Why A Jet Drive?

[Submarine Tom]

I very recently posted asking for input on the merits of a jet drive for a

small, high-performance submarine I am designing to build. My thread was

poisoned and derailed by a jaded, pessimistic, paranoid, close-minded,

know-it-all who apparently has no knowledge of jet-drives.

I have abandon the thread.

Here, http://www.psubs.org/projects/1237684922/r300submersible/

a fluid mechanics engineer has built an impressive, expensive sub with a

jet drive. He did it for safety around the grand-kids he says. His vehicle

compells me to ask those in the know, why a jet drive? If you are pro jet

drives, please let me know why.

My sub is low drag, requires 500 Newtons thrust to achieve 15 knots with

an 8 kw motor and enough batteries and compressed air to fly around for

an hour with an additional hour reserve. That's with ~90% efficiency.

What kind of efficiency can I expect from a jet-drive? Can counter-rotating

forces of torque be cancelled by stator vanes? Are there other benefits to

jet-drives, especially for my application?

Please note: if John (AKA "jehardiman") responds on this new thread I will

abandon it too and would ask you to personal message me your input.

Thanking you, once again, in advance,

Tom

[Submarine Tom]

That's kind of what I thought too, no good points to a jet drive.

Inefficient sums it up in a word.

Silly that the R300, with all it's costly construction, used one.

What a waste. He claims 25 knots at the top end. He's dreaming.

Props it is for my sub!

Cheers, Tom

[Ad Hoc]

Jets versus props is a bit like asking which is best, steel or aluminium or GRP.

Everyone has their own views, however there are as with steel, ally etc each has their own pro's and con's and must be viewed objectively and in the context of the design.

Generalising here, however, jets are usually best at higher froude numbers, simply because the efficiency is better and they are more suited to low length displacement ratio designs. BUT, how many people care that much about the efficiency if all the other 'desirables' of the design have been met and she performs as 'desired'?

Almost everyone here who contributes fails to appreciate that the final design is the desire of the client. If the client wanted the boat made from Switz cheese and engines run with little mice running around a treat mill, its crazy, yes. But and this is the point. If the client wanted that, then it matters not what the efficiency is or if aluminium is better than the cheese. It is all about what the client wants. So, how does one design the boat to perform made from the cheese, that is the interesting part.

As such looking at the pro's and con's of both jets and prop's, which, for your boat, has more pro's than con's...it is a simple at that.

Design is all about finding the solution to the problem, (or SOR)...there is no real right or wrong answer, only different solutions.

It becomes derailed, threads etc, by those who have never designed a boat before and/or only have computer programs to back up their very weak, if any, technical knowledge (or very myopic) and the fact they are only looking at one aspect, because that is all the program can do, and not the "whole design", with is the end product.

When i get into my car, there are some bits i don't like..but does it make the whole car crap..of course not. It is all about the holistic design....not whether the a/c switch is where i want it, or the accelerator peddle being slightly bigger etc.

Same is true in designing boats, well, those that actually are trained and qualified naval architects and design for real. It is all about the holistic product working as one to achieve the desired goal, by whatever means.

[Frosty]

Jets versus props is a bit like asking which is best, steel or aluminium or GRP.

Everyone has their own views, however there are as with steel, ally etc each has their own pro's and con's and must be viewed objectively and in the context of the design.

Generalising here, however, jets are usually best at higher froude numbers, simply because the efficiency is better and they are more suited to low length displacement ratio designs. BUT, how many people care that much about the efficiency if all the other 'desirables' of the design have been met and she performs as 'desired'?

Almost everyone here who contributes fails to appreciate that the final design is the desire of the client. If the client wanted the boat made from Switz cheese and engines run with little mice running around a treat mill, its crazy, yes. But and this is the point. If the client wanted that, then it matters not what the efficiency is or if aluminium is better than the cheese. It is all about what the client wants. So, how does one design the boat to perform made from the cheese, that is the interesting part.

As such looking at the pro's and con's of both jets and prop's, which, for your boat, has more pro's than con's...it is a simple at that.

Design is all about finding the solution to the problem, (or SOR)...there is no real right or wrong answer, only different solutions.

