Use pressurized water as manuevering thruster?

Discussion in 'Propulsion' started by ldrumond, Jun 18, 2006.

  1. Guest625101138

    Guest625101138 Previous Member

    Ad Hoc
    I could find hundreds of videos and photos of jets discharging above the water level but since you referenced KMW I thought the proof would be more compelling to you if I chose a ship, which I believe is fitted with KMW jets.

    Watch this video till the very end (once it is at design cruising speed) and tell me what you see. And then answer this simple question "Are the jets operating in free air above the water level?"
    http://www.youtube.com/watch?v=V_TMaTBW1ho

    Once you have provided the correct answer I would appreciate acknowledgement that you were wrong. I know that is wishful thinking but it should at least make you stop posting your nonsense to this thread.

    Rick W
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    These jets are not above the waterline.

    When you design a vessel with a waterjet, the shaft line is in line with the static dwl. When the vessel si underway, once speed has been attended, the transom is ventilated; that means it is clear and dry. So when you look down, just as in this video, it looks like it is above water. But in profile view, the shaft-line is on the waterline..ie zero head...it is not ABOVE!
     
  3. CTSAUTOMATION
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    CTSAUTOMATION Junior Member

    sorry no pics but if you google cyclone pump from jabsco you should find it.
     
  4. Guest625101138

    Guest625101138 Previous Member

    Ad Hoc
    FINALLY YOU HAVE GOT IT.

    It took a long time but you finally understand what I pointed out in my original response to CTSAUTOMATION. If you want efficient operation the jet must discharge in free air - not under the water.

    If you do not appreciate this then the simple test is to get a hose at full bore and compare the thrust to restrain it in free air compared with the situation when it is plunged beneath the water surface.

    Rick W
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    When you design a vessel with a waterjet, the shaft line is in line with the static dwl., how does that related to...."...Ideally you will jet above the waterline ...."

    You truly have a bizarre set of definitions, such as what is a waterline, and you consistently do not try to establish assumptions or facts before you rush off like a bull in a china shop...not to mention ignoring your own assertions like vessels with waterjets below the waterline which you claim cannot be done etc.
     
  6. Guest625101138

    Guest625101138 Previous Member

    Ad Hoc
    I am happy that you now acknowledge that the jet needs to discharge in free air for best efficiency. That was all I was pointing out to CTSAUTOMATION - remember he had set his below the waterline where it will lose thrust due to entrainment.

    My concern is that it took so much nonsense to convince you of this. As long as CTSAUTOMATION appreciates the salient point - water entrainment in the jet stream will reduce thrust.

    Of course the practicality of a high velocity discharge jet in free air near the bow of a boat in a marina may be a concern. However the flow will droop and a flush nozzle will likely have a slight downward trajectory to start with.

    Rick W.
     
  7. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    "...My concern is that it took so much nonsense to convince you of this..."

    I'm sorry, but it is you who has been talking nonsense, because you do not use the same definitions as everyone else. A waterline is simply that, a waterline, measure form the deepest location of the hull to the location of the water where the vessel is floating....where does it say, it is taken from the bottom of the transom....????

    You need to use the correct terminology first before you rush off into technical discussions. Which you consistently fail to do on this and other posts.

    Buy a book on terminology..then maybe people may understand what you are talking about...whether it is nonsense or not.
     
  8. Guest625101138

    Guest625101138 Previous Member

    Ad Hoc
    You obviously cannot read what I write - maybe you can read my quote that you posted.

    I cannot see any mention here of waterline in the quote I made regarding efficiency.

    The waterline was quoted in the Hamilton jet history. If you have a problem with them using the term waterline inaccurately then take it up with them:
    http://www.hamiltonjet.co.nz/hamiltonjet_waterjet/waterjet_history
    For my mind I know what they meant. If you want to be such a pedantic fellow then have a go at them. They may appreciate your insight more than I do but then I doubt it.

    If you care to apologise for you misinterpretation you may gain a little credibility.

    Rick W
     
  9. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    "If you have a view that placing the nozzle below the waterline helps then this is incorrect.....this is the reason jet boats discharge above... It is more efficient."

    Hmmmm..lets me see..waterline, and oh yes, mentioning boats..wow, so you go from a waterline referring to boats to er...something completely different in one sentance....oh yes...and there it is too....effecinecy!...so there you have it.

    Reading between the lines you cannot be a professionally trained naval architect. Because there is never is bizarre changing the definitions to suit owns own bizzaro views. It is clear and absolute.

    Definitions are the first thing one learns....you chop and change and select at your peril. You may think you know what you're talking about...but it is clear to others with formal training that you do not know such correct terminology and their exact definitions, nor when to use them.

    "If you don't say what you mean, you wont mean what you say..."
     
  10. Guest625101138

    Guest625101138 Previous Member

    Ad Hoc
    Now you are in the gutter. Selective quoting to try to put words on paper that I never intended - incredibly childish and very poor form. I hope others will see how silly you really are.

    You have proven yourself to be ignorant on many issues that you claim to be expert in and now you are showing that you will act dishonestly to try to score a point.

    Rick W
     
  11. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    So in other words, you don't stand by what you are saying when exposed to being misrepresenting and misleading others when definitions that ahve no place in text books etc.

    Thanks for that.

    Very unprofessional....still haven't my Qs anwsered too....!!
     
  12. cor
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    cor Senior Member

    Enough arguing, for best efficiency water jets should discharge into free air.
     
  13. CTSAUTOMATION
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    CTSAUTOMATION Junior Member

    Bow thrusters

    Hi guys my point was in reply to earlier on in the threads about bow manouvering thrusters only not propulsion. Wildo quoted me a system that would install 40mm pipe (inch and half i think- swimming pool size like mine) and suggested the outlets to be 100mm below the water line. I made the decision to go smaller as i felt they had over sized the system for a bow thruster. I only need a gentil-aid to help berth my boat not spin on its axis. The pump they use is a similler one to a davis swimming pool pump (240v not 12vdc) I am considering using a swimming pool pump 500lmin at 6 bar and rewind the motor to 12vdc. I know a local motor rewinder who can do this for me. I am interested in your valid feedback. On the wildo web site you can see the installation examples of a jet below the water line. I feel this is for boat comfort in tight berthing situations as to not disturb other users.
    Best regards gentlemen.
     
  14. Guest625101138

    Guest625101138 Previous Member

    The large 100mm nozzle will reduce the exit velocity and thereby reduces the entrainment. So efficiency loss is not as great.

    The hazard of high velocity discharge above the waterline has been discussed previously on this thread at post #46. The reduced efficiency of underwater outlet has to be weighed up in these terms.

    The flow power is 4.8kW at the stated design conditions. This will require motor power of 6-7kW depending on the pump efficiency. This is very high power for a 12