[YachtForums]

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.

[Ben]

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.

Originally Posted by YachtForums

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.

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.

[Ben]

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.

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.

[YachtForums]

Originally Posted by Ben

Thanks for all that Carl, interesting reading.

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.

Originally Posted by Ben

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

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.

Originally Posted by Ben

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.

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

Originally Posted by Ben

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.

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.

Originally Posted by Ben

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?

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.

Originally Posted by Ben

Obviously this is not my area of expertise.

Contraire! Your questions are good.

[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...

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

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.

[Codger]

Originally Posted by YachtForums

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.

There are already materials that can accurately and with consistant repeatability change their form simply in response to pressure, and or centrifugal force. No requirement for flyweights or other external configuration triggers. Been awhile since I looked in to these and not sure about availability, but wouldn't these materials take care of this application?
Don't recall the manufacturer but was definately U.S. Perhaps been tried already..

[brian eiland]

Power for the Jet Pump & Multi-staging

Originally Posted by YachtForums

In either case, jet-pumps need torque to create pressure, not RPM's, however both must be present to reach reasonable levels of efficiency.

So just like so many other applications, the torque curves of the electric motor are a better match to the needs of the propulsor (propeller, jet, whatever) than the internal combustion engine. We know electric motors develop their best torques at starting and low RPM verses the upper RPMs.

It may also be that a number of the existing jet-pump units will require a bit of redesigning as the big switch from 2-cycle to 4-cycle engines is dictated by the new anti-pollution regulations.
__________________________________________________ ___

Originally Posted by Ben

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.

Originally Posted by YachtForums

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.

Originally Posted by YachtForums

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….

The Volvo Duo-Prop design has been quite successful. I was even suggesting this duo-prop configuration in a forward facing manner on the kevlar belt-driven props aboard my 65 catamaran design back in 1989, well before Volvo’s fairly recent introduction of their new IPS system.

But correct me if I’m wrong, isn’t the theory behind the duo-prop a function of the second (countra-rotating trailing prop) to recover some of the spin energy imparted to the water by the leading prop, and of course to get more blade area into an overall smaller diameter prop system;…..and concurrently keep prop tip speeds within reason in higher revolution systems?? Aren’t these attributes of the duo-prop a bit different than those associated with a dual-stage axial flow pump??

In a dual-stage water jet pump it doesn’t seem to make sense that one could utilize two same-diameter impellers of a different pitch (progressive pitch system). As you noted in this quote;

Originally Posted by YachtForums

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.

Unlike the ‘compressible flow’ of an air jet engine, the water medium is an incompressible flow, thus the second stage would probably involve a different dia impeller and housing for the trailing stage. When you add in these mechanical complications, the additional ‘tip losses’, and the very real practical problems likely with ‘fouling’ the dual stage units, it really looks likely that the rim-drive route is a more feasible solution.

__________________________________________________ _

In either case of a duo-prop or a dual-stage jet pump, most likely the two props or impellers will be rotated in contra-rotating directions. Mechanically this is most easily accomplished by driving the concentric shafts via a pinion gear arrangement where the engine-driving shaft needs to be perpendicular to the concentric shafts of the ‘propulsors’. In the great majority of these situations the engine power unit is located ‘over’ the jet pump unit, thus the engine itself will most likely need to operate with its drive shaft in a vertical orientation.

I personally have always had an uneasiness about the potential lubrication problems that a 4-cycle engine might encounter when they are operated in this orientation. Yes I do realize that most all of our current crop of outboard engines are converting to this configuration, but are there any good studies on the long-term life, wear-and-tear factors of this arrangement?? I’ve heard very favorable comments on the smoothness of their operation, and the non-smoking attributes, but not a lot on their ‘uneven-wear’ possibilities, etc.

I thought it would be interesting to look around at some alternative engine packages that might be utilized to power a small RIB jet boat, particularly any higher power-density possibilities. Obliviously the power-head of the latest outboard engines are possibilities, and they operate in a vertical mode. Most of the current crop of PWC engines are designed for horizontal operation, and as 4-cycle units they are quite a bit heavier than their 2-cycle predecessors.

