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Jet Drive vs. Prop

Discussion in 'Technical Discussion' started by brian eiland, Feb 27, 2006.

  1. brian eiland

    brian eiland Senior Member

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    I did a search on this forum prior to starting this new thread, and I found another subject thread entitled "Jet vs Prop" The problem is most of this discussion was centered around larger yacht systems.

    I seek to get a discussion going of smaller units as utilized on vessels in the 10-30' foot range. I've included a couple of quotes from that other subject thread that might be applicable to these smaller units:

    This initial posting is already longer than I originally planned so I will continue the discussion in a separate posting on a particular item I viewed at the Miami show, the JetPac
  2. AMG

    AMG YF Moderator

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  3. brian eiland

    brian eiland Senior Member

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    JetPac from Sword Technology

    At the Miami show this year I took a look at JetPac, a relatively new patented marine propulsion system. I didn't spend that much time in their booth, but I done a little research since I got home, and I present here some quotes from their literature and website that might deserve more discussion. I would particularly be interested in Carl's observations as he appears to have lots of experience with 'water jets'.
    __________________________
    Excerpts from their paper "A Short Treatise on Design Considerations of the OPS Jet Propulsion System":
    Most engine failures are due to over gearing or overpropping, my estimate is that over 90% of boats are over-propped after delivery, most due to overweight, dirty bottoms or bad center of gravity (“C of G”). Many problems are caused by the owners preference for high speed at lower RPM....

    If the engines have the capacity to be overloaded (as in all prop driven boats) a simple C of G change (like a large load in the front of a bowrider) can grenade the engine in a short period of time, with no verifiable installation fault This results in a full warrantee problem, dissatisfaction, expense and ruined reputation.

    Although we have regularly experienced similar performance with equally powered planing boats, conventional inboard jet boats have a bow down attitude that robs them of a considerable advantage.

    Observing this has made it hard to understand the prevailing opinion that jet drives are less efficient than propellers. This opinion has been accepted as gospel for many years based on an analysis “Hydrodynamics Aspects of Internal Pump Jet Propulsion” (University of Michigan, 1964) H.C. Kim claimed the efficiency from a water jet is far less then that of a well-designed propeller system. Kim’s analysis was even reproduced in the 1988 version of “Principals of Naval Architecture” by the Society of Naval Architects and Marine Engineers (SNAME). This analysis was incomplete and the resulting data misleading.

    A study in 1992 conducted by Naval Architects Donald Blount and Robert Bartee dispelled Kim’s analysis in “Design of Propulsion Systems for High speed Craft” (Marine Technology, SNAME, Oct. 1997). This analysis revealed that a propeller-driven boat will have a hull efficiency of 92%, while the water-jet driven boat will have a hull efficiency of 110% at speeds over 25 knots.

    Normal inboard jets are made to adapt to engines forward of the water jet. This means the jet drive shaft has to be higher than ideal because of the engine crankshaft height. Although jets should be fitted with a reduction to be efficient, most are fitted directly to the engine. If the jet were fitted as close to the bottom of the boat as possible, efficiency would be much higher for these reasons:
    1) Frictional losses on the inlet and outlet would be less, giving greater efficiency.

    2) Jet outlet would be lower on the transom and thrust line would therefore be lowered. (A low thrust line is desirable because it moves the active C of G aft giving less of a nose down attitude to the boat).

    3) The lower thrust line also makes the boat more stable by cutting down the boat teeter caused by directional changes of the nozzle and this would reduce wandering at all speeds.

    4) Inlet size would be reduced; this would enhance the efficiency of the boat by reducing the hook effect caused by putting a large hole in the most critical part of the hull.

    Generally, the correct size of jet is not fitted to a boat......

    Mercury and OMC have been working on jets for more than 30 years, and virtually all their experimentation, to my knowledge, has been done on inboard/outboard gasoline direct drive, (small diameter, high RPM, high pressure units) or two cycle outboard type jets. These approaches are unacceptable to us, because, it has been established over many years, that a larger diameter, slower turning, low-pressure jet performs much more efficiently.

    The further aft the C of G, the faster the boat. This is a major part of the outboards performance advantage (a bracket increases this advantage). Because the outboard is completely over the transom, in a bracket design, the passenger’s location is further aft also, further enhancing the performance.


    _______________________________
    Excerts from another portion of their website speaking to the 'advantages':
    a)...under Reliability discussion
    It starts with the use of an automotive engine because most of us find that our automobile is ready to go anytime we need to use it.

