Jet Boat Speed Calculator
The jet boat speed calculator estimates the maximum top speed of your jet-driven vessel. By utilizing a modified version of Crouch's formula, it calculates the relationship between engine horsepower, total gross weight, and the hydrodynamic efficiency of your specific hull design.
Understanding Jet Boat Performance
Calculating the speed of a jet boat requires a slightly different approach than calculating standard prop-driven vessels. Jet drives inherently experience a 20% to 30% power loss due to the friction and mechanics of pumping water through an impeller and stator inside a confined tube.
The Math: Crouch's Formula
This calculator uses Crouch's Formula: Speed = Constant × √(Horsepower / Weight). Because of the aforementioned jet pump power loss, the hydrodynamic "Constant" (C) must be adjusted downward compared to an outboard or sterndrive boat of the same size.
- Weight is critical: Jet boats are incredibly sensitive to weight changes. Adding four adult passengers to a 21-foot jet boat can easily scrub 5–8 MPH off the top speed, as the pump has to push more hull out of the water to maintain a plane.
- Cavitation matters: This calculator assumes your impeller and wear ring are in perfect condition. A worn wear ring allows water to bleed past the impeller blades, drastically reducing thrust and lowering your real-world top speed.