Where the number comes from
Search any boating forum for waterjets and you will find the same warning: a jet costs you about 30 percent of your horsepower. The number is real, but it belongs to a specific machine: the outboard jet conversion. When an outboard motor is converted to a jet, the propeller gearcase is replaced with a compact jet lower unit, and the manufacturer re-rates the package. A 150 HP powerhead is sold as a "105 jet". A 115 HP powerhead becomes an "80 jet". The gap between those numbers is roughly 30 percent, and it is printed on the cowling, so it spread.
That re-rating describes the equivalent propeller performance the converted outboard delivers, with a small general-purpose pump grafted onto a leg that was designed for a propeller: a compromise intake angle, a compact impeller running far from its ideal loading, and one fixed geometry asked to cover every engine and hull the unit might meet. The 30 percent is a property of that conversion, not a law of waterjet physics.
An inboard axial flow jet is a different machine
An inboard axial flow waterjet is not an accessory bolted where a propeller used to be. It is the propulsor the boat is designed around: a straight intake duct shaped for the hull, an axial impeller matched to the engine, a stator to recover swirl, and a nozzle sized for the jet velocity the application needs.
The key point is power absorption. The impeller diameter, trailing edge pitch and shaft speed set how much torque the pump absorbs. Sized correctly, the pump absorption curve crosses the engine power curve exactly at rated power and rated rpm, so the engine works at its design point and delivers its full rated power into the water. Nothing is written off at the transom. This is why Agilis sizes the impeller pitch and nozzle for the specific engine of each installation rather than selling a fixed catalogue step.
Where a jet actually loses energy
Every propulsor loses energy, and honest comparison happens at the level of overall propulsive efficiency at the speed you actually run. In a waterjet the losses sit in three places: hydraulic losses in the impeller, stator and nozzle, losses in the intake duct, and the residual kinetic energy that leaves in the jet stream.
A propeller has its own account: appendage drag from the shaft, strut and rudder, which a flush intake removes entirely, and steeply falling efficiency when it ventilates or cavitates. The balance depends on speed. Below planing speeds a well matched propeller is generally more efficient. As planing speed rises the comparison narrows, and at higher planing speeds a well designed jet becomes fully competitive while keeping the practical advantages that justify it in the first place: no exposed rotating parts, full thrust available at zero speed, shallow water operation and precise low speed control.
For Agilis drives, published performance predictions come from our parametric pump model and the Savitsky planing method. They are model data, stated with tolerances. We publish them that way deliberately: a stated method with stated tolerances is worth more than a slogan percentage.
What to take away
If someone quotes the 30 percent rule, ask which machine they mean. For outboard jet conversions the re-rating is real and printed on the unit. For an inboard axial flow jet sized to its engine, the engine delivers its rated power into the pump, and the meaningful question becomes propulsive efficiency at your cruise and top speed, on your hull, at your displacement.
That question has a numeric answer for your specific boat: enter the hull and engine in our configurator and see the indicative prediction for an Agilis drive sized to your installation.