Jet drive disadvantages fixed by new design

Discussion in 'Jet Drives' started by expedition, Oct 6, 2009.

  1. baeckmo
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    baeckmo Hydrodynamics

    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.
     
  2. apex1

    apex1 Guest

    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.:p

    Regards
    Richard
     
  3. Murray Peterson
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    Murray Peterson Structural Engineer

    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?
     
  4. Doc Nozzle
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    Doc Nozzle Thrust Whisperer

    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.
     
  5. baeckmo
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    baeckmo Hydrodynamics

    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: ????????
     
  6. anthony goodson
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    anthony goodson Senior Member

    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.
     
  7. drmiller100
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    drmiller100 Junior Member

    You have access to someone with some pretty good understanding of pumps. Who is it?

    Your interpretation misses the most interesting opportunities however.
    Thrust and horsepower are not directly related for a pump.

    If we take a HUGE pump, with HUGE volumes of water, and a HUGE nozzle with little pressure, we can make HUGE amounts of thrust. We can make significantly more thrust at zero hull speed then you can with a direct drive prop, because if you try to spin the prop very fast it cavitates.

    However, your formula (which is not correct by the way, but serves to illustrate basic concepts) indicates that at higher hull speeds, you want more pressure, smaller nozzles if you hope to come close to propeller efficiencies.

    In the archives of this forum there are a few formulas from about a year ago which are well worth reading.
     
  8. baeckmo
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    baeckmo Hydrodynamics

    Quite correct, and this HUGE capacity pump is aah, well......a propeller!
     
  9. drmiller100
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    drmiller100 Junior Member

    Well, there is that.

    Ultimately I am a believer a propeller is more efficient then a jet pump. The goal is to make the jetpump approach propeller efficiencies.
     
  10. baeckmo
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    baeckmo Hydrodynamics

    Oh yes, fully concur! That is what started the process here. And the most urgent development for the jet is better inlets; less total energy loss, much reduced velocity gradients in impeller inlet plane, less throat drag at off-design speeds. In addition to that, impellers with better cavitation performance for moderate-to-low speeds.

    Just jump the wagon, get on with it!!
     
  11. drmiller100
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    drmiller100 Junior Member

    I don't think there is significant gains to be found in an intake, unless your design is fundamentally messed up.

    At low speeds, the intake just needs to feed enough water to keep the impellers from cavitating. Eventually, all impellers will cavitate, just like a propeller will cavitate in open water.

    Lets look at the limits.

    If we make the intake too small we get cavitation.

    Lets look at too big of an intake. At zero boat speed, if we make the intake "too big" nothing bad will happen. At very slight boat speeds, if we make the intake facing forward, and make the intake "too big" you will produce a "negative pressure" in the intake, helping to pull the boat forward.

    At high speed, if we make the intake too big, we have a significant change in velocity in the water as the water slows down.
    This translates to "work done over time."

    Where does the energy go? It gets converted into pressurizing the water in the intake, which feeds the impeller, which gets converted into thrust.

    Now granted, at high speeds the math is going to work out that you want a smaller intake to go faster as any racer will tell you. Any racer will also tell you there is a happy spot for maximum thrust which can be measured in PSI of inlet pressure. This changes somewhat as a function of MPH of the boat, but not much.

    The endurance jet boat racers can tell you the perfect PSI. From memory, I want to say it is 50 psi or so, but that is probably not the real number.
     
  12. apex1

    apex1 Guest

    Ähh, sorry,

    how wise do you think is it, to teach the expert?
     
  13. baeckmo
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    baeckmo Hydrodynamics

    Ooops, I seem to have overestimated you,"drmiller". You have no idea about what is going on in a waterjet or in its intake, that is clear from your post above. May I recommend that you study the subject in any of the textbooks available on marine propulsion or search the net for "waterjet inlet".

    I have no problems explaining at length to someone who is trying to understand, but I just get fed-up with ignorants pretending they know. I've had my fair share of ****** today, they eat ones energy, enough is enough. And thanks Apex, you have a better strategy with them than I......!
     

  14. drmiller100
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    drmiller100 Junior Member

    Ok, then help me learn.

    What is the rationale as to why there is significant gains to be found in the intake?

    If the boat is moving, can we agree we are going to see some pressure in the intake?
     
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