Jet Drive vs. Prop

Power for the Jet Pump & Multi-staging

Power for the Jet Pump & Multi-staging

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.
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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;

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.

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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

Torque Power

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.

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.

Questions About a JetPac Claim

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?

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??

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.

Rim-Drive Impellers

It certainly appears as though the rim-driven propulsor concept has a lot of potential as a component in a jet pump drive system……ie, a rim driven impeller (“a new era of impeller design” as noted by YachtForums). Now we know there is considerable work being done on rim-drive technology, particularly those incorporating permanent-magnet electric motor components to power-up the rim. These electrical driven rim-drives may prove too advanced, overly complicated, and/or too expensive for small PWC or RIB jet applications.

So what if we look back at mechanically driven units. I would imagine it would not be too difficult to design up a kevlar-belt driven unit similar to either of those depicted at Peripheral Journal Propeller Drive utilizing suitable bearings and minimal water seals as the whole ‘rim cylinder’ and belt-drive mechanism might be contained in a ‘water box’ as noted in AIR Fertigung product info.pdf. The input drive shaft would exit the ‘water box’ to be hooked up to whatever motor was chosen to drive the jet pump unit. The water seal incorporated at the drive shaft’s exit point would guarantee the ultimate water integrity of the boat in case of failure or slow leak of a seal at the rim itself. And the water box concept would allow for the removal of the whole impeller drive unit for inspection or repair even while the vessel was afloat. Kevlar belt drives are a proven entity.

One variation of the rim-driven impeller itself might consist of a fixed-bladed model where the blades were ‘fixed’ to the inner circumference of the ‘rim cylinder’. Per note:

So I assume these fixed blades might best appear a bit more elongated or ‘screw like’ to obtain some overlap, maybe??

My next thought went to why not add a progressive pitch to these fixed blades, so that the water was accelerated some additional amount. Refer:

The rim-driven impeller may offer some alternatives to this equation that might allow for progressive pitch of the blades. My first thought was that the inner circumference of the rim drive ‘cylinder’ might be constricted in diameter along its length to compensate for the change in volume between the progressively pitched blades. Next I thought, maybe some sort of inner hub attached to the blade tips configured as a cone to account for the volume changes.

But wait a minute, both of those solutions restrict me to only processing the same volume of water that I would with the non-progressive pitched blades. How about making use of that that ‘center channel stream’ of water that would exist in the free space between the inner blade tips of the rim drive impeller? Could this be the source of my extra ‘feed water’ to fill the void crated by the progressive pitch?? This ‘free space’ down the center channel of the rim drive impeller might offer several virtues;
1) provide ‘feed water’ for progressive pitch blades
2) provide extra water to stave off early aeration of the total flow
3) provide a path of less resistance for ‘excessive’ flow that the inlet gullet might have allowed, thus cutting down on resistance to the boat’s forward progress, and making adjustments to the inlet gullet volume less critical.
4) provide for a greater overall water processing capability than a same diameter hub driven impeller jet pump (more water = more thrust)

With respect to the aeration I mention in #2, I reference this quote:

Besides the hydrodynamics of the situation, the rim-driven impeller obviously offers a lot less potential for fouling at both its hub-less center and at the zero-gap, blade-to-rimwall area of a jet pump unit.

Those are some thoughts of mine on fixed-pitched blade rim drive impellers at the moment. There may be some arguments for variable-pitch blade impellers, but that complicates matters for the ‘small jet pump units’. (maybe not as much so as in the case with hub driven jet pumps, but more complicated, never the less).

I look forward to the comments

Pliant Duct Material

BTW, Carl I’m still looking for more discussion on that ‘pliant material’ you’ve mentioned on several occasions

Were those both the same material for the inlet gullet and the venturi? Is it, or are they, both still ‘classified’ ??

Designing a Hi-Thrust 10 HP Jet online

Just ran across this on-line design process

http://www.marinejet.com/view/53

quoted....
Over the next few weeks NAMJ will show you the entire process of producing a new jet from scratch.

NAMJ will go through the entire research and development process of design, building test models, testing, outcome of testing and then to production.

We will try to post the results as they happen.

Monday (5/15/06) will be the first day of posting.

So keep coming back to the web site to see the progress of the new low horsepower small jet line.

Jet Pump Inducer

From another forum I ran across this posting that I thought might be interesting to this thread discussion.

Pliant Duct Materials?

Been quite awhile since I revisited this thread, and don't know that I have that much time to pursue it now. But I did happen across another 'pliant material' research;

"Agile new plastics change shape with heat"
Researchers have invented a class of materials so remarkable for their agility in changing shape as they react to heat, they might be described as acrobatic plastics. The new materials, known as "triple-shape materials," can assume three different shapes, each shape depending on how much heat is applied.

http://www.manufacturingcenter.com/dfx/news/stories/feature-2.asp?

Intellijet press release

Redmond, WA September 11, 2007 -- IntelliJet Marine, Inc. has been granted U.S. Patent # 7,241,193 for its Variable Marine Jet propulsion technology. The company believes the IntelliJet™ technology will answer the expressed desire from boaters for greater reliability, more economical operation, and increased maneuverability and safety.

The company is currently testing the IntelliJet™ and preparing to go into production.

The technology incorporates continuously variable power transmission (CVT), now becoming standard in automobiles, and simplifies operations by eliminating the need for reverse gearing or a reversing bucket. "Existing boats utilize a fixed drive system that hasn't been seen in automobiles since the Model T. The IntelliJet™ is naturally expected to enhance fuel economy and engine life in boats, just as transmissions have in automobiles," says Jeff Jordan, President of IntelliJet Marine, Inc.

Management expects the IntelliJet™ system to offer better efficiency than conventional jets and propellers over a wide range of operating speeds and loads.

The technical details are well documented in issued patents, white papers, demo videos, and PowerPoint presentations to industry groups, available at www.iiJet.com.

....previous background
Existing jet boats have the identical problem as the first generation of jet airplanes. Although they are fast and maneuverable, their initial acceleration is so poor they can barely pull water-skiers out of the water. They can be designed to either go fast with poor acceleration, or to provide acceleration at the trade-off of low top speed.

If it were not for these operating range restrictions virtually all boats would be water-jet powered. Jets are safer than outdrives (no prop in the water). They are mechanically simpler than outdrives. They are more maneuverable than outdrives because the jet outlet is directionally controlled. But, historically, the jet was sized for speed and lacked the low speed thrust required for docking and acceleration.

IntelliJet Marine answers these needs. And the result is just as revolutionary as it was in turbo-jet airplanes. Their innovative technologies improve jet performance by up to 80% at low boat speeds, while also increasing top speed, fuel efficiency and cruising range. These patented methods mark the most significant advance in marine propulsion systems in many years.