One unit that caught my attention was this RotaMax engine, a concept that has had a long maturity age with the Mazda auto manufacturer….pretty well proven by this time, and hi-power density. I might question its torque capabilities? Wankel basics

Or how about this little number Dyna-Cam

[Ben]

Originally Posted by brian eiland

One unit that caught my attention was this RotaMax engine, a concept that has had a long maturity age with the Mazda auto manufacturer….pretty well proven by this time, and hi-power density. I might question its torque capabilities? [/url]

Interesting idea Brian. Rotary engines can produce high power figures for their size (and also tend to be very smooth running), but as you have said, they have even less low rev torque than a piston engine, probably having characteristics closer to that of a turbine.

You'd probably need to gear it down and add some forced induction.

If you added a reduction gearbox, you might as well add a 90deg. elbow and lie the engine down as it was designed to run.

I think torque would be the first hurdle.

[Guilly]

I have tested twice the JetPac, in different boats (A rib and a workboat), and I've found Swordmarine statements are quite correct. Acceleration is pretty good, maneouvrability also and fuel consumption most reasonable. I didn't perform a full load test to check the suffering or not of the engine.

[tartanski]

Originally Posted by Ben

Interesting idea Brian. Rotary engines can produce high power figures for their size (and also tend to be very smooth running), but as you have said, they have even less low rev torque than a piston engine, probably having characteristics closer to that of a turbine.

You'd probably need to gear it down and add some forced induction.

If you added a reduction gearbox, you might as well add a 90deg. elbow and lie the engine down as it was designed to run.

I think torque would be the first hurdle.

As far as I know Cube for Cube Rotary engines have far more torque than Reciprocating engines. Can it be they have torque characteristics like a Electric motor?

Just check out the tiny rotary engines in the mazda RX-8 1.3 liter!!

[Ben]

Hmmm, yes I think you've got me there, cube for cube they win hands down.

Because of the way the rotary works it's a little more difficult to compare than it seems.
The current (1.3ltr) mazda engine produces the torque of a 2ltr engine, and the power of a 3ltr engine, but also uses the fuel of a 3ltr engine.
The Wankel rotary has a power stroke for every rotation of the drive shaft, compared with a power stroke on every second rotation on a (4 stroke) piston engine, leading some to suggest that the Wankel engine is in effect twice it's measured capacity (which somewhat explains the fuel use).

The mazda engine produces it's maximum power at something like 7500rpm and max torque at 5500rpm, suggesting it needs to wind up before it gives it's best.

I could be wrong.

[brian eiland]

Originally Posted by tartanski

As far as I know Cube for Cube Rotary engines have far more torque than Reciprocating engines. Can it be they have torque characteristics like a Electric motor?

I rather doubt this is true. One big indicator of the torque capabilities of the prime engine source itself would be to look at the ‘transmission gearing’ that accompanies its application. In general if it requires a lot of gearing to start motion from a slow speed, then the max-torque characteristics of the prime source must be at the upper RPM range rather than down lower.

Originally Posted by Ben

…The mazda engine produces it's maximum power at something like 7500rpm and max torque at 5500rpm, suggesting it needs to wind up before it gives it's best.

The lastest generation Mazda RX-8 engines, Renesis, - an engine boasting innovative technologies such as side intake/side exhaust porting - is a 654 cc x two rotor unit that produces 177kW (ECE at 8200 rpm and 211Nm (ECE) of torque at 5500 rpm when combined with a six-speed manual transmission.)
[http://www.rotaryengineillustrated.com/

Internal combustion engines in general are not good low-end torque producers. The older diesel engines with their ‘longer-stroke’ certainly outshone the gas engines. But nowadays many of the diesels are more compact with shorter strokes and turbo charging to give the compression ratios required to ignite the diesel fuel. Even their torque outputs come at higher RPMs. And incidentally this means higher piston speeds resulting in shorter engine life.

Electric motors on the other hand are noted for their low-end torque characteristics. I’ll let a more knowledgeable electrical person explain why. But look at the applications in cranes, trains, and previously big buses to name a few. Cranes need that initial hoisting capability, trains need that ‘start rolling torque’ to get heavy things rolling, and buses use to use electric motors to drive the wheels until the computerized multi-speed transmissions allowed for connecting the diesel engine to the wheels without the diesel-electric interface.

BTW, electric motors are much better suited to driving our cars, as their torque characteristics are a much better match to the auto’s needs…plus we could regenerate upon braking. Too bad we aren’t pursuing this with greater vigor. Maybe the current fuel prices will force the issue. I tried to get an Asian/American development program on this subject going back in ’97, but couldn’t arouse any interest in the USA. Sorry got off the jet-pump drive subject a bit.

[brian eiland]

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….