It becomes derailed, threads etc, by those who have never designed a boat before and/or only have computer programs to back up their very weak, if any, technical knowledge (or very myopic) and the fact they are only looking at one aspect, because that is all the program can do, and not the "whole design", with is the end product.

When i get into my car, there are some bits i don't like..but does it make the whole car crap..of course not. It is all about the holistic design....not whether the a/c switch is where i want it, or the accelerator peddle being slightly bigger etc.

Same is true in designing boats, well, those that actually are trained and qualified naval architects and design for real. It is all about the holistic product working as one to achieve the desired goal, by whatever means.

The guy asked about a jet for his sub, he got your resume and your dislike for your accelerator pedal in your car.

Your opinion of us "almost everyone here" is eye opening, did you study for your superiority complex or is it a gift.

If the guy wants to design a sub he can do wether you agree or not.

It makes me wonder if you have any designing to do as your here most of the time.

Im retired,-- all the time in the world,-- retired at 38 you wont be able to do that.
Now if you would read his question again he explains adequately and gave lots of numbers for you to play with.

Try not to mention how good you think you are. "Most of us here" already know.

[Ad Hoc]

"..Your opinion of us "almost everyone here" is eye opening, did you study for your superiority complex or is it a gift..."

Nope, just by reading others replies..formed my own opinion, you can take it or leave, as I'm sure you have your own too. But i wonder how many fully qualified naval architects there are, compared to wannabes and amateurs, care to guess?.

Going back to his question:
"..I very recently posted asking for input on the merits of a jet drive for a
small, high-performance submarine I am designing to build..."

There are no numbers, just opinions required, which is given above. Without any hard data, that is all one can give, opinions, unless you are in receipt of more information than anyone else, please share?

"..It makes me wonder if you have any designing to do as your here most of the time..."..time differences and the magic of pop up emails is quite amazing..either i choose to reply or not...and this 'time issue' is important to the posting because..???

[CDK]

Most subs I've seen look like a cucumber with a shrouded prop at the end. There were lots of them in '40-'45 and they kept on building them that way. The torpedo's had the same shape, just scaled down, so that probably proved to be the best way to propel a submerged vessel.

The only refinement that comes to mind is a duoprop system because it eliminates the rotational forces so there is no need to compensate these with fins. But on the other hand, fins or rudders are necessary anyway....

Whether or not a shrouded prop should be called a prop or a jet drive is a matter of personal taste: both use a rotating object within a confined space.

[Submarine Tom]

You guys crack me up.

I found lots of info on related threads.

I should have looked there in the first place.

Tom

[Doc Nozzle]

You guys crack me up.

I found lots of info on related threads.

I should have looked there in the first place.

Tom

This site is the "Odd Duck" of boat forums! You would think with the word "design" in the name of the website that most of the members would be somewhat OPEN to new and/or creative ideas or questions.

Hmmmmm...:(

[brian eiland]

Hi Tom,
If this subject still interest you, you might have a look over at this subject thread on another forum. The webmaster of this forum, Carl, has lots of experience with this subject. Posting a question there just might draw him out into the discussion.

http://www.yachtforums.com/forums/technical-discussion/4470-jet-drive-vs-prop.html

[Submarine Tom]

Thank you Brian, I'll have a look.

-Tom

[Submarine Tom]

Brian,

Thank you. Really interesting reading. Just what I was looking for.

Too bad the thread came to a halt 1 1/2 years ago.

Any further reading you'd recommend?

Cheers, Tom

[apex1]

PM Baeckmo,

thats all you need.

Regards
Richard

[Submarine Tom]

I wish it were that simple Richard.

I've emailed and PM'ed him, no response.

So, I thought I'd try a post.

Thanks though,

Tom

P.S. Wait a minute, who's Baeckmo, Brian Eiland?

[baeckmo]

Ok Tom, got your message and have checked the link you referred to earlier. I will write a short tech explanation to you. Very briefly one can say that the Cliff-something guy is loosing a substantial amount of operating radius because an enclosed propulsor (~waterjet) has very low total propulsion efficiency in this application. I would say that the selected inlet arrangement is making things even worse. Unfortunately I find no technical info on that page, so there is nothing to base a calculation upon, except for pics on the jet nozzle diameter. The job is admirable, but he is lacking basic understanding of propulsion mechanisms.

