Impellers 101

PWC and Jet boat impellers are a little more complicated to understand than a regular boat propeller. Below is some information to help you better decide whether to buy new a impeller for your craft, or to simply have yours modified or repaired.

First, let’s take a look at the jet-pump and the part it plays with your craft’s performance. The impeller by itself will only scatter water, and is highly inefficient. A ducted impeller produces greater efficiency than its open counter-part (propeller). The duct controls water and forces it backwards as opposed to a propeller which allows water (or air) to slip outwards. Impellers (and jet-pumps) work on the principal of positive and negative pressure. As the blades rotate, they push water back (and outwards due to centrifugal force). At the same time, water must rush in to fill the space left behind the blades. This results in a pressure differential between the two sides of the blade: a positive pressure, or pushing effect on the blade face and a negative pressure, or pulling effect, on the blade back. This occurs on all the blades around the full circle of rotation. Thrust is created by water being drawn into the impeller and accelerated out the back. However, due to the spiraling effect (vortex) of water leaving the trailing edge of the blade it must pass through straightening vanes (or stators as some call them) to "true", or “straighten” its trajectory. The vanes also increase velocity by "catapulting" water, similar to the way a "kick" works on the trailing edge of a blade. After the water leaves the vane section, it then travels through the venturi (or nozzle) before finally exiting the pump as thrust. The venturi is reduced in size and causes the water to accelerate. This is where you get the "jet" in pumps. The steering nozzle then deflects thrust for steering the craft.

Aftermarket impeller manufacturers primary goal is to make impellers that offer better performance than the impeller included with each new watercraft. While the O.E.M. Watercraft manufacturers now all use some form of stainless and or progressive pitch impeller, they are still trying to find an ideal balance for their customers, giving them acceptable acceleration, as well as competitive top speed out of their craft. This leaves options for companies like us to custom tune an impeller to suit our customers desires. Additionally, engine modifications will dictate the need for an impeller better matched for the torque and rpm available from said modifications. This will result in increased speed and/or acceleration in most circumstances. Here are a few of the most common variables that affect the performance of jet boats & personal watercraft.

Weight
Every PWC and Jet boat is unique in hull design and horsepower. Most people with a larger PWC or boat want more bottom-end power to get up on plane faster. Although the manufactures try to equip each craft with a good all-around performing impeller, heavy loads make it harder for the engine to respond as quickly. We can adjust your impeller to change how your craft performs, or we can help you decide which aftermarket impeller will best suit your needs. In some cases, we can also pitch the impeller to gain a couple of mph out of a craft.

Desired performance
The OEM impeller is designed to accommodate a wide variety of users. Every craft is a little different, as is the riders' performance desires, you may have a great performing impeller in the craft now, but as stated in the Weight section above, you may want to squeeze every bit of top speed or bottom end out of it. You might also decide to increase the horsepower. If so, you will likely need to either have your impeller repitched or buy a different pitch impeller

Horsepower
A common idea is that the more horsepower you have, a higher pitch impeller is needed. That is not necessarily the case because you’ll get maximum performance if you allow the engine to rev at the engine RPM where maximum horsepower is produced. Many engine “tunes” actually produce max horsepower at a higher RPM than stock, so you’ll need a slightly lower pitch impeller to allow the engine to rev to that level. We have tested many impellers on a wide variety of watercraft, so we can help you with the best possible impeller recommendation available.

Altitude
A lot of people that have always ridden their ski/boat at high elevation lakes don’t realize how much performance they’re giving up by not having the ideal impeller that is pitched for their boat at the lakes they play at! The elevation of lakes at a higher altitude plays a major role in watercraft performance. Lakes even as low as 1500’ will affect the engine’s horsepower. Certainly, if you play at lakes up at 4-6000’, you’ll want to either have your impeller repitched to accommodate the change, or contact us for our recommendation for an ideal performance impeller. Using the impeller that was designed for sea level performance essentially limits the engine’s RPM, and produces poor performance compared to the same craft run at sea-level. Bottom line for people in Utah, Colorado, and many other states with higher elevation lakes- allowing your engine to rev at the ideal RPM really makes a huge difference in performance- It's like a whole different watercraft!

Stainless steel: The first and arguably, the biggest gain in efficiency was the use of stainless steel in place of aluminum. The more efficient thinner blades, and superior strength increased the area available to create thrust.

Overlapping blades: Another giant big step was over-lapping blades, which gave an increased blade area to accelerate water while increasing vacuum, critical to bringing more water up into the gullet and thus producing more thrust. We’ve found that more overlap is ideal for improved “hook-up”, while less overlap has proven better top speed numbers. Of course, there’s a balance with both designs. The trick is finding maximum speed with an impeller, while still maintaining efficiency (hook up) in rougher conditions.

