Why A Jet Drive?

[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]

Originally Posted by 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

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]

Originally Posted by 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 .

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

Originally Posted by anthony goodson

,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!