At Impros, we take the responsibility of educating people about the many variables of PWC impellers very seriously. Quite simply, the more you understand them, the better decision you will be able to make when you decide to have yours modified, repaired, or replaced. We want you to completely understand the options you have, so when the time comes, you will have a good idea of which style and pitch is best for your application. The following are some basic, yet important factors that we feel you should know.
Weight
Each watercraft is unique in hull design and horsepower. Some are designed to carry 1 person, and today, there are some available that can carry up to 4 people and a lot of cargo. With a varying amount of riders and cargo, the weight being carried on some craft can fluctuate to more than 500 pounds. The more weight being carried, the more bottom end you will need to get up on plane at a faster rate. Although each craft is equipped with a good all-around performing impeller, heavy loads make it harder for the engine to respond as quickly. We can usually adjust the impeller to accommodate your particular situation. Pulling a water skier or knee boarder is a good example of when additional low-end thrust is beneficial. The desired performance with larger craft used for recreation is usually more bottom-end power, which can be achieved by reducing the pitch of the impeller, thus limiting the volume allowed to pass through the pump. Sometimes we can also squeeze a little speed out of a craft carrying substantially less weight than it was designed to by raising the pitch of the impeller. The bottom line is, if you normally carry more or less weight than the craft was designed for, Impros can probably help.
Desired Performance
When watercraft and impeller manufactures design an impeller, their goal is to reach the absolute best overall performance and efficiency possible for the range of PWC pumps that the impeller applies to, and provide you with both bottom-end and top speed without having to compromise. Unfortunately, that is not always the case, the OEM impeller is designed to accommodate a wide variety of users, so they usually make a good middle-of-the-road impeller. 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 may also decide to add more horsepower, if so, you will get to a point when that great impeller is simply not aggressive enough to provide the added performance to match the new power. So be careful when looking at the numbers used for pitch, they can be deceiving. Take a look at the Q&A section of our site, where you can find a lot more information regarding impellers and how to make the right choice.
Horsepower
With the many different manufacturers and models of craft available today, there are also different size motors. A common assumption in this industry is, the more horsepower you have, the higher pitch impeller needed. That is not necessarily the case, when an impeller is designed. The amount of horsepower is figured in with the weight, peak rpm level, and many other factors. There are many critical things to consider within the design of an impeller that determine the overall load and RPM the watercraft will have. The pitch of an impeller can be deceiving, because of the many geometric differences in impeller designs, even from one manufacture, there is no correlation in pitch between various impeller models. They do not represent your craft’s potential performance at acceleration and top speed, but is simply the pitch progression of the outer blade angle. For instance, comparing the stock impeller’s pitch to a performance design from Skat-Trak to gauge the aggressiveness of the impeller would be an inaccurate reference. The recommended pitch of each specific design is based on the performance traits of that specific impeller model that is found to be most efficient on each craft, with various horsepower ratings in mind. The relationship between the impeller and pump exit nozzle diameter is also critical to the performance of the watercraft. All of the impeller recommendations are based on each craft’s stock nozzle diameter. We test many impellers on a wide variety of watercraft, so we can inform you of the best possible recommendation available.
Altitude
Altitude can play a major role in watercraft performance. Considerable changes in elevation merit having Impros reduce the pitch of the existing impeller to allow the motor to spin up to its peak level. Using the impeller that was designed for sea level performance restricts the motor, because the lack of oxygen to your engine will not allow it to run at it’s peak performance combined with the sea level impeller. If you are higher than 4000 feet above sea level, we recommend that you choose 1 pitch combination lower than what the recommendation charts generally list. This will allow your engine’s Rpm’s to come up quicker and higher than that of the original impeller in stock form, or the normal recommended impeller.
Here is a great article written by Kevin Cameron that also provides a good understanding of the Jet pump’s part in moving your watercraft.
JET PUMP FUNDAMENTALS…
To better understand how impellers work, we must examine the jet-pump, because the impeller by itself will only scatter water, and is highly inefficient. The current state of impeller development is somewhat evolutionary, as opposed to revolutionary. But still one fact remains, a ducted propeller (shrouded) produces greater efficiency than its open counter-part. The reason is simple. 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, or the push/pull concept. As a blade rotates, it pushes water back (and outwards due to centrifugal force). At the same time, water must rush in to fill the space left behind the blade. 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 action, of course, 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 stators (straightening vanes) to “true” its trajectory. Stators also increase velocity by “catapulting” water, similar to the way a “kick” works on the trailing edge of a blade. To further enhance velocity, water passes through the venturi before finally exiting the pump as thrust. The venturi works on the principle that a restriction or reduction in line size will cause water to accelerate if the same volume is to be realized at the other end of the restriction/reduction. This is where you get the “jet” in pumps. Newbie’s think the steering nozzle size is what dictates thrust. The steering nozzle is only to vector or deflect thrust for your direction.
Impeller design and efficiency is strongly linked to the other components that make up the jet-pump, i.e., gullet volumetric area, laminar transition of the intake housing, stator blade area and angle of trajectory, venturi rate of compression, venturi “bowl” area, exiting orifice dimension, mass and weight of the hull, and pump placement or depth within the same.