Progressive pitch: Smaller pitch gives greater acceleration, but reduces top speed. Larger pitch decreases acceleration, but increases top speed- to a point. Many people think that the leading edge pitch represents bottom end, and the trailing edge pitch represents top speed. But it’s just not as easy as that. You’re still limited by the volume the “opening windows” allow into the pump, so by dropping pitch, you’re limiting volume, which limits the ability to build as much thrust and therefore top speed. A progressive pitch impeller (literally all of the current impeller designs) allow the impeller to grab a given mass of water per blade at a given pitch angle (the lower pitch number) and transition it into a more aggressive pitch (the larger number). This concept works much like a catapult. At the same time, a smaller pitched leading edge reduces laminar separation due to a lower pitch angle. Laminar separation results in cavitation, or the separation of air from water. A larger pitched leading edge can grab too much water, thus over-loading the engine and reducing acceleration. Most progressive pitch impellers root angle (where the blade attaches to the hub) is constant, but the outer edges of the blade are not. This is where the term “progressive” comes from. This system is made up of three basic principles- acceleration, centrifugal force and velocity. As water enters the leading edge of the blade, it is accelerated. During transition to the trailing edge, the constant chord of the blade near the hub and the increasing size of the hub, work with centrifugal force to push (and pull) water toward the outside edge of the blade. This results in a collective action that increases the velocity of the water exiting the blade. Although you cannot compress water, this design sort of emulates compression.

Kicks: This is a drastic increase of pitch at the blades trailing edge. Water is catapulted to increase velocity, and has shown improvements in “hook up”. Because you are essentially increasing pitch, it does affect engine RPM, so you may need to reduce oversall pitch when adding a kick.

Radial Edge Design: Most every recent impeller design is made with this idea in mind. The radial edge, or sometimes known as a Swirl, Mariner reactor and Hooker design has been around for many years in Australian Jet Boat Racing. A swept leading edge will slice through water, reducing cavitation, as opposed to a straight, perpendicular to the hub leading edge that "chops" through water, thus increasing laminar separation at the tip. Also, a swept design can offer more blade area that results in more vacuum.

Rake: In the ‘90’s, an impeller manufacturer called Nu-Jet (now defunct), introduced an impeller featuring an aggressive rake, with-out overlapping blades. The Nu-Jet Destroyer had merit, similar to a chopper prop in outboard performance circles, but with the outer edge of the blade cut-off. Without overlapping blades, this impeller did not create the vacuum necessary to keep a personal watercraft traveling at 60 mph glued to the water unless you were in ideal, smooth water conditions. With the right pitch, this design produced impressive acceleration and top speed in smooth water, but lost performance in rough water due to the loss in vacuum. The rake of the Nu-Jet impeller was so aggressive that it was basically impossible to have overlapping blades, to gain the vacuum needed to make it a successful design.

Here are some more terms that are relevant to pump and impeller technology for your reference...

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 outside diameter, or part of the blade nearest the liner or wear ring.

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

Blade back: The side of the blade facing the front of the pump, aka the negative pressure side of the blade.

Blade root : Where 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.

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

Gullett: 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 (the blades are attached to it).

Impeller: A propeller that resides in a pump housing and creates thrust. Current generation impellers are a combination of the Archimedian screw (similar to a helicoil) and Conoidal propeller (sections of the helicoil removed).

Intake grate: A grill 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: The veined, cylindrical component which directs the flow of thrust created by the impeller.

Leading Edge: The part of the impeller blade nearest the front of the pump. Also a common term for the best, and coincidentally the name of this company :)

Mixed flow: This design has only been used in a few craft, like the older Kawasaki JS 550, and Ultra 150. It is a pump with 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 by another blade when viewed from the front or rear of the impeller.

Pitch: On a boat propeller, it is the theoretical travel of the impeller through a mass per revolution. This is not accurate with PWC impeller designs. It is simply 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. As of 2023, almost all impellers are made this way.

Pump nozzle: A cone-shaped device attached to the back of the jet pump that pressurizes the flow of water.

Ride plate: A cover for the bottom side of the pump, and the craft "rides" on this plate.

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

Stator vanes: The straightening vanes located immediately behind the trailing edge of the impeller that re-direct spiraling flow into a straighter trajectory.

Straight pitch: The pitch is constant from the leading edge to the trailing edge of the impeller. Rarely used anymore.

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: The part of the impeller blade 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.

Ventilation: The induction of air into the intake gullet due to excess speed or lift, or an air leak (e.g. through the carbon ring seal, ride shoe, hull penetration), thus breaking vacuum.

Venturi: Another name for the nozzle, that compresses (accelerates) water to a greater velocity.