Boat Speed Calculator
Estimate your boat's maximum speed using Crouch's formula and its theoretical hull speed based on waterline length.
Understanding and Estimating Boat Speed
The Boat Speed Calculator is a comprehensive tool for boaters and marine enthusiasts to estimate a boat's performance. It calculates two key metrics: the theoretical maximum speed for planing hulls using Crouch's formula, and the classic hull speed for displacement hulls. This provides a fuller picture of a boat's potential performance based on its design and power.
🛥️ How to Use the Calculator
To estimate your boat's speed capabilities:
- Enter Engine Power (HP): Input the total horsepower of your engine(s).
- Enter Boat Displacement: This is the weight of your boat. Enter the value and select the unit (lbs or kg).
- Enter Waterline Length (LWL): Input the length of the hull that is in the water.
- Select Hull Type: Choose a hull type from the dropdown. This selects a "Crouch Constant" (C), which is crucial for estimating the maximum speed of planing hulls.
- Calculate: Click the "Estimate Speed" button.
The calculator displays three results: the estimated Maximum Speed (from Crouch's formula), the theoretical Hull Speed, and the Speed/Length Ratio (SLR).
Two Key Speed Concepts
1. Hull Speed (for Displacement Hulls)
As a displacement hull boat moves, it creates a wave. The length of this wave is tied to the boat's speed. Hull speed is the point where the bow wave's length equals the boat's waterline length. Exceeding this speed requires immense power as the boat tries to climb its own bow wave.
Hull Speed (knots) = 1.34 × √LWL (in feet)
2. Maximum Speed (Crouch's Formula for Planing Hulls)
Planing hulls are designed to rise up and skim on top of the water, breaking free of the hull speed limitation. Crouch's formula provides a good estimate for the top speed of these types of boats.
Max Speed (mph) = C / √(Displacement (lbs) / SHP)
- C (Crouch Constant): An empirical value based on the hull's design and efficiency. A higher C value indicates a more efficient, high-performance hull.
- Displacement: The weight of the boat in pounds.
- SHP (Shaft Horsepower): The actual power delivered to the propeller, which is typically slightly less than the rated engine HP due to drivetrain losses. This calculator uses an average loss factor to estimate SHP from your engine HP input.
3. Speed/Length Ratio (SLR)
SLR is a dimensionless number that helps classify a boat's performance type. It relates the boat's speed to its waterline length.
SLR = Speed (knots) / √LWL (in feet)
- SLR < 1.34: Displacement Mode (moving through water).
- SLR 1.34 - 2.5: Semi-Displacement / Semi-Planing Mode.
- SLR > 2.5: Planing Mode (skimming on top of water).
💡 Frequently Asked Questions (FAQ)
- Which speed is more relevant for my boat?
- If you have a sailboat, trawler, or heavy cruiser (a displacement hull), the Hull Speed is your practical maximum. If you have a speedboat, runabout, or modern cruiser (a planing hull), the Maximum Speed from Crouch's formula is a better estimate of your top speed potential.
- Why is the calculator result an estimate?
- These formulas provide excellent theoretical estimates but do not account for real-world factors like wind, water conditions (chop), propeller efficiency, boat load (extra gear and passengers), or the cleanliness of the hull. These factors can all affect a boat's actual top speed.