    A propeller puts a heavier load on the engine if the boat is heavier and can easily overload the engine leading to premature engine failure. A water jet protects the engine from changes in boat weight. It presents a predictable load to the engine and that load does NOT change with changes in the boat.

    Water jet systems typically are more reliable than propeller systems because they are less complex and the engines are protected from overloads more adequately. This advantage can be lost if the water jet components are made of aluminum and are more subject to erosion, corrosion and wear. Two cycle inboard water jet systems are made of aluminum and use an outboard power head through a complex drive system leading to degraded reliability. JetPac™ water jet components are ALL 316 stainless steel and are highly resistant to erosion and corrosion. The JetPac™ is designed to provide reliability.

    b)...under Top Speed discussion
    ...you might have to accept poor acceleration characteristics at low speed and some difficulty getting the boat on plane to be sure of reaching top speed. One drive setup (propeller, gear ratio, etc) usually cannot achieve maximum performance across the full range of speeds in a specific boat.

    If top speed is your ultimate priority you may want to consider an outboard or I/O drive system because there are very limited choices in water jets for top speed.

    JetPac™ can be an attractive choice for you if your family goes with you in the boat or it is a commercial boat. We have compromised top speed (usually by a few mph) to give you excellent acceleration, powerful towing capability, very attractive fuel economy, and high reliability.

    The two cycle water jet and the diesel inboard water jet do not perform quite as well because the engine and water jet weight are ahead of the transom degrading the performance of the hull on plane


    c)....under Acceleration heading
    A water jet does not permit water to escape off the tips of the blade. Any water that goes in the front of the water jet must come out the nozzle. That makes the water jet more efficient. Smaller diameter water jets operate at higher speeds and higher pressures and do not move as much water as larger diameter water jets. A large diameter water jet creates more thrust because it moves much larger volumes of water


    d)....under Handling heading
    Water jets also vary considerably in handling. Two cycle water jets are not as responsive as any of the other systems because the small diameter water jet operating at high rpm and pressure does not move enough water to provide crisp response to steering changes. Inboard water jets, while having larger diameter jets and moving more water, have a disadvantage because the steering nozzle is usually at or just behind the boat transom. This does not give it the same steering leverage as an I/O or outboard where the propeller is usually 24 inches behind the transom providing more steering leverage

    _______________________________
    I've chosen these excerpts above as I hi-lited them in my reading of the subject matter. There is the full text of these discussions at Sword Marine's website

    Interestingly I was initially attracted to their technology as a result of seeing a illustration that indicated they utilized a 'kevlar belt drive' item that I have long thought applicable to marine drives, ref on my website

    Attached Files:

  4. KCook

    KCook Senior Member

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    "a larger diameter, slower turning, low-pressure jet performs much more efficiently"

    This may be the key consideration. Of course over the years this topic has come up on other sites. The jet fans (who seem to have more depth in their understanding) state that the usual little pleasure boat jets (Sea-Doo, etc) burn so much gas because they are too high pressure.

    Kelly
    Last edited: Feb 27, 2006
  5. YachtForums

    YachtForums Publisher/Admin

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    I have no problem with the preceding hypothesis or statements. It’s quite accurate, however I take exception with…

    This is not correct. It’s actually quite the contrary. Jet pumps will encounter a greater degree of loading and unloading because they are recessed inside the hull, as opposed to a prop placed underneath the hull. The higher the x-factor, the sooner ventilation is incurred. This is compounded by intake gullet vacuum, which is essentially artificial weight… and is QUITE significant with jet pumps.

    Let me give an example… in IHBA Drag racing (using jet boats), when the pump unloads at high speed, you’re essentially driving a kite. You’ve lost a huge amount of downforce and vacuum (or weight). The forward velocity remains relatively unchanged for the moment and the hull still has the same level of wind speed passing under it. You can do the math from here…

    With their exposed, prop driven counterparts, when the prop unloads… the downforce on the hull remains constant. However, this can result in some wild stern walking! Both of these scenarios, at the speeds these boats are traveling, can result in the boat swapping ends. No need to go into the horrific details that follow.

    This is NOT entirely correct. There are multiple variables. This has more to do with the boats length, displacement and hull design. Conventional rudders generally offer greater deflection, because they have greater travel than the typical jet pump steering nozzle allows. The reason pump nozzles have limited travel is because vectored thrust can hydraulic at the venturi’s orifice under extreme angles and high pressure. This is a matter of controlling hydraulics and optimizing the connection between the venturi and the nozzle.