If you have any (realistic) figures on expected hull drag, cruising speed, motor power and rpm's for your rig, please send so they can be incorporated as example.

[Submarine Tom]

I thought I gave them to you, but here they are:

Max Drag: ~500 Newtons at 16 knots

Mass: ~300 kg

Cruising speed: 5 - 10 knots

Max shaft power: ~8kw

RPM: as needed

Dive time: One hour max

No surface operation, buoyancy is +/- 1 kg

Prime objective: FUN!

-Tom

[TollyWally]

"Prime objective: FUN!"
LOL,
It's good to have the end objective clearly in focus.

[baeckmo]

Right Tom, let’s start the week with a quick check on your basic data:

Va=8.23 m/s (Advance speed 16 kn) No correction for wake, for simplicity.
Pdyn=33866 Pa (Dynamic pressure = Va^2*density/2)
Af=0.4 m2 (Frontal area, dia 650 mm plus small canopy) Fins and rudders not included.
Cd=0.07 (Drag coefficient. Best shape, l/d ~5.5 has Cd=0.04, but in reality 0.06>0.09)
R=948 N (Hull drag=Af*pdyn*Cd). Note; bare hull drag, your estimate is too optimistic.

Now we continue to check a propeller for this setup, we use daddy’s method; pencil, paper and Taylor prop diagrams (we will correct for attainable speed later).

Paq=7804 W (Va*R, actual effective power)
Etap~0.7 (Propeller efficiency, first estimate to find range, select within 0.6>0.8).
From Taylor prop diagram, example: 4 bladed, BAR 1.00, find the optimum P/D for etap=0.7. (Note that this is NOT the optimum propeller!) That operating point corresponds to:

Bp=6.2 (load coefficient, = n*sqr (Pshaft)/Va^2.5). Dimensions here n=rpm, Psh=hp and Va=knots.
Delta=97 (de=n*D/Va). D=prop dia in feet.
P/D=1.35

This gives us the required shaft power: Psh=11149 W (=Paq/etap), =15.15 hp. Obviously far beyond what you have available. You have a shaft power of about 7600 W (=10.3 hp), assuming transmission losses of 5 %. Now as there are no waves involved in submerged operation, the shaft power is f(Va^3), so the possible max speed with your motor will be 16*(7600/11149)^0.33, which boils down to 14 knot with your 8 kW motor. The corresponding drag is then 726 N.

Then back to Mr Taylor and the Bp number: with Va=14 knot and Psh=10.3 hp, we find n=6.2*14^2.5/sqr(10.3); which is 1437 rpm. Using n and Va in the delta expression, you find the prop dia 0.954´, or 288 mm. Pitch is 389 mm. The tip speed is then 21.7 m/s, which secures non-cavitating operation.

Next, we will have look at the situation with a jet installation like the one in the link. Please give me your guesstimate of the nozzle diameter of that rig!

[Submarine Tom]

Baeckmo,

Thank you.

The nozzle diameter can be whatever you want it to be.

You've got some facts wrong however.

Shaft power was given as 8 kw after losses.

Sub diameter is 0.45 m

Drag is 500 - 600 Newtons max

There is no canopy.

Prop'ed, rpm's would be 1200 and diameter 0.5 m, pitch 0.5 m.

-Tom

[baeckmo]

That's ok Tom, my intention here is not to deliver a marine engineering consultancy job particularly for one specific project, but to show the principal procedure leading to a design goal. I feel quite convinced that you have the basic skills to do the math with the numbers relevant for your project! (But I do wonder how you squeeze an adult shoulder-width of 500 mm+ into a 450 mm tube though.....).

So, for the study of the jet version, I will stay with the basic figures used in the propeller case; it is just a question of a proportional scaling. The quantitative conclusions are still valid.

As for the nozzle dia, I will optimize this for the situation as per above. My question was concerning the Cliff R -sub, which btw has a non-optimum shape.

"I'll be back"

[brian eiland]

Brian,

Thank you. Really interesting reading. Just what I was looking for.

Too bad the thread came to a halt 1 1/2 years ago.
Any further reading you'd recommend?