I don’t know that I fully understand this claim, or its insinuation. It seems to me that in many of the PWC the jet units (and their impellers) are located about as low in the hull as the bottom edge of the impeller would allow?

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).

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.

Are they claiming that a ‘lower’ jet unit might even be more efficient? Is this to insinuate that the jet unit might actually be lower than the hull bottom?

I can’t believe this would be true, as at that point you have now placed some of the jet unit out in the water flow under the hull’s bottom, and thus increased the drag factor, just as with a conventional prop and strut support. One of the virtues of the jet unit is that it be capable of creating enough ‘suction’ to draw its water supply up into its inlet rather than projecting a drag creating inlet scoop down into the water flow, and/or trying to ‘ram charge’ its inlet, right??

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

I found this observation interesting. While working in SE Asia for a few years in the late 90’s, I was involved with the water-jetting of gas/oil pipelines and fiber optic cable into the seabed at the beach approaches. While most of our other competitive outfits involved with similar work almost all utilized ‘hi-pressure’ flows to facilitate cutting the trenches underwater, our hydraulic specialist designed low-pressure, hi-volume flow systems. Ours were the most efficient at doing the jobs.

[EnigmaNZ]

Interesting discussion. Growing up in New Zealand as a teen, 35 years ago, we had a 3 jet boats in succession, the first was a 13'6" with single stage Hamilton jet with a 3L V6, my stepfater like it so much he went for a new 18' with a 351 Ford V8 driving a 3 stage jet, the 753. It had a 7 inch venturi and the bulk of the unit was outside the stern, 3 impellors turned in the same direction with stators inbetween to straighten the flow. I don't recall any problem with getting on plane, it did have a nose up altitude, and spun on a dime at high speed, while at low speeds it pivoted around the bow, very impressive at the time. No doubt due to the exit being about 3 feet from the back of the boat. I think it maxed out at about 45 to 48 mph. With the unit mounted so far back the engine was also right at the stern comparable to a stern drive installation.

Due to the commercial use we upgraded to a larger 21' with heavy duty hull, Ford 351, and the 10 inch 1031 single stage jet, it didn't have the speed, about 35 mph+ but it could dig itself out of a hole even when heavily laden. Again the engine was rear mounted as the impeller was external to the hull.

The attractive features were shallow water ability without snagging bottom, rocks etc, turn in it's own lenght maneuvuring, and being able to drive straight up onto the trailor, something I could do successfully from the first time as a teen. Line up, blimp the throttle, and you were up on the trailer as easy as. Towing skiers with the 18' presented no problem, not tried with the 21'.

Variable inlet and exit, variable pitch fan, do we need all this added complication. I'm reminded of the high tech Japan adds to it's cars to improve handling, eg, 4WD, 4WS, electronic damping, etc, while a good European car handled as well just with a well designed drive train. All the Japanese hybrids designed to increase gas milage (wait till the battery pack needs replacing) while Europe just designs more effecient diesels. Give me a jet unit that is sturdy, low maintainence, good proformance over a wide range even if a little down on what can be acheived with hi tech addons.

Oh, interesting site BTW. Love big boats

[Carcharadon]

Did some searching and came across this appropo discussion


Gentlemen, we have been having an active discussion on another BB about the adaptation of a jet drive to a submarine.


http://groups.yahoo.com/group/vulcania_submarine/

Quick background; Pat Regan who has a functional Disney Nautilus submarine wants to adapt a jet ski pump to the sub. He has the belief that similar to a Jet Ski the pump develops its greatest thrust at the surface. I, however, don’t think it would lose any /much thrust placed 16 inches below the surface.

The logic he presents seems sound, that the whole jet column is related to maximum thrust and that in air the thrust is greater than underwater because it encounters less friction leaving the Nozzle than under water.

Another view is that once the water leaves the jet nozzle, thrust has already been developed and it is immaterial what happens to the water after since the kinetic energy for thrust has already been imparted to the water and any remaining energy dissipates whether in a jet column or up against a wall. Realizing of course that the Nautilus will never plane like a Jet Ski or jet boat is there any advantage to having the jet located on the surface or will there be a loss of thrust if located 16 inches below the surface. The pump will never load at Jet Ski speeds.

To summarize, does a jet pump lose thrust submerged 16 inches below the surface as opposed to “at” the surface.

Can you shed any light on this?

Tom, otherwise tewrecks on Pat’s BB thanks

I have a jet model sub

http://www.rcgroups.com/forums/member.php?u=40728