Jet, Jet Pump, Waterjet, Jet Drive vs Prop

[brian eiland]

First, the title of this thread... I sought to try and put most of the key words in the title such that a web search of this subject matter would pick it up. Secondly, I did a search within this forum and quite a number of different threads on this subject, but divided out into a great number of individual ones. Thought I might try to bring a number of these discussions together into one information pile.

So as to not alienate the individual webmasters, much of these discussion will likely appear on two separate forums.
I started this out at YachtForums.com with....

Jet, Jet Drive Vs Prop

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:

Originally Posted by Capt J

Aside from the shallower draft as mentioned earlier, jets have two major disadvantages to props.
1. Jets are usually around 40% less efficient at putting the power to the water compared to props
2. Low speed manueverability, you really have to pour on the power to maneuver the boat in docking situations....

Originally Posted by YachtForums

...don't know how I've missed this thread! Too many thing to do around here I guess. Capt. J... welcome aboard! I'm curious, how did you arrive at the stats of jet pumps being 40% less efficient than conventional props?

Originally Posted by YachtForums

... nothing could be further from the truth. Properly designed jet-pumps are absolute models of concentrated thrust. Quite simply, a ducted prop is more efficient than a non-ducted (or shrouded) prop, unless ofcouse... the drag created by the shroud outweighs the benefits of encapsulation. This isn't the case with a jet-pump on boats.

This being said... I have to clarify one thing. Application is critical!!! There are applications better for pumps and application better for props. In depends entirely on the operating parameters of the craft. An offshore race boat would be the wrong application for a pump, just like a Bravo drive would be the wrong application for yacht

Originally Posted by Capt J

...Unless a jet is spinning at the speed it is designed to run at, it is very in-efficient below that because the housing is designed for a certain amount of water to flow through it and the size of the nozzle is designed according to that. Another reason a 9' waverunner with 155hp only does 65 mph. Another reason a 70hp outboard is re-rated around 40hp when a jet lower unit is installed.

Originally Posted by YachtForums

You'll notice as speed and RPM's increased, the efficiency of the pump increased exponentially and quickly caught up to the propped hull. The pump was only just beginning to come into its spectrum. To me, this indicated the pump may not have been the best choice for the operating speeds of this hull. That doesn't mean it's a less efficient propulsion system.

Also, referencing the outboard reduction... you need to examine the jet-drives that are adapted to these lower units. They are centrifugal pumps. There are no similarities between this and what we're discussing.

Originally Posted by Capt J

The reason the jets are not as efficient at lower speeds is because they have to be designed to achieve 2350 rpms at full throttle and the impellor has to have the correct pitch to allow the engine to achieve full throttle rpm, also the pump nozzle has to be large enough for that volume of water. Therefore at cruise rpm 1950, the jet is not nearly as efficient as a propellor. A jet works great with a turbine because a turbine is designed to spin a certain rpm all of the time. When a pump is combined with a diesel engine it is not an efficient system because of the nature of the diesel engine.

Originally Posted by YachtForums

When pumps enter their designed operating spectrum, they will excel... to a certain threshold. After this point, they will become less efficient due to aeration and lacking the vacuum to bring water up to the intake gullet at higher speeds. As speed increases and the hull developes more lift, (riding higher on the water), pumps are prone to inducing more air, resulting in a reduction in vacuum. This is also compounded by water trying to by-pass the intake gullet.

This has been my point all along... it's not that jet pumps are less efficient. They are actually more efficient in many applications, especially when the craft will be operating at or near their optimum output.

I can tell you this.... there ARE configurations that are more efficient for pumps than what we see commercially, for example surface piercing pumps. These are pumps that are placed partially beneath the hull's surface. In this scenario, water doesn't have to make an abrupt turn up to the impeller (via the intake gullet), but rather... water encounters the impeller directly as it passes under the hull.

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

[brian eiland]

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

[brian eiland]

Reply from YachtForums

Originally Posted by YachtForums

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


Quote:
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.


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.


Quote:
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.


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.


Quote:
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


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.

[brian eiland]

YachtForums wrote:
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.

[brian eiland]

Originally Posted by Orion

Has anyone experience of this jet system? www.marinejettech.com

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

Quote:
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.

The intelligent inlet duct adjusts to recover the velocity head of the incoming water at all planing speeds and at all throttle positions. This higher inlet efficiency is most important in designs based on larger jets. Larger jets, in turn, are desirable because they produce more thrust at low boat speeds. This parallels engine development in commercial aircraft where high-bypass turbofans move more air through a larger jet for shorter takeoffs.