Cheers, Tom
Yes that is too bad. I got sidetracked onto other matters such as drawing up my Dynarig motorsailer, and Carl has his hands full moderating his Yachtforums. Sometimes he avoids participating directly in technical discussions, but rather watches from the sidelines not wanting to steer the discussions one way or the other.

You'll notice I tried to draw him out on the 'pliant material' question, but never was successful doing so. He may be a bit less busy right now with the downturn in the "YACHT' business, so if you were to pose a question in the discussion it might elicit a response from him?

[Submarine Tom]

Baeckmo,

Thank you again.

I don't have any technical data for the R300 of Cliff's other than his

nozzle articulation is 20 degrees max.

If the R300 has a "non-optimal shape", what would be an optimal shape?

-Tom

[baeckmo]

Ok Tom, here attached some more numbercrunching, showing the basics of jet calc procedure. Purists among us will complain that I have not taken hull wake and boundary layers into consideration. There may also be some mudslinging caused by my selection of inlet and nozzle efficiencies, not to talk about how the stars were positioned in relation to the gravity lines at the time of launching. But I omitted the deeper digging into these holes for the sake of increased clarity.

It would certainly be interesting to see how far a "simple" numerical method like JavaFoil or similar, might take the analysis of local hull pressures and boundary layer profile. I also believe that those lightly loaded propellers might be predicted by these programs, although they fail where we have higher blade solidities and/or mach numbers above about 0.3., as in high speed flows with gas mixed into the water.

Next, I will be back with the basic tools for analysis to explain why we get the results shown in the attached comparison.

--. -- -.--

[baeckmo]

**** happens, here the file attached!

[Guest625101138]

Tom
At the risk of repeating what I pointed out many months ago.

I have attached the details of a prop design that basically meets your original specification of 500N thrust and 0.5m diameter. You will see efficiency is 87%.

You will not get design curves for marine props like this. The BAR is about 10% (I did not calculate the actual value). You will not find curves for marine props with BAR less than 20% and in practice it is hard to find actual marine props with BAR less than 30%. These are typically made for easily driven yachts in auxiliary drive application.

So when you are aiming to make the most efficient drive for your application you will not find it in the normal box of gear available to marine engineers. What you are aiming to do is outside their experience.

I have reduced the shaft speed for the design because with 1200rpm you get ridiculously thin blades that would not take the loads or you are pushing much more blade area than you really need so there is an efficiency loss.

The only reasons you would use a jet on a submarine is to eliminate the exposure from an external rotating blade either for damage to the blades, fouling of the blades or injury to people. Having a jet with a tiny nozzle relative to the size of the craft comes at a huge cost in efficiency.

Anyone who has basic understanding of physics can conclude this.

If jets were the most efficient choice you would see them commonly used on submarines. This is not the case. You do see counter-rotating props but these serve a number of roles that are not constraints in your application where you have nominated a prop with larger diameter than the diameter of the hull and the speed is quite low by comparison with planing hulls.

Rick W

[baeckmo]

Yep Rick, that was what I was looking for when it comes to those lightly loaded props for high efficiency, where we have little of empirical data, fine! Now, d'you know if the JavaFoil can be tricked into the treatment of 3D-bodies, say a body of longitudinal shape like a symmetrical NACA profile, with cylindrical transverse shape?

For Tom's sub I would personally sacrify a few percent of top efficiency in order to get a smaller, less vulnerable prop. Submarining is a dangeruos undertaking, getting stuck with a prop entangled in weed is not enjoyable, but that is slightly off topic here, of course....

--. -- -.--

[Guest625101138]

JavaFoil only gives coefficients in 2D fields. There is a correction for aspect ratio but that only applies to induced drag.

I do not know if there is a similarity relationship between 2D section and body of revolution of that section but I expect there would be some relationship. (if anyone knows I would appreciate a reference). The drag coefficients based on surface area produced for 2D in JavaFoil are higher than measured on a body of revolution of the section.

You can use JavaFoil to look at relative changes in drag from one shape to another as well as influence of Re#. The Cd can change quite dramatically for relatively small changes in Re#. You can also play with the pressure profile to move the point of flow transition further aft for a particular Re# to lower drag.

