Jet, Jet Pump, Waterjet, Jet Drive vs Prop

[Riverjet502]

Gentelmen, When you are talking about preloading an impeller you are referring to an inducer. This does help your hole shot especially when working with a loader in the intake, but doesn't always produce a higher top end. This works very well at reducing cavation. One thing to remember or think about from the above posts is that there were three styles of pumps mentioned above; axle flow, mixed flow, and centrifigul pumps these are not apple to apple comparisons. Each has its own nitch so to say where it works best for the craft it is installed in.

 

[Black Swan 01]

Wow, this thread is getting old. Fortunately, the subject matter is still much on the minds of many.

With regard to the comment about boat weight not changing the load placed on a jet... how much does the static weight of a boat change when it becomes airborne? Or conversely, how much additional torque load is placed on a jet drive system when one adds, say, 500 lbs. of diesel fuel to a boat?

I think that was more of what the statement referred to, that increasing the mass of the boat did not increase "load" on the jet. Increased mass (weight) of any given hull will, of course, change the performance of that hull based by additional hull resistance due to additional "wetted area", and certainly, we have seen differences in "mileage" , affected (sometimes severely) by overloading, but does additional mass in any given hull change the torque load on a jet? Not according to anything I've read from any of the manufacturers.

And I'm not sure that airborne operation is an important design criteria when choosing a propulsion system for a fast ferry.

However, you are absolutely correct, the torque load on a jet drive does change when you take the boat out of the water.

(BTW, the day my 11000 lb. 30 ft. Reinell Flybridge with the twin diesels becomes airborne or starts chine walking due to excessive speed, I'll post a picture of my shorts!

For the most part, and I may be stretching here a bit, I don't perceive that the majority of contributors here are primarily concerned about launching their boats into the air. I'm sure that there are probably a good many sites that are more attenuated to the "go faster" crowd.

Meanwhile, the relationship between thrust, flow, pressure, horsepower, torque, and the myriad of
factors surrounding this type of propulsion system still defy our attempts to simplify it. The one thing I have noticed from all the manufacturers websites and case studies is a propensity towards throwing a lot more power at jet drive systems. Very few (none) are touting economy or "efficiency", (think I'll leave the lid on that can of worms), the key buzzword seems to be "performance".

Heck, my boat would "perform" better if I threw a couple af 500HP diesels at it too!

So what it seems to come down to is that Jet drive propulsion becomes a choice that is made by weighing a certain set of criteria. In my own case, the choice will be made based on my own set of criteria, which, in no particular order seem to be:

1. shallow draft operation
2. ease /simplicity of installation
3. reliability/ serviceability
4. safety/environmental impact
5. reduced operational maintenance - no fluids to change, cool, or recycle
6. all that expensive stuff that hangs down under my boat goes away
7. reduced weight - 2 jets weigh less than 1 veedrive+1 shaft+1prop+1 strut+1 rudder+1 thru-hull hardware group

The process is underway. The boat is on the hard, I'll keep you all posted.

 

[alexlebrit]

Just to pull it up from the depths once more - and to twist it out of shape on the way.

What about centrifugal pumps as opposed to axial pumps? Why do I ask? because I'm still on the continued search for an alternative to a propellor for a pedal powered boat. Traditional jet drives are deemed unsuitable because of the slow rotating speed achievable by the pedaller, but I'm wondering about centrifugal type pumps instead which seem to be more "powerful" at slower rotating speeds.

 

[john zimmerlee]

While we're on the subject, I'm in the final prototype of my foot-controlled electric powered personal watercraft - Stream Dancer. I'm using a 30 amp trolling motor on each side turning 18" long 5.25" diameter augers. This seems to address the shallow water need (6") and lessens the cavitation/ventilation problems of standard props. I briefly worked with centrifugal pumps but they have no reversibility without cuffs, so I abandoned them quickly. The system is quite simple and can be seen at www.streamdancer.com

I'm still open to suggestions on decreasing drag and increasing efficiency. Electric-only fishing tournaments are popping up all over the place and interest is extremely high in this watercraft.

