Glossary of Terms

 
There are many facets of impeller design that are critical to producing thrust.  Here are several terms and explanations relevant to impeller technology for your reference purposes... 

Angle of attack: This is a line relevant to the surface of the water and the angle the hull is achieving on plane. 

Axial flow: A jet pump that pushes the water out in line with the pump centerline. Guide vanes are used to straighten the flow of water from the impeller, and eliminate torque reaction which can cause the craft to roll during acceleration.

Blade tip: The part of the blade nearest the liner or wear ring.

Blade face: The side of the blade facing the rear of the pump, known as the positive pressure side of the blade. 

Blade back: The side of the blade facing the front of the pump, known as the negative pressure side of the blade.
 
Blade root: The point at which the blade attaches to the hub.

Cavitation: The separation or implosion of air and water and the heat associated. This creates cavities or pockets of air, damaging the impeller, pump and performance of the craft.

Cup: A radius on the blade face at the outside edge of the blade that controls deflection and accelerates water.

Diameter: The overall width of the impeller from blade tip to blade tip.

Driveshaft: The shaft that connects the engine to the jet-pump and transfers torque to the impeller.

Gullet: The area forward of the impeller known as the intake housing to channel water toward the impeller via vacuum.

Hub: The center of the impeller that fits over the drive shaft.

Impeller: A propeller that resides in a pump housing and creates thrust for a personal watercraft. Impellers, better known as propellers when unshrouded or not placed within a duct, are a product of an extensive evolutionary process. Current generation impellers are a combination of the Archimedian screw (similar to a helicoil) and Conoidal propeller (sections of the helicoil removed).

Intake grate or scoop grate: A grill-like component that mounts on the bottom of a hull and feeds water to the jet pump while also preventing foreign objects from entering

Intake stuffer: A funneling device that mounts in the intake tract to increase velocity

Jet pump or pump: The veined, cylindrical component which directs the flow of thrust created by the 
impeller.

Kick: The area nearest the trailing edge of the blade that adds more pitch relative to its original chord.

Leading edge: That part of the impeller nearest the front of the pump.

Mixed flow: A pump with an impeller in a tapered housing, where water flow has some incline to the impeller access. This adds centrifugal force to the water as it is pressurized by the water. The pump housing collects the pressurized water and directs it aft in a high-speed stream.

Overlapping blades: The amount of blade surface covered or hidden by another blade when viewed from the front or rear of the impeller.

Parabolic rake: The off-center development of a concave area on the blade.

Pitch: The theoretical travel of the impeller through a mass per revolution. Common with boat propellors, but not as accurate with PWC impeller designs. This is the actual blade angle measured along the outside diameter of each blade. 

Progressive pitch: The pitch increases from the leading edge to the trailing edge. Most impeller made now are designed this way.

Pump nozzle or nozzle: A cone-shaped device which attaches to the back of the jet pump and pressurizes the flow of water.

Rake: The angle of the impeller blade in correspondence to the impeller shaft or hub.

Ride plate: A flat piece of metal that covers the cut-out for the pump. The craft "rides" on this plate.

Slip: The difference between actual and theoretical travel of a blade and the loss of efficiency created.

Stators: The directing vanes located immediately aft of the trailing edge of the impeller that re-direct spiraling flow into straighter trajectory.

Straight pitch: The pitch is constant from the leading edge to the trailing edge of the impeller. Rarely used with current impellers made.

Sweep/skew: The radius of the leading edge in relation to the hub.

Top loader: A type of intake grate that utilizes a wedge perpendicular to the grate's parallel bars, in order to scoop more water up into the top half of the pump. This loads the pump more evenly.

Trailing edge: That part of the impeller nearest the rear of the pump.

Variable pitch: The pitch increases from the leading edge to the trailing edge, and from the hub to outer tip.

Ventillation: The induction of air into the intake gullet due to excess speed or lift, thus breaking vacuum.

Venturi: The shroud aft of the stators that compresses (accelerates) water to a greater velocity.

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