Jet drive disadvantages fixed by new design

[baeckmo]

Well, Anthony, I will not aim for a diplomatic career; I think Murray is man enough to take an honest word if you provide a sensible, engineering background for it. Real knowledge is served al dente........ software comes foaming!

Anyway, a short note on the swinging reverse bucket (in addition to the importance of not blowing aerated water back into the inlet): With the bucket rotating together with the nozzle, it can only provide vectoring fwd-aft along the nozzle line, while a non-rotating bucket gives full 360 degrees vectoring around the horizon, PLUS the bonus that the side vector is always turning the boat the same way the helm is turned. No matter if the vessel is making way fwd, aft or maintaining position.

[apex1]

Originally Posted by anthony goodson

Thanks apex1 for the constructive comments,the "quote " was intended to be a humorous aside ,I can't explain that in your language i'm afraid ,[does anybody know the German for humerous aside?]

No need to translate thanks. But humor does´nt travel good through the net. So, sorry that I did´nt see that in the same context as you.

And do´nt forget to buy the whole outfit when you purchase the bike! That includes the jacket, helmet AND the knowledge.

Regards
Richard

[Murray Peterson]

Thanks for everyone's comments. However I would still like to ask baeckmo a couple of questions.

You give an example of:-

Effective power is 3163 * 15 [W] (=47.45 kW). With 100 kW spent, the overall efficiency is thus 47.45%.

The trust makes sence to be but what does the "15" represent? It is not the output velocity (25.5). It has the same numeric value as the advanced velocity but that doesen't make sence for a power output calculation. If it is the speed of the boat, then it implies that the loss calculation includes hydrodynamic losses from propelling the boat through the water. So what does it represent?

I still don't understand what an eta greater than 1 means. The eta for the above example is 1.43. What does this mean? It appears the equation would have to be incorrect?

[Doc Nozzle]

Originally Posted by baeckmo

Murray, I'm afraid I have to spoil your plans for revolutionizing the jet propulsion. Your reasoning on the variable nozzle shows that you are not familiar with the basic hydrodynamics in pump- and jet technology.

Thrust comes from a momentum increase, ie (volume flow) * (fluid density) * (velocity change), while the power to pump this lot is the product of volume flow times pressure increase. The propulsion efficiency is the work done per time unit related to the power spent. After some arithmetics we face a jet momentum efficiency that reads: eta=2*(advance velocity)/(nozzle velocity+advance velocity).

I have read and re-read this information and even after you fixed your equation I have to say that I respectfully disagree. I do not feel that you can tie jet or nozzle thrust efficiency to advance velocity. May I ask for you to clarify something for me? Is your definition of "advance velocity" the same as "hull speed"? Because that is how I am interpreting your comments. And I could invent a 100% efficient jet drive but attach it to a coal barge and it would have a horribly low efficiency using your strange equation. The performance of the jet thrust and the hydrodynamics of the boat it is attached to are two separate issues.

"Momentum efficiency"???? maybe that is useful in rocket design, but for boating I would think that it is more useful and practical to discuss a jet pump (or nozzle) in how well it converts mechanical input power into static thrust. Whether the resulting thrust coming out of a jet pump can move the vessel it is attached to is not the fault of the jet pump - it is the fault of the human who married the two together.

[baeckmo]

Ok, here we go:

The effective power, that is the actual power used for moving the vessel at a certain speed against its resistance at that speed. In the example the maximum delivered thrust at 15 m/s would be 3163 N. So it would be able to propel a vessel with the corresponding resistance at this speed.

Alas, the effective power is 3163 N * 15 m/s. The units "Force" in N (which is kg*m/s2) times "Velocity" in m/s has the dimension W, which is the SI unit for power! This is the power that will be dissipated from just moving the boat, towing it with the force 3163 N.

And in my book 2*15/(25.5+15) makes 0.7407, not 1.43!!!! Multiply this with the pump efficiency (0.8) and you get 0.59 or 59 %. The difference between those 59 and the 47.45 in my example is due to the inlet and nozzle losses; the inlet causing the major loss!

The "eta" here is figuring under other pseudonyms as well, some use the term "jet" efficiency (but that would be confusing when discussing water jet propulsion), in some texts you find "theoretical" eff. aso. I prefer the "momentum" efficiency because it describes the inherent and unavoidable losses in the slipstream after the vehicle, no matter what kind of "momentum increaser" (pump, propeller, paddle wheel, oar.......).

The idea of studying the momentum process and the pump separately, is that it allows the designer to find the best balance between the two. Another example: If we use a 15" propeller to produce 3163 N at 15 m/s, the velocity in the immediate slipstream is 16.9 m/s. This corresponds to Vj, and the momentum efficiency would be 0.94. To get to the total efficiency we have to know the pump efficiency of a propeller, which is low due to the absence of housing and stator vanes et c.. But due to the high momentum efficiency the total is good.

As for DocNozzle: ????????

[anthony goodson]

Hi Murray
Unlike some of us on here you have a summer season approaching and the opportunity to supplement these theories with an empirical approach.I can understand your reluctance to completely refurbish and modify your jet ,particularly the intake ,but in my ,limited, experience the actual physical work involved needn't be that great. The problems tend to originate around two areas on the intake. The forward end where it is faired into the hull and the gullet or throat where it splits the water. You haven't said which jet you have but I'l guess Hamilton 751 or Doen ,its not crucial which jet it is really, now Hamilton will have optimised this intake on the jet but they are unlikely to have put it in the boat and this is critical , the height ,angle of attack and smooth entry from the hull are all very important .If you drill and tap the duct and connect a simple manometer to it with a tube ,you can read your intake performance. as a rough yardstick pressure is drag and vacuum us unnecessary work at what you want to call your optimum speed ,Small modifications with bodyfiller will make temporary changes to the shape of the intake and it doesnt need much. I am not saying this is a panacea but it is a useful guide to what is going on This is very cheap to do and if in the end you decide to leave things as they are then leave the manometer on and it will tell you when you have weed in the jet.