There have been a variety of impeller designs introduced through-out the development of the jet ski years, and later the personal watercraft era. Many of the designs have good technical merit but in actual application, do not work as effectively as theorized. The following details some of the major leaps forward in impeller design including a few that are not so good…
STAINLESS STEEL:
The first big leap forward was the use of stainless steel in place of aluminum. This decreased the necessary amount of metal needed for strength and thus increased the area available to create thrust. This also decreased the hub diameter making for a larger area in which water could pass through the pump.
OVER-LAPPING BLADES:
The next 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. There comes a point of redundancy with overlapping blades in that increasing blade length much further brings us full circle back into complete convolution. (Leonardo de Vinci)
PROGRESSIVE PITCH:
Smaller pitch gives greater acceleration, but reduces top speed. Larger pitch decreases acceleration, but increases top speed. By combining smaller pitch at the leading edge and transitioning to a larger pitch at the trailing edge, you effectively get the best of both worlds. But progressive pitch has limitations when coupled with over-lapping blades! There comes a point where the leading edge of the blade begins shutting off area to the blade behind it. This becomes more pronounced in a helicoil design.
Progressive pitch technology allows 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. If the leading edge angle is too agressive it creates a paddle effect that “slaps” water as
opposed to transitioning the water along the blade.
If you examine a progressive pitch impeller from the side, you will see the pitch angle of the blade is constant where it is attached to the hub, but the outer edges of the blade are not. This is where the term PROGRESSIVE comes from. The reason this system works is because it connects 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 water is not compressable, this system somewhat emulates compression.
KICKS:
Once water begins acceleration from the leading edge to the trailing edge, it can be catapulted (nominally) to increase velocity. There comes a point of diminishing returns on this as well, i.e., reduced rpm, cavitation, etc.
SWEEPS:
Mariner introduced this design a couple of years ago to the watercraft industry. However, they are not to be credited with the origination of the technology. It has been around for many years in Australian Jet Boat Racing. The design has good technical merit but inherent limitations. 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 (the industry standard), thus increasing laminar separation at the tip. Also, a swept design can offer more blade area that results in more vacuum. Unfortunately, this design is not conducive to progressive pitch, which is far more valuable.
NARROW HUBS:
This dates back to the stainless instead of aluminum theory. A narrow hub allows more water though and gives more blade area for acceleration. (a real no-brainer) What is most valuable about narrow hub designs is the reduction of blow-out at the leading edge of the hub.
COMPOSITES:
You probably never heard of this one. But the product was made available by a pioneer in the jet-pump industry and has strong technical merit if connected to the proper design and material. Composites are lighter, thus allowing faster acceleration and potentially increased RPM’s, due to less rotating mass. American Turbine brought this to the market only to find that the watercraft industry was dominated by names like Skat-Trak and Solas, and nobody paid attention to them, so they pulled out. It’s unfortunate that many new technologies do not find their place in the market because of consumer skepticism and lack of education.
RAKE:
Recently, an unheard of manufacturer, Nu-Jet, introduced an impeller featuring an aggressive rake, with-out overlapping blades. The Nu-Jet Destroyer has 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 may not create the vacuum necessary to keep a personal watercraft traveling at 60 mph glued to the water. Vacuum is the most essential ingredient in jet-pump performance and watercraft handling. With the right pitch, this design could produce greater acceleration and top speed in smooth water, but may limit performance in rough water due to the loss in vacuum. Their ads reference “backlash from over-lapping blades”. In theory, this is true, similar to “blade-slap” with rotors on a helicopter when descending. In a jet pump application, this would only take place if the volume available between the blade-face and the blade-back at the entrance… exceeded that area available at expulsion. The rake of the Nu-Jet impeller is so aggressive that it would be impossible to have overlapping blades given the hub length available.
NEW TECHNOLOGY…
Aftermarket impeller manufacturers are somewhat limited in what they can develop. Their primary goal is to make impellers that offer better performance than the impeller included with each new watercraft. Most of the O.E.M.’s have included some form of stainless and or progressive pitch impeller that was chosen to give the best all around performance for a given craft, based on the torque available and the rpm produced by a given powerplant. In most cases, 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.
However, current generation jet pump configurations and placements are the real limiting factor. When the leading transportation manufacturers begin incorporating more advanced pump designs, i.e., dual-stage axial flow pumps, surface piercing jet pump drives, variable geometry venturis, vacuum enhanced intake gullets, and some of the other technologies that we pioneered into production vessels, we will begin to see an entirely new era of impeller designs, sizes, materials, applications and results.
A more important area to examine for now, regarding current generation impellers, has not been addressed by any of the manufacturers. Here are some of those areas…
1.The pressure differential between the area located immediately in front of and directly behind the leading edge of every blade at the root. This problem manifests itself in the form of cavitation burns in this area.
2.Controlling hydraulics where the outside edge of the blades meet the inner liner wall. Remember, water is not just forced backwards on a blade, but travels outwards as well. It is the impact of water against the inner liner wall that substantially reduces the over-all efficiency of current single stage axial-flow pumps.
3.”TRUE” Variable Pitch Impellers. This can accomplished via mechanical means and activated by variables in hydrodynamic pressure. Centrifugal force can activate blade rotation and hydrodynamic pressure can control the angle of attack.
It is also possible to create a true variable pitch impeller with no moving parts. The future of pump and impeller design is in REFLEX TECHNOLOGY. Military research programs developed these materials and we’ve worked with them extensively. While they are still proprietary technologies, and thus regulated and protected at this date, there are a variety of future applications in commercial venues.