I have attached some examples of how it can be used to make improvements to shape. One is a NACA 16 series 22% section, then there is a modified 4-series 22% section and then that section adjusted using the design function. I improved the CdA from 0.0087 to .0051 to .0049 and the later is even for a thicker section so higher volume for surface area. These manual adjustments align with what you can see coming out of optimising routines such as those that produce the laminar flow hulls.

In practice you see Cdv values from just under 0.02 to higher than 0.04. The optimum fineness ratio is dependent on Re#.

A body that is changing direction frequently is not likely to maintain laminar flow over much of the body for any period of time so this will increase drag.

Rick W

[baeckmo]

Pity it does not work with bodies of revolution yet. As far as I can see, Toms sub is opeating far into the supercritical Re range with fully developed turbulent bl's. I take it that the Cd you refer to is the pure friction drag coefficient? My interest in the bl mapping here was in order to have the prop resp. wj inlet wake adapted.

And certainly a sub will operate with angles of attack that have consequences for the total drag!

[anthony goodson]

Rick W ,excellent analysis as usual ,takes me ages to understand and I find it really fascinating ,but a tad ill informed about water jets in subs ,our entire fleet of T class subs with the exception of HMS Trafalgar have water jet propulsion ,also many others around the world. In your list of reasons for using WJ's in subs,you missed the most important one of all ,low noise ie stealth.A water jet has a very low underwater signature apart from dead astern. However your other arguments are validated by the fact that a nuclear reactor is neccesary to make this system viable ,so unless ST goes really upmarket with his project you are probably still his best bet .

[Guest625101138]

Rick W ,excellent analysis as usual ,takes me ages to understand and I find it really fascinating ,but a tad ill informed about water jets in subs ,our entire fleet of T class subs with the exception of HMS Trafalgar have water jet propulsion ,also many others around the world. In your list of reasons for using WJ's in subs,you missed the most important one of all ,low noise ie stealth.A water jet has a very low underwater signature apart from dead astern. However your other arguments are validated by the fact that a nuclear reactor is neccesary to make this system viable ,so unless ST goes really upmarket with his project you are probably still his best bet .

Anthony
Thanks for the update. I know noise is something the sub designers have been trying to improve but did not know that jets were in use on them.

I tried to find some layouts to get an idea of the proportions. If you have any ship cut-away drawings of the pump system I am interested to see them. The efficiency can be reasonable if velocity ratio is low but will still not match a large diameter open water prop.


Rick W

[apex1]

,but a tad ill informed about water jets in subs , .

Let me reduce it to this!


That is the common contribution of our muscle driven (and playstation backed) GURU of propulsion!

Ill informed, and estimating.

A amateur playing boaty boaty!

[Guest625101138]

There is boundary layer information provided in JavaFoil. Again it has to be taken as trends rather than absolute because it is only 2D and the pressure gradients are higher than in the 3D case.

The attached compares the boundary layer for different Re#. You can see there is flow separation at the tail occurring as the Re# reduces. I expect that separation would be similar for 3D but at slightly higher Re# than the 2D case.

At the design speed for a 500kg vessel of 22% thickness the boundary layer is approximately 1 mm thick where intake ports would be located. It becomes thicker when separation occurs at lower Re#.

Maybe Tom can ask Martin Hepperle to produce JavaBoR so he can optimise his shape. The underlying maths is long lost for me.

The last image is an approximation of the lowest drag form for Re# around 10E7 and JavaProp correlates well with this. Again the result relies on stable flow conditions not ducking and diving. You will see the Cda is quite a lot lower than the previous shapes. This has a maximum thickness of 30%.

The Cdv for this shape is about 0.008. For 500kg at 8.2m/s this give drag of 170N. Using the 2D Cda from JavaFoil the drag is 300N. The 170N is lower than you see in the real world but then there are no hulls or marine shapes that approximate this. The only laminar flow hulls that have been built that I know of are in the lower Re# range and these are finer with thickness around 20% or a bit lower.

The laminar flow hull shape for Re# above 10E8 is not dissimilar to the shape of large whales so provides some credence to the low drag of these shapes. All the laminar flow hulls aim to provide a pressure gradient that shifts the transition as far aft as possible. At low Re# the maximum thickness is behind centre but moves forward at higher Re#.