Any comments or suggestions are welcome and appreciated,

email@streamdancer.com

John Zimmerlee 770-565-4420

 

[FAST FRED]

"I seek to get a discussion going of smaller units as utilized on vessels in the 10-30' foot range."

In PBB there was a similar discussion of jet vs prop , and what caught my eye was a statement that the water accelerated by the hull on a 40 f'er was about 4 inches thick.

While an inlet is fine for capturing a very small portion of this accelerated water , and increasing the jets efficiency, it is probably only capturing 5 or 10% of the energy already paid for.
Atkins reverse deadrise setup would seem to overcome this and use almost 100% of the "feed water" (accelerated by the forward portion of the boats movement ) to feed the prop .The Gullet is 100% the beam of the boat.

Perhaps this explains the claimed efficiency at semi displacement speeds .

A high velocity prop stream (Atkin used small diameter props and 1-1 gearing) would operate in the already accelerated stream for better efficiency?


Certainly easier to change prop pitch , than have a variety of impellers cast, to optimize.

But this is only for SL 3 or perhaps SL 4 , not for the high speed folks.

http://www.robbwhite.com/rescue.minor.html

Is an example of a boat that seems to have exceptional performance.

FF

 

[brian eiland]

Cross-Reference discussion

I seek to get a discussion going of smaller units as utilized on vessels in the 10-30' foot range

Hello Fred,
Regretably I don't have time to enter this discussion at the time, but I might suggest you post a similar inquiry on this other forum as sometimes there are many interested parties that do not cross over in the discussions. The forum moderator of this other site has quite a bit of knowledge in the smaller jet units, particularly if you can draw him into the discussions.
Jet Drive vs Prop
http://www.yachtforums.com/forums/technical-discussion/4470-jet-drive-vs-prop.html

 

[dooley]

Looking for some information on how to change lower unit from prop to jet unit. Recently purchased a 19' alumaweld formula vee and it has a 1995 150 hp 2.0L mercury that came with a jet unit. have done some out drive work but have never changed from lower unit to jet b-4.This appears to be pretty staight foward but does anybody have any insight to this.

 

[brian eiland]

electro-magnetic propulsor as part of a waterjet propulsion unit

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.

Abstract

A concept investigation has been carried out into the possibility of using a tip driven electro-magnetic propulsor as part of a waterjet propulsion unit. The primary advantage is that there is no need to insert a drive shaft within the waterjet inflow. This significantly reduces cyclic variations in the propulsor inflow and removes an area of flow separation around the shaft. It also provides the designer with greater freedom as to the types of propulsion systems available and where they can be placed within the ship.

The viability of the concept has been examined by considering the performance of an axial flow electromagnetic thruster developed for the ROV market and numerically studying its performance within a typical waterjet inflow. A study was also conducted to examine the scalability of such propulsion units. It is concluded that for a typical size of waterjet with an input power requirement of 110 kW and diameter of 0.25m the maximum delivered power at 2,200 rpm would be 90 kW. This design would have the benefits of no shaft induced losses and reduced cyclic blade loadings and should deliver a higher thrust than comparable conventional waterjet units using geared electric motor drives
http://eprints.soton.ac.uk/22746/

 

[Guest625101138]

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

If your system has to bring the water flow up to hull speed before ejecting it then it will have a theoretical peak efficiency of 50%. Same as what a squid can get.

The advantage of a squid drive is its rocket mode in the initial thrust as the water is taken in when the velocity is at zero. THe first thrust is very efficicent so gives good escape mode. One shot wonder though. I designed a rocket system that ran on a 5 litre 3000psi air cylinder that would rocket a person to impressive speed. It stored water in a pvc tube that was ejected at high velocity. Again just one shot.

With a jet you want to just accelerate the water backward rather then bringing it up to boat speed before ejecting. This is the same as a prop that just accelerates the static water a bit faster in the reverse direction.

