How accurate is Crouch's Formula calculation?

Crouch's Formula provides a theoretical estimate based on power-to-weight ratio. It is generally accurate within ±10% for planing hulls but varies significantly based on hull efficiency and water conditions.

What is reliable propeller slip percentage?

Typical propeller slip ranges from 10-15% for efficient planing hulls. Heavy loads or fouled bottoms can increase slip to 20% or more. High-performance racing boats may achieve slip as low as 5-8%.

What is Crouch's Constant?

Crouch's Constant (C) represents the hull efficiency. Higher values mean a faster, more efficient hull. Displacement hulls typically range from 150-180, while high-performance racing hulls can exceed 280-300.

Boat Speed Calculator

Estimate your boat's top speed using two industry-standard methods: Power-to-Weight Ratio or Propeller RPM.

Boat Details

Boat Type

Automatically sets Crouch's Constant (C). Typical range applied.

Crouch's Constant (C) - Hull Efficiency240

Heavy (100)Racing (350)

Adjust manually if needed. Higher number = more efficient hull.

Total Boat Weight (lbs)

Total Shaft Horsepower (HP)

Display Units

Estimated Top Speed

57.4MPH

Speed Range53 - 62 MPH

Based on Crouch's Constant range for Planing Hull.

Power-to-Weight Ratio

17.5 lbs/hp

Results are estimates and vary due to water conditions, hull condition, and load distribution.

Method 1: Power-Based (Crouch's Formula)

Best for estimating the potential top speed of a boat based on its design and power. This method uses Crouch's Formula, a widely accepted empirical formula in naval architecture.

Speed (MPH) = C ÷ √(Weight ÷ HP)

It relies on Crouch's Constant (C), which represents hull efficiency. Lighter, faster hulls have higher constants, while heavier displacement hulls have lower ones.

Method 2: Propeller-Based (RPM & Pitch)

Best for finding the theoretical mechanical speed of your drivetrain. It calculates how fast the prop would move through a solid solid, then subtracts "Slip" to account for water efficiency.

RPM: Engine revolutions per minute.
Pitch: The distance a propeller would move forward in one revolution through a solid.
Slip: The percentage of lost distance due to fluid dynamics (typically 10-20%).

Typical Crouch's Constants

Boat Type	Typical Range (C)	Description
Displacement Hull	150 - 180	Heavy cruisers, trawlers, sailboats
Semi-Displacement	180 - 220	Downeast boats, medium cruisers
Planing Hull	220 - 260	Runabouts, center consoles, bowriders
Racing Hull	260 - 300+	Hydroplanes, bass boats, racing cats

Why do these results differ?

The Power-based calculator gives a theoretical maximum based on physics estimates. The Prop-based calculator looks at mechanical limits. Real-world speed is affected by wind, current, hull cleanliness, and load distribution. Always treat these numbers as estimates.

Slow Down for Safety

High speeds are only safe when your boat is properly loaded. Check if your current passenger and gear load is within USCG safety limits.

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