[YachtForums]

I just found an article I wrote on an old CD that might be a good reference for this thread. It's generic in nature and repeats some of the things I've already posted, but it's some good reference material. Here it is...

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IMPELLERS: DESIGN & DEVELOPMENT
by Carl Camper

Impellers, better known as propellers when unshrouded or not placed within a duct, are a product of an extensive evolutionary process. Original conceptions date back to Leonardo da Vinci's aerial screw and subsequent applied dynamics for marine use. Current generation impellers are a combination of the Archimedian screw (similar to a helicoil) and Conoidal propeller (sections of the helicoil removed).

There are many facets of impeller design that are critical to producing thrust. The following terms and explanations are listed for reference purposes...

1) LEADING EDGE: That part of the impeller blade nearest the front of the pump.

2) TRAILING EDGE: That part of the impeller blade nearest the rear of the pump.

3) BLADE TIP: The part of the blade nearest the liner or wear ring, or the outside edge of the blade.

4) BLADE FACE: The side of the blade facing the rear of the pump, known as the positive pressure side of the blade.

5) BLADE BACK: The side of the blade facing the front of the pump, known as the negative pressure side of the blade.

6) BLADE ROOT: The point at which the blade attaches to the hub.

7) HUB: The center of the impeller that fits over the drive shaft.

8) PITCH: The theoretical travel of the impeller through a mass per revolution.(usually measured in inches)

9) STRAIGHT PITCH: The pitch is constant from the leading edge to the trailing edge of the impeller.

10) PROGRESSIVE PITCH: The pitch increases from the leading edge to the trailing edge.

11) VARIABLE PITCH: The pitch increases from the leading edge to the trailing edge, and from the hub to outer tip.

12) RAKE: The angle of the impeller blade in correspondence to the impeller shaft or hub.

13) PARABOLIC RAKE: The off-center development of a concave area on the blade.

14) DIAMETER: The overall width of the impeller from blade tip to blade tip.

15) ROTATION: Clockwise or Counter-Clockwise.

16) OVERLAPPING BLADES: The amount of blade surface covered or hidden by another blade when viewed from the front or rear of the impeller.

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

18) SWEEP /SKEW: The radius of the leading edge in relation to the hub.

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

20) NUMBER OF BLADES: This is self-explanatory.

21) BLADE THICKNESS: This is also self-explanatory.

22) ANGLE OF ATTACK: This is a line relevant to the surface of the water and the angle the hull is achieving on plane.

23) BLADE LENGTH: The distance from the leading edge to the trailing edge.

24) BASE ANGLE: The pitch angle of the blade where it meets the hub.

25) SURFACE AREA: The total amount of surface available per blade, measured from the leading edge to the trailing edge and from the hub to the outer edge of the blade.

Several terms relevant to impeller technology:

1) STATORS: The directing vanes located within the pump immediately aft of the trailing edge of the impeller that re-direct spiraling flow into straighter trajectory.

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

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

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

5) CAVITATION: The separation or implosion of air and water and the heat associated.

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

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

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 yaw 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 years. 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 decreases the hub diameter making for a larger blade area and decreases the blade thickness for more volume between the blades.

OVER-LAPPING BLADES: The next big step was over-lapping blades, which gave an increased blade area to accelerate more 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.

Specific pitch numbers of impellers vary from one company to another and some of the new designs make the transition numbers irrelevant, i.e., narrow hub and high rake impellers. There is no scale factor applicable because of the current state of evolution, and the lack of accurate pitch interpretation amongst the different manufacturers.

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: An American company introduced this design a couple of years ago, 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/bronze 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 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, introduced an impeller featuring an aggressive rake, with-out overlapping blades. The design 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 this 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 most of the O.E.M.’s, which are now using 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.