You should get good efficiency with a multivane ducted prop. Javaprop has a ducted option so you can use it to determine the efficiency.

So the answer is YES. I think you are missing something. You have to devise a system that does not need to accelerate the water to boat speed before ejecting it.

I have conceived a squegee drive that is like a hose pump but very short tube. The tube ducts water longitudinally through the hull as short as possible and fits into an arc in the bottom that rollers on a rotatating spider squegee water through. If it could be engineered well to avoid back leakage it would make a perfect jet. It has high theoretical efficiency. Better than 70%.

Rick W

 

[Guillermo]

...possibility of using a tip driven electro-magnetic propulsor as part of a waterjet propulsion unit.

It makes a lot of sense. Thanks for the info, Brian.

Cheers.

 

[Capt Dave]

Brian
Have been building water jets for 15 years in composites (GRP) in 2 sizes 300mm & 370mm with larger unit in development as well as a smaller unit 228 mm using injection molding (In Thailand)
The Electro-magnetic concept is interesting but I think Electrolyses may be a concern.Has anybody studied this 77317343

Abstract

A concept investigation has been carried out into the possibility of using a tip driven electro-magnetic propulsor as part of a waterjet propulsion unit. The primary advantage is that there is no need to insert a drive shaft within the waterjet inflow. This significantly reduces cyclic variations in the propulsor inflow and removes an area of flow separation around the shaft. It also provides the designer with greater freedom as to the types of propulsion systems available and where they can be placed within the ship.

The viability of the concept has been examined by considering the performance of an axial flow electromagnetic thruster developed for the ROV market and numerically studying its performance within a typical waterjet inflow. A study was also conducted to examine the scalability of such propulsion units. It is concluded that for a typical size of waterjet with an input power requirement of 110 kW and diameter of 0.25m the maximum delivered power at 2,200 rpm would be 90 kW. This design would have the benefits of no shaft induced losses and reduced cyclic blade loadings and should deliver a higher thrust than comparable conventional waterjet units using geared electric motor drives
http://eprints.soton.ac.uk/22746/

 

[brian eiland]

Electric PWC, the Green Samba

Electric Personal Watercraft (PWC)

Got a note from a fellow just today asking if I had seen this... I had not.

It's not the first to try its hand at an all-electric personal watercraft, but the Ohio-based Silveira Group is claiming that its new so-called Green Samba will be the first "viable" vehicle of its kind. To that end, the company says that the Green Samba will deliver the same 65 mph performance of the fastest 260 bhp sit-down PWCs, and do so while remaining completely silent thanks to a pair of twin direct drive electric propulsion pods. As the company is quick to point out, that also has the added benefit of eliminating one of the biggest complaints about PWCs from the public: their noise. Of course, the other key to viability is the price, and Silveira is unfortunately doing decidedly less talking about that right now, saying only that it will be able to "ballpark a retail price" when it finishes work on the latest prototype in August.......Gizmag

Wonder what the details are on the propulsion unit(s) ??

 

[brian eiland]

Unique Propulsion Units

...just found this:

When we wrote up the Samba in April 2009, it had already been through four prototypes, and several more have seen the water since then. The latest iteration has seen the shape and construction refined to such an extent that according to Rodrigo Silveira, just 65 bhp is now required to match the performance of the biggest 240-260 bhp Sea-Doo, Kawasaki and Yamaha designs. But the aim had also been to eventually replace the 800cc twin-cylinder internal combustion engines the company was using in the development process with electric propulsion.

In the last twelve months, the company almost gave up on that process. "I can tell you that making a green watercraft is not easy," Silveira told Gizmag.

"Our experiments were becoming more discouraging. We knew that the public would not accept lesser performance to go green – being a good citizen does not compensate for being ten mph slower than your friends on their internal combustion skis."

