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]

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/te...e-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]

Originally Posted by brian eiland

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/

[Rick Willoughby]

Originally Posted by john zimmerlee

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]

Originally Posted by brian eiland

...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

Originally Posted by brian eiland

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/