This combination of larger jet size, efficient inlet duct, and variable nozzle allows a 50 to 80% increase in low speed thrust, while increasing top speed and maintaining higher propulsion efficiency at all boat speeds and accelerations. These three innovations work together to approach the limits of propulsion efficiency.

Larger Jet Size
increases propulsion efficiency using technology demonstrated in development of larger jets in aerospace industry.

Intelligent Inlet Duct
automatically adjusts to recover the power of the incoming water at all planing speeds and at all throttle positions

Variable Rectangular Steering Nozzle
allows simultaneous control of nozzle area and steering direction to maintain peak efficiency over wide ranges of boat speed, pump shaft speed and steering vectors.

Why It All Works Together
Bigger jets are desirable because they create higher thrust. But the bigger the jet, the more power that is lost in the ordinary inlet duct. This power loss has to be made up by the motor and the pump.

The adjustable inlet duct reduces this power loss. And, as the inlet duct becomes more efficient, it increases pressure on the nozzle, which results in higher flow through the system.

But, higher flow through the system results in reduced pump efficiency. Hence the need for the variable nozzle to regulate the system flow for pump efficiency.

Summary: Using the combination of these three innovations means a high volume of water, an efficient inlet duct and an efficient pump operation under all operating conditions.

The Patents

To view the patents, go to: http://www.uspto.gov/patft/index.html

Click on Patent Number Search.

Enter a patent number noted below.

#5,658,176 “Marine Jet Propulsion System”

#5,679,035 “Marine Jet Propulsion Nozzle
and Method”

#5,683,276 “Marine Jet Propulsion Inlet
Duct and Method”




_____________________________________________
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, and by the webmaster Carl. He begins with some jet-pump fundamentals http://www.yachtforums.com/forums/13462-post27.html
……some excerpts…

Originally Posted by YachtForums

Impellers (and jet-pumps) work on the principal of positive and negative pressure, or a push/pull concept. As a blade rotates, it pushes water back (and outwards due to centrifugal and accelerated 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 backside of the blade. This action 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. 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. This is where you get the "jet" in pumps. Finally, a steering nozzle is used 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., the intake gullet, its volumetric area, the laminar transition of the intake housing, stator blade area (including 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.

The intake gullet is the recessed area within the hull leading up to the entrance of the jet pump. This area plays a vital role in jet pump efficiency. There are a multitude of factors that determine its length, size, shape and depth. For instance, a larger vessel with greater displacement may choose an intake gullet design with a more gradual rake leading up to the jet pump entrance. This maximizes the amount of water available for acceleration. In this scenario, intake gullet vacuum is not as critical because the weight of the hull (and the depth of the pump) will keep the intake cavity primed. In contrast, a light, high speed hull that rides closer to the water's surface, may use an intake gullet with a more aggressive rake and a reduced intake gullet area. This decrease in cavity size, increases the vacuum (or negative pressure zone) at the intake, which helps reduce ventilation brought on by a higher speed planing hulls that operate near the water's surface.

Ultimately, the best intake gullet design would be variable in size. In other words... larger for acceleration and smaller for high speed operation, to maximize intake vacuum when aeration is present.

Originally Posted by tantetruus

Do varible gullets exist??

Originally Posted by YachtForums

Not really, although it is possible to mechanically reduce the volumetric area of the cavity without having having an adverse effect on laminar flow. I've done a considerable amount of analysis in this area and it yields great improvements in efficiency, but the mechanical means of altering the gullet would not prove economically viable.

Originally Posted by Codger

There was a variable intake for waterjets in development during the mid 90s. All that I saw of it was a paper-napkin drawing during a conversation that I probably shouldn't have been having. Looked like a NACA duct with a flush spring loaded slider. As speed increased the plate moved aft and reduced the opening size. There was a drag vane inside the throat that was part of the actuator mechanism. Sorry, that's all that I recall about it.

Originally Posted by YachtForums

Yes, I'm familiar with the system. In the end, we developed an inner liner that was positioned on the top of the gullet (and only the top). It used a soft durameter plastic that was flexible enough to pull away from its seated housing as vacuum increased, creating a bubble shape. Because pumps run fully loaded at idle to medium speeds, the pressure of the water kept the "skin" pressed firmly into its housing.

As speed increases sufficiently, water can not make the abrupt turn into the intake gullet as quickly as it can at slower speeds. The result is, a negative pressure air pocket is formed on the top of the intake cavity. This negative pressure zone pulled the skin away from its seated position, thus reducing the volumetric area of the intake cavity, which ultimately increases vacuum and therefore efficiency at higher speeds. The beauty of the system was... no moving parts.