    As for smaller diameter orifices under higher pressure not being as effective… False. A jet driven boat CAN turn faster because it does not have an appendage protruding beneath the surface that will hold a designated track. Pressurized thrust with sufficient deflection can provide equal or better better turning. Again, the hull plays an equally critical role in the performance of either propulsion system, or it’s form of thrust vectoring. Certainly the mass of water being moved is important, but pressure and deflection play important roles as well.

    This is not entirely correct either. A shorter distance between two leverage points provides a tighter turning radius. A greater distance can provide increased leverage, but this does not equate to faster turning. Vectored thrust under high pressure, will more than make up for the leverage lost from a further forward exit point. Again, this can have as much to do with the hull than the factors cited.

    Also, when referencing the leverage an outboard can create... say on an extended bracket, that same outboard uses a skeg on the lower using that will reduce side slip. Jet boats generally don't have an appendage of this type, allowing more slip at the stern and thus a quicker change in heading.
  6. YachtForums

    YachtForums Publisher/Admin

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    Brian,

    A little disclaimer... I've been away from this for years. I'm juggling about 100,000 lines of code, a dozen different software packages and trying to stay on top of the yachting industry at the same time.

    I have to give credit to JetPac. I haven't met them or tried their product, but it sounds like they have a MUCH better understanding than most.

    Best!

    Carl
  7. YachtForums

    YachtForums Publisher/Admin

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    I think this is more a by-product of their operational speeds... which tend to be Wide Open Throttle most of the time. In recent years, manufacturers such as Honda and Bombardier (maybe others) have began super-charging and turbo-charging watercraft. Lots more power... and lot more fuel.

    I'm still wondering how they've addressed dowsing a glowing supercharger with water... and the ensuing effect on performance, i.e., steam in the engine compartment or turbo lag with constant on/off throttle operation... but that's another topic. ;)
  8. KCook

    KCook Senior Member

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    The difference is not huge, but still there. Throttle a Yamaha SR230 (2900 lb bowrider, not PWC) back to 35mph and it sucks 12 gal/hr. A 21' I/O bowrider (actually more weight) at the same speed uses 10.5 gal/hr.

    Kelly
  9. YachtForums

    YachtForums Publisher/Admin

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    We're getting off-subject here, but fuel economy can be directly proportionate to engine displacement. The I/O you're speaking of is probably connected to an automotive style, 4-stroke block... where as Yamaha and Bombardier Jet boats have used 2-stroke engines from PWC's for many years.

    In recent years, I think Bombardier adapted Mercury power plants for their jet-boats, but I believe these are also, high output 2-strokes. I'm REALLY not sure on this. I haven't looked at these types of boats in years! I'd like to reserve the right to pre-confess... I may be wrong. ;)

    My point is, comparing a high output 2-stroke, that is requiring more RPM's (and fuel) to sustain the torque of a large displacement 4-stroke is not a fair comparison. In either case, jet-pumps need torque to create pressure, not RPM's, however both must be present to reach reasonable levels of efficiency.
  10. KCook

    KCook Senior Member

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    This particular Yamaha uses two 998cc 4-stroke motors (motorcycle derived). Indeed, I would expect 2-strokes to burn even more fuel.

    Kelly
  11. orion

    orion Senior Member

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    Has anyone experience of this jet system?

    www.marinejettech.com

    Teoretical it sounds good to me.
    Could it make a revolution in the pleasure boat market?
  12. Arniev

    Arniev Senior Member

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    The IntelliJet technology is very interesting.
    Am wondering if there are actual products in the market that are actually using it, since the technology was initially introduced in 2004.
    :)
  13. brian eiland

    brian eiland Senior Member

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    I thought it might be worthwhile to post some text portions of their website here since they are relatively brief, and too often in the past I have had occasion to try and link to a website for more info on the subject matter only to find it no longer available for one reason or another.
    _____________________________________________
    …from www.marinejettech.com


    _____________________________________________
    So next I went looking for more info on this ‘adjustable inlet and outlet’ subject as related to waterjet propulsion, and found some very nice discussions right here on this forum, and by our webmaster Carl. He begins with some jet-pump fundamentals http://www.yachtforums.com/forums/13462-post27.html
    ……some excerpts…
    Brian observes: Now this is getting real interesting. We certainly have agreement from all parties that a variable inlet and outlet can remarkable improve the jet-pump performance!!


    I must leave this posting unfinished for a few hours while I do something else.
  14. YachtForums

    YachtForums Publisher/Admin

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    Brian,

    Thank you for resurrecting this thread. I meant to expand on the Intelligent technology, but I got caught up in other things. It never seems like a good time (for instance, I'm packing boxes right now and moving over the weekend). I need to more closely examine what they are doing, but I've got some definative thoughts on what I've seen so far. I’ll post on this a little later.