Rick W

[baeckmo]

I'm afraid that a boundary layer thickness of 1 mm on a vessel moving with a Re of 1.6 to 1.8 E7, and a length of 2-2.5 m is not near a realistic value. We should see something like 28 mm of turbulent thickness in the rear part of the body, and the shape of the boundary streamline is not congruent with what is observed on similar bodyshapes in windtunnels. See f.e. Hoerner or Schlichting. The streamlines appear all too much alike the theoretical non-viscous source-sink flow.

So unfortunately there is either a serious bug in your software, or a serious handling error. In either case the output is of no use for prediction purposes, it may in fact lead to dangerous mistakes! Before someone is getting in serious trouble by using ANY output from this source, the error must be localized!

[apex1]

I'm afraid that a boundary layer thickness of 1 mm on a vessel moving with a Re of 1.6 to 1.8 E7, and a length of 2-2.5 m is not near a realistic value. We should see something like 28 mm of turbulent thickness in the rear part of the body, and the shape of the boundary streamline is not congruent with what is observed on similar bodyshapes in windtunnels. See f.e. Hoerner or Schlichting. The streamlines appear all too much alike the theoretical non-viscous source-sink flow.

So unfortunately there is either a serious bug in your software, or a serious handling error. In either case the output is of no use for prediction purposes, it may in fact lead to dangerous mistakes! Before someone is getting in serious trouble by using ANY output from this source, the error must be localized!

Hmm

one could say that was the more conveniant sort of: fuc.k this idiots playstation "advice".

[anthony goodson]

Rick W good morning
Details of submarine propulsion systems are not easily obtainable ,for obvious reasons ,but if you Google "Rolls Royce submarine water jets" you will see that the latest British submarine ,one of three to be built ,is powered by a new version their water jet .although it's main claim to fame will probably be the end of "hot bunking". I accept of course that you are quite right about the superior efficiency of the propeller in this application.

[baeckmo]

Tom, one more word of warning when it comes to comparing the drag of different shapes. When the functional criterion is a specified VOLUME, the performance must be compared in terms of "drag/volume", not drag per transverse section area or total surface area, as done above!!!

In the examples, with a diameter/length ratio of 0.22, and your diameter target of 0.45 m, the enclosed volume of a body of revolution with such a shape would be less than your specified 0.3 m3, causing this sub never to surface again, once launched. To reach your volume target, the overall dimensions of the craft have to be increased, simultaneously increasing surface area and thus final drag.

This is not to say that the short low-drag shapes are out of question per se, but the drag with your volume capacity will be greater than indicated just by the calculated drag figures shown.

A more pragmatic comment: those shapes with a continuous curvature midships are extremely sensitive to longitudinal cog for trim in surface mode. This is a good reason for having a cylindrical midships section, connecting a spherical or ellipsoid nose with a conical rear.

--. -- -.--

[Submarine Tom]

Baeckmo,

I never specified a volume of 0.3 cubic meters. Where did you get that number from?

I stipulated I am not interested in surface operation so I'm not sure why you're addressing it.

If you find Cliff's R300 to be of less than optimal shape, what shape would consider better?

Can you see any advantages in using a jet drive for my sub?

What about using a larger diameter jet?

-Tom

[baeckmo]

Hi Tom, been gone for a week....now back to harass ya! In your note #15 you specify a weight of 300 kg. In order to float those kg's your hull has to displace at least 300 l of water. That is 0.3 m3. Unless you plan to leave the sub parked onto the sea bottom and never reach the surface again, the total volume of your vessel must be greater! Otherwise you have to winch her home!!!!

See why I adressed the subject ......?

There is no way a waterjet of this power can get you even close to the efficiency (=operational range) of a decent propeller.

[Submarine Tom]

Welcome back baeckmo,

Longitudinal cogging is not a concern as there will be minimal surface operation, no need to address it.

Objects that are neutrally buoyant don't sink to the bottom nor do I intend to park on the bottom.

In sea water, a 300kg vessel with a volume of 0.3 m3 will have positive buoyancy, but don't concern yourself with this.

No dialogue on your opinion that the R300 could have a more optimal shape? Like what?

You have no comment on ANY other advantages a jet might have in my application over a prop?

That was my original query.