Originally Posted by YachtForums

On the subject of intake gullets, which are only one aspect of jet-pump integration and configuration, I should expand on the venturi...

Of all the components that make up a jet pump, the venturi is by far the most critical component in dimension, shape and size. It is the final stage of acceleration that water will receive prior to expulsion. The venturi, for those of you not familiar, is the shroud located just after the stator blades (directing vanes) and the part of the jet pump that the steering nozzle or thrust deflectors are most commonly connected to.

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!!
(to be continued....)

[john zimmerlee]

I need help!
My electric powered personal watercraft prototype was commended by the 2005 International Concept Boat Competition, but it was powered by trolling motors which require too much draft.

It appears that electric power is not sufficient enough to drive jet pumps and they don't work well at low speed anyway.

The craft currently drafts only 5" of water and I would prefer to have propulsion that will work in the same depth.

Paddle wheels make too much noise spalshing above the waterline.

But this is what I'm thinking: Build a catamaran with a tunnel down the centerline. Turn two paddlewheels on their side and embed one in each hull about half way in . . . exposing half of the wheels in the tunnel. The wheels will turn counter-rotating . . . much like a cake batter mixer . . . sucking the water through the tunnel. I think this is the way vane pumps are built.
If the width of the wheels are about the same as the depth of the tunnel, and the top of the tunnel is below the waterline, it should not cavitate and should have propulsion wherever the the boat can draft.

Obviously, I'm missing something. It seems too logical and I can't find where any one is using this, so please tell me where my thinking is flawed.

John Zimmerlee
Marietta GA
www.streamdancer.com

[brian eiland]

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

Originally Posted by Originally Posted by YachtForums

On the subject of intake gullets, which are only one aspect of jet-pump integration and configuration, I should expand on the venturi...

Of all the components that make up a jet pump, the venturi is by far the most critical component in dimension, shape and size. It is the final stage of acceleration that water will receive prior to expulsion. The venturi, for those of you not familiar, is the shroud located just after the stator blades (directing vanes) and the part of the jet pump that the steering nozzle or thrust deflectors are most commonly connected to.

The exiting size of the venturi's orifice is generally half the size of the dimensional area of the intake gullet footprint, or a 2-to-1 reduction. Quite simply, the venturi is a reducer or compressor, and in the case of water, which can not be compressed, it is an accelerator. The venturi is one of the most important links or stages in jet pump design. Without it, the jet pump as we know it... would be rendered benign.

Increasing the venturi's expulsion size will decrease backpressure, and allow water to be processed more rapidly, thus moving the hull (mass) forward at a faster rate due to more available thrust, but sacrifices top speed because of reduced compression. Decreasing the venturi's expulsion size will create more backpressure, which results in less water being processed, but increases the velocity at which it exits. This results in higher speeds, but does not give the mass of water necessary for greater acceleration. Venturi designs are usually a compromise to give maximum acceleration and top speed.

The real reason that an adjustable venturi is necessary and holds so much value is that because pumps do not run fully loaded at higher speeds.

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

Originally Posted by Originally Posted by YachtForums

In early 1984, our research team began conceptualizing and theorizing the potential of an adjustable venturi and later developed the V.G.V. (variable geometry venturi) This unit operated on the principles mentioned above but utilized hydraulics to control orifice diameter, which was necessary given the huge amounts of thrust created on the research vehicles we developed. In 1987, a very unique material was made available, current regulated (electrical stimuli), that lined the inner walls of a venturi (or bowl) and controlled exact camber and orifice dimension. This material has future applications i.e., artificial limbs, robotics, etc. Unfortunately, it is under regulation for now and there is no access to it.......
Inner wall flex and fluctuation is critical as well. The reason that I mention flex is because it is conceivable to utilize a material with built in flex to accomplish some desirable characteristics.

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??

Originally Posted by Originally Posted by YachtForums

One of our first VGV’s was an adjustable venturi that utilized inner bowl “feathers” actuated by an aperture that closes concentrically. While it was mechanically a very cool-looking contraption, much like the afterburning tail-feathers on a fighter jet, it was hydrodynamically incorrect. The reason is simple, while it reduced orifice size it also increased the rate of compression while failing to control trajectory. Properly configured and controlled, the device had great merit

Originally Posted by Originally Posted by YachtForums

A properly designed venturi can yield significant acceleration gains and top speed gains. A really good design will become exponentially more efficient with speed. In other words, the faster you go.. the more efficient it becomes! Venturis work on thrust and pressure. Wherever you have thrust you have the potential to create vacuum. Wherever you have pressure, you have energy. And in the case of venturis, that pressure can control a multitude of variables… and this entire process can be executed with NO MOVING PARTS!

