Jet, Jet Pump, Waterjet, Jet Drive vs Prop

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:

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

 

JetPac from Sword Technology

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'.
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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.

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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
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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

 

Reply from YachtForums

 

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.

 

Jet Pump Technologies

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.
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…from [B]www.marinejettech.com[/B]

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”




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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…

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....)

 

vs paddlewheel or vane pump

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

 

Jet Pump Technologies (continued)

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

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

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

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

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

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

 

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?

 

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 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:

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:

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).

 

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

 

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.

 

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.

 

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.

 

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

david_smyth_ogst@mac.com

 

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.