    Carl
  15. brian eiland

    brian eiland Senior Member

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    Jet Pump Technologies (continued)

    ....guess I will have to continue this discussion in this new posting...

    As I said before it does appear that the real secret to increasing jet-pump efficiency is to incorporate a varible inlet and outlet.

    Here is the mention of that 'pliant material' again.

    Is this material still so classified as to not be available in the commerical market??

    Since it was a pliant type of material, was there some upper range of HorsePower that might limit the utilization of this particular material??


    Here again is that "no moving parts" quote that caught my eye on two occassions. I have not had time to look at IntelliJET's patents yet, but I suspect they are an electro-mechanical device to control these orfices...usually complicated, and not all that dependable. I really want to know more about these pliant solutions, if possible??
  16. YachtForums

    YachtForums Publisher/Admin

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    I took a look at the patents issued for Intellijet. It was interesting to note they were issued to a Palm Bay, Florida location where some of my own development work eventually ended up.

    I don’t have the software currently installed to display the TIFF images/drawings from the patent office, but in reading through their patents, it looks like they’re on the right track. However, I do have some reservations about what I’ve seen in the promotional video on their website…

    A. Variable Intake Gullet; Sliding Plate

    I like the idea of a sliding plate to reduce the intake gullet size, but there are two important caveats….

    1. Sand intrusion with any type of moving mechanism on the bottom of a hull. We encountered this repeatedly with deployable pumps and foils on the vessels we designed. In all our testing, we could not be fully prepared for the “real world” punishment a craft could be subject to, i.e. high speed beaching, running aground or hitting a submerged/floating object.

    2. Intellijet’s sliding plate is designed to increase vacuum at higher speeds by decreasing the size of the uptake entrance. Short of a better mechanical means, this will work, but it’s really not the ultimate answer. In order to effectively increase vacuum, you MUST decrease the volumetric area within the cavity, specifically where ventilation is induced, which is near the top of the intake gullet. Decreasing the size of the “opening” should also be done in conjunction with an overall reduction of the volumetric area of the intake gullet. Mechanically… this is very difficult to create!

    3. Intellijet’s sliding plate moves forward to decrease the opening at high speed. Ultimately, moving the intake forward can have both good and bad effects.

    The good… moving the lip of the intake plate forward CAN help optimize flow to the top of the impeller, as well as reducing the abrupt uptake of water at higher speeds, by creating a more direct path to the impeller.

    The bad… the more forward the intake area is, the sooner ventilation will be incurred at higher speeds. For example, as speed increases, a hull develops more lift and the point of lift moves further aft. The higher the hull rides on the water; the sooner ventilation will be incurred by the jet pump intake. Moving the intake forward, where more air is present, can reduce uptake efficiency. Another caveat here... by moving the plate forward, you are diverting flow higher on the impeller, which may starve the lower portion of the impeller.

    In a nutshell… the intake gullet should become smaller with increased speed, because this increases vacuum and decreases aeration. Ultimately, the opening should move further aft, away from potential interruptions in vacuum. The pump should move further aft and assume a surfacing position. The combination of both should also create a path of least resistance to the flow of water for greatest efficiency at higher speeds.

    B. Variable Pitch Impeller Blades…

    Variable pitch blades and the mechanism for controlling the same are nothing new, but this is the first time I’ve seen it in a jet pump! This is one of the Holy Grails to increasing efficiency, whether it’s for jet pumps or non-ducted props. BUT, it’s not without some drawbacks…

    1. In order to accommodate the mechanism/gears/cams for controlling pitch, the size of the impellor hub must increase in size proportionately. The larger the hub, the greater the obstruction to flow. In an axial flow pump, this can be a problem. This is the main reason a rim-driven prop is a more effective design.

    2. The benefit of variable pitch is obvious, but with a jet pump… it is even more important. Increasing pitch can increase vacuum… critical to high-speed pump performance, BUT… variable pitch blades DO NOT overlap each other. It would be physically impossible for the blades to go from forward, to neutral or reverse. Over-lapping blades are one of the most beneficial ingredients in pump efficiency, along with radial edge (swept) blade designs.

    Intellijet’s variable pitch blades allow forward, neutral and reverse, without shifting gears, changing RPM or utilizing a reverse bucket. This is ALL good, but it’s not the Holy Grail of jet pump design.