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??

[Guillermo]

Brian,
A client of mine installed a 150 HP diesel JetPac on his working boat with nice success. You may find more info at thread: http://boatdesign.net/forums/showthread.php?t=11073

Probably you already know these high thrust jets suitable for slow speed operation: http://www.marinejet.com/ Any comments on them?

[brian eiland]

Originally Posted by YachtForums
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.

Originally Posted by YachtForums
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

Originally Posted by Innomare
One exception is the jet drive, though, where the propeller (actually impeller) is so efficient because it hardly has any blade tip losses. I wonder when, or if they'll come up with an electrically driven rim-drive waterjet propulsion. It seems like the best of both worlds to me. No shaft going into the waterjet…

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 as I note at RunningTideYachts/Power.

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:

Originally Posted by YachtForums
Over-lapping blades are one of the most beneficial ingredients in pump efficiency, along with radial edge (swept) blade designs.

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

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:

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…

Originally Posted by YachtForums
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.

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:

Originally Posted by YachtForums
The real reason that an adjustable venturi is necessary and holds so much value is that because pumps do not run fully loaded at higher speeds. They ventilate, thus inducing air into the equation. Because the amount of water available for compression at higher speeds is reduced, due to the introduction of air, there is less water density available for thrust

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).

[Guillermo]

May we even think in contra-rotating rim-driven propellers?

[ed fitz]

I am new to this discussion and am not an engineer. I would like to propose a different type of jet unit. If you were to take a drive shaft and attach it directly to a diesel engine (no transmission). Then take the shaft and run it into the jet water holding area,with the inlet directly below, then through a cutlas bearing ,it would be easy enough to weld it in, and then install a small propeller on the shaft then allowing the shaft to continue into a 12 inch tunnel. At the end of the shaft would be a 11.9 inch propeller. This would create an axel flow propulsion. This propeller could become a dual prop or a counter rotating prop, or some variation of the above. This tunnel would extend 12 to 14 inches behind the boat. The small propeller would direct the water flow toward the tunnel and the main propulsion attempting to minimize airation. It would apear to me that with the proper engine etc. that a pair of 12 inch units would be sufficient to push 10 to 15 tons of boat. I have purposly left out many details (parts). I am looking for comments on the general propulsion Idea. The unit would be inexpensive and relatively easy to maintain and repair. At the present time the four or five jet manufactures are getting 15K for a jet. This is totally rediculous.

[u4ea32]

That's basically how they work. I don't see your distinction.

Many drives have essentially a properllor forward of the impellor. Probably all successful drag jet boats do this, and have done so for years.

The capitalist system is competitive. There are many, many manufacturers of jet drives in many different countries. If the price seems high, and no competitor can undercut it, then I suggest that there are complexities and
costs that you don't appreciate.

[ed fitz]

I thank you for your ansewer.I did not know that the drag boats used a prop foreward of the impeller. As to your comments on price: I returned yesterday from Benton Ark. visiting N.American Marine Jet. They are essentially building military jets. The are built to be bullit proof and last foreever,the are extra heavy etc. I have looked at Hamilton,Doen,Ultra jets. Each is built with the military in mind and not recreational boaters. If you look at a Jacuzzi jet it has an impeller up to about 8 or 9" and sells for about 5K. This is aimed at the recreational boater. All of the other mfg. start at 12" (which seems to be the minimum the military wants) and go upward. These jets start at 15K,and are extremly heavy,bullit proof. These mfg seem to me to be offering the general public copies of military jets. I would apreciate a rebuttal.

[u4ea32]

Ed, please email me, as I think we are both doing the same research

david_smyth_ogst@mac.com

[ed fitz]

If you are looking for a jet you might want to check G S A auctions the sell rib boats with jets. They are usually dual cummings engines with Doen or Hamilton jets. They are sold many times in southern Cal. Also Military Surplus and Government Surplus Auction sell jets,especially coast guard jets with Yanmar engins. Most of these jets are in the 12" range and would probably be ideal for a 12 knot boat. I am looking for an 11" or 12" jet that is not specifically designed for the military,i.e. very heavy and bullet proof. The closest thing I have seen is the Kamawa. It looks like the best impeller design.