"We wanted to use an electric motor but the amount of energy required to get a jet ski on the plane was so large that it was using too much of the battery capacity. Combustion engines give you the torque you need to get to planing very quickly, without using too much of the available stored energy. We refined everything we could but to get the engine performance we wanted, and sufficient range, we would have needed 150 pounds of batteries which would have impinged on the weight and hence maneuverability too much. We had some sleepless nights wondering if our objectives were achievable with the current state of battery technology."

As the man who split the atom is oft quoted, "we didn't have any money, so we had to think" and it was in going back to the objective and rethinking it that the breakthrough came for Rodrigo. With limited resources and the high burn-rate of funds associated with intensive R&D, Silveira acknowledges that he was on the verge of giving up when the breakthrough came.

"We realized that we had been focusing on the motor, not the pump. In the same way that automotive electric propulsion is just about to go through a revolution of design because it doesn't require the same centrally located ICE we have had for the last 100 years, we began exploring what could be achieved with different propulsion methods."

"Once we began looking, we came across a propulsion technology originally developed for military applications that had not been previously used in watercraft and we have now acquired an exclusive license for using the technology on watercraft. After testing, we are now confident it makes our objectives possible both weight and price wise."

"With twin electric shaft-less drives of far less modest horsepower output, we are expecting about the same top speed as with the 65 bhp gas engine, which will give us 65 mph. In some ways the US Coast Guard's limit of 65 mph for watercraft has worked to our advantage as otherwise we'd probably be seeing 80 mph watercraft by now. Aiming at a non-moving target of 65 mph has been a huge advantage."

"The PWC is a huge water pump and the existing manufacturers have ignored the inefficiencies of the jet drive system currently in use by pouring horsepower and fuel on it. The inefficiencies in the horsepower-thrust conversion are quite obscene and that's were we've been focusing our energy."

"The propulsion pods we're using have an electric motor self contained within them. Each one has one moving part on the internal wall of the thrust pod, so it's a direct drive and it has eliminated many of the losses and made the system extremely efficient. What started out as a 6 horsepower motor became two 12 horsepower pods that are far more efficient."

"Our tank tests have proven to us that we can achieve between 34.9 and 36.5 pounds of thrust per horsepower compared to 28 pounds of thrust from a gasoline engine. By going electric and having a self contained pod that isn't connected to anything else other than a power source and a controller, we have achieved a similar freedom of design as automotive designers are now experiencing with electric motors. It means we don't need a big fat motor and a drive train in line with the motor."

"We started out with a single, centrally-located propulsion unit, but along the way, we realized if we made it into a dual pod system we would gain maneuverability by being able to get far more angle on the thrust pods than by using the traditional centrally-located nozzle steering. Then to take full advantage of turning the propulsion units, we had to completely different hull configuration."

"The redesign allowed us to put the pods below the planing line, which allows a more efficient intake and even better performance. With traditional design you can overload the pump, so you have a limitation on the amount of water going into the intake tunnel, but with these pumps we realized that the more we feed the pump, the better the performance.”

One of the benefits from using a lightweight, low-powered craft with such efficient drives is that the range of the unit has now skyrocketed and Rodrigo expects to get more than three hours usage from a full charge – well in excess of the 250 horsepower sit-down PWCs.

"Our testing so far shows that by getting the two pods on either side of the center line, you can get amazing turning ability. The first tests showed it was a bit too sensitive – it'd tip you off too easily because you just couldn't hold on. The prototype we have planned for August brings all the strengths into focus and… we're confident now we have a viable next generation PWC in every respect.."

http://www.gizmag.com/green-samba-first-viable-electric-pwc/15141/

 

[brian eiland]

I was thinking it sounded more like these two subjects I brought up before:

Posting #9, Rim-Driven Impellers

Posting #23, Electro-Magnetic Propulsor as Part of a Waterjet Propulsion Unit

 

[speedboats]

And according to their propaganda...

we came across a propulsion technology originally developed for military applications that had not been previously used in watercraft and we have now acquired an exclusive license for using the technology on watercraft.