to 2.2. The exact value depends on hull shape, weight, power, trim, and sea state.
Using Froude Number
The Froude number compares boat speed with wave speed. It is a helpful physics ratio. Low values show displacement behavior. Middle values show semi displacement behavior. High values suggest planing influence. This calculator uses the chosen Froude number and waterline length to estimate a target speed. It also calculates the actual Froude number for a trial speed.
Reading the Results
The result panel gives classic hull speed, adjusted semi displacement speed, Froude target speed, speed length ratio, and speed in several units. These values are planning estimates. They are not a guarantee of performance. Real boats need enough power to reach the calculated speed. They also need correct propeller selection and safe trim.
Practical Use
Use this tool during early design checks. It also helps owners compare possible cruising speeds. Enter a conservative coefficient first. Then test a higher value. Compare both results with known engine power and fuel use. If the predicted speed seems unrealistic, reduce the coefficient or Froude number. Always confirm final choices with sea trials or professional naval advice.
Limits and Safety
No simple calculator can read every hull detail. Hard chines, round bilges, tunnels, appendages, and loading can change drag. Wind and waves also matter. Treat the answer as a structured estimate. Leave margin for fuel, weather, and reserve power. For commercial work, use tank data, builder trials, or a qualified marine designer before making decisions and passenger comfort checks.
FAQs
What is semi displacement hull speed?
It is an estimated speed range for hulls that partly lift while still moving through water. It sits above normal displacement speed but below full planing behavior.
Why does waterline length matter?
Waterline length affects wave creation. Longer waterlines usually allow higher speeds before resistance rises strongly. That is why hull speed formulas use length.
What coefficient should I use?
Use 1.5 for conservative estimates. Use 1.7 to 1.9 for many semi displacement boats. Use higher values only when hull form and power support them.
What is Froude number?
Froude number compares boat speed with gravity wave behavior. It helps show whether the boat is acting as displacement, semi displacement, or planing influenced.
Is this calculator suitable for final design?
No. It is best for planning and comparison. Final design should use detailed resistance data, power curves, propeller checks, and professional review.
Why is trial speed included?
Trial speed lets you test a planned cruise speed. The calculator converts it and shows its actual Froude number and speed length ratio.
Can the result predict fuel use?
No. Fuel use depends on engine load, propeller efficiency, hull condition, trim, wind, waves, and displacement. This tool only estimates speed behavior.
Why add a safety margin?
A safety margin gives a more conservative planning speed. It allows room for loading, rough water, fouling, engine limits, and other real conditions.