    In addition to NOT having over-lapping blades, the other problem I have with the entire variable pitch concept within a jet pump is how to control hydraulics between the trailing edge of the blades and the leading edge of the stators. When blades change pitch, the distance between these point’s changes dramatically… not to mention how important it is that the angle of stators match the pitch of the blades. That’s an entirely new subject that I won’t go into.

    C. Variable Geometry Venturi…

    I don’t have time to read the patent (in its entirety) on how Intellijet is accomplishing a variable geometry venturi, but I can tell you this is VERY difficult to do mechanically. Not only must the diameter of venturi’s exiting orifice change accordingly, the rate of compression inside the bowl must change as well. For acceleration, the exiting orifice should be larger and the rate of compression (acceleration!) should be more abrupt. As speed increases, the orifice should become smaller and the rate of compression (acceleration!) should be more relaxed. This entire sequence must work in conjunction with the level of aeration present in the bowl, which should also match the venturi’s tail cone. NOT easy to do!

    And last… in reading Intellijet’s website, they speak of presenting this technology to the Naval Surface Warfare Center. I can tell you this much… The Department of the Navy has had an effective means and technology in this area for MANY years, but it’s not an area of critical need. If it was, I wouldn’t be the admin of YF... I’d still be standing in front of congressional committees presenting proposals for continued funding.
  17. Ben

    Ben Senior Member

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    Thanks for all that Carl, interesting reading.:)

    Now that Carl has shared all his top secret knowledge, I might see if I can get myself a high paying research job.;)

    Got a few (more serious) questions for you, and other knowledgeable members.

    1st. How about using variable pitch blades that still overlap, allowing forward thrust only?

    2nd. How about having variable pitch stators that match the pitch of the blades, if not angle matched, then flow angle matched?
    This would also keep the distance between the stator and the blade much more constant.

    3rd. Has anyone considered using a screw instead of a blade for accelerating the water?
    In my mind it could offer more bite with less revs, possibly cleaner flow?
    You could even use a small worm drive in the centre of the screw to stretch it out or compress it, thus changing the pitch.


    Obviously this is not my area of expertise.:eek:
  18. Ben

    Ben Senior Member

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    After going out on a limb with the last question, I thought I'd add another that suggests using air compressing technology on non-compressing water.

    Would it be possible to combine the use of a movable inlet spike like on the Lockheed SR-71 to change the shape and size of the inlet pathway?

    I'm thinking I could combine automotive turbocharger/supercharger technology with jet engine flow technology and end up with a efficient jet boat.:D

    There is probably a good reason why this wouldn't work (ie, water is not air), I'm just thinking outloud.


    Back to the drawing board.
  19. YachtForums

    YachtForums Publisher/Admin

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    Glad you have an interest. I should also thank Brian for his diligence on the subject, while apologizing for not expanding on this thread in a more expedient time frame. I’ve just had too much on my plate over the past few months.

    Variable pitch, combined with over-lapping impeller blades would be great, but this is sort of an oxymoron, because the tolerance between the tip of the blades and the inner walls of the duct will change with variable pitch.

    It’s mechanically VERY difficult to do. This would mean the stators must elongate and physically change shape.

    In reality, impellers are essentially a worm (screw) drive, on a shorter scale. There has been substantial research in the area of worm drives, but not for performance. It’s greatest value was in reducing acoustic signatures. A shrouded drive with an expulsion point far aft of a stator section resulted in an unturbulated flow that was hard to detect. But, worm drives are not good accelerators, because you cannot progressively increase the pitch. As the volume between the blades increases, there is no source of fluid to draw from. Unlike a turbine that can compress air via a system of increased blade rpm and pitch, water cannot be compressed… only accelerated. However, it is conceivable to “feed” or draw in water at specific points on the exterior of the worm drive to fill the void. The SR-71 Blackbird used a similar system to draw air into the afterburner, because the amount of air present at higher altitudes was reduced.

    A MUCH better solution to all of this is a dual stage axial flow pump. This configuration is not unlike Volvo’s Duo-Prop designs. Twin fixed pitch impellers can also greatly increase uptake vacuum.

    Yes! The precursor to this concept originated on the Russian Messerschmitt 262, the world’s first operational jet fighter. It was called the “movable onion”. Later, Lockheed’s chief designer Kelly Johnson adapted it for the SR-71. We played with several variations of this idea, but in the end, our development protocol was for simple, rugged designs with very few moving parts.

    Contraire! Your questions are good. :)
  20. YachtForums

    YachtForums Publisher/Admin

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    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...

    ********************************************

    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.

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