All the best,

Tom

[baeckmo]

OK, guess you have control over "floatability". I mentioned it more to show that there is no general "best shape", but that optimisation will follow different paths, depending on the object or function that is to be contained. That said, if the enclosed volume is the deciding parameter, a l/d ratio of ~5.5 to 6 is near optimum. Optimum shapes were studied when airships were developed around 1910........ See fex a classic: G. Fuhrmann, diss Göttingen 1910, (=Jahrbuch d Motorluftschiff-Studienges. 1911/12 pp 63).

The usual reasons to use a confined propulsor (waterjet) in the low speed range, where its propulsion efficiency is lower than a corresponding open propeller are:

1/ No rotating parts outside the pump housing; less risk for damage.
2/ Excellent maneouvrability with nozzle moving in two dimensions and split duct reverse bucket. However, the r-bucket is difficult to "hide" from flow in fwd position in a submerged vessel, and may cause very high drag.
3/ Wj rotor may work with higher rpms than its free prop cousin, simplifying transmission if main engine or motor is high speed. Note that wj's often fail due to cavitation caused by too high rpms, though!
4/ Pressure wave (=sound) radiation can be significantly lower than with a free prop

[Submarine Tom]

baeckmo, thank you. I have an expanding number of specific questions

before I will be content to leave this research. I sense you are not

interested in specifics but knowing where I'm going with this, is there a

particular book (or books) you would recommend that may help me answer

my ponderings? Diameter, RPM, inlet designs, torque, matching drag/

velocity/thrust curves optimally, etc, etc.

-Tom

[Submarine Tom]

baeckmo,

Okay, so you don't want to share that information with me, that's okay.

What do you think of a nose inlet with internal channelling to a larger

impeller and a matching larger nozzle?

Positive feed pressure could be achieved, increased efficiency from a

larger, longer impeller. Motor cooling would be made available.

What is considered "high" RPM which could induce cavitation?

How large an impeller would be needed to hit my 500 Newton bogey with

8kw? What kind of prespinning benefits might be gained with directional

vanes in the gullet?

-Tom

[baeckmo]

Sorry Tom, don't get pissed; its not that I don't want to continue this communication!!! Its just that right now I have a tight schedule with deliveries for those who pay my invoices, so please have some patience!

There is no "Popular waterjet design manual" as far as I know. It is a question of blending turbomachine fluid mechanics with ship hydrodynamics in a double optimizing process, so you need an understanding in both disciplines. I'll prepare some additional info on inlets and efficiency limits later, but probably no time to do so before X-mas.

[Submarine Tom]

No problem baeckmo, I'm not pissed.

It's just that I've noticed you're not really into dialogue on this thread

but chatty on other threads. Proirities I guess.

Have a good Christmas.

-Tom

[Submarine Tom]

baeckmo,

Are you really coming back to my questions?

If not, please let me know, don't just leave me hanging.

If you don't have anything to add, or don't know the answers, let me know.

How was your Christmas?

-Tom

[baeckmo]

Yep, Tom I am preparing some sketches for you plus some answers; I'll be back in due time with those. Don't feel "betrayed" because I dealt shortly with the pwc jet issue; there is in fact a common wisdom regarding inlet area sizing.

I am also involved with a research project on the cold adaptation of dogs, and we have now an intensive data collection period, that keeps me away from much of the waterjet fiddling right now.

And thanks, still topped up with good stuff from the X-mas menue now that we prepare for the new year. So:

Happy New Year to you!!

[Submarine Tom]

And the very same to you my friend!

Glad to hear you haven't lost interest.

Standing-by...

-Tom

[baeckmo]

Ok Tom, attached another headache, dealing with the pump technology. After some digging in old sediments I found a test report on a pump with performance close to the preliminary optimum I calculated before.

Next I will deal with the inlets, beeing the root of ethernal misunderstandings among boaters (and, in fact among some waterjet manufacturers as well!!!!).

[Submarine Tom]

baeckmo,

Thanks for your recent efforts.

It appears the inefficiency of a jet drive may not be outweighed by the advantages (i.e. directable thrust, concealed moving parts, and other unknown potential positive benefits).

Also, the time delay and lack of dialogue aren't helping me to make an informed decision about attempting a jet drive in my unique application.

So, I am abandoning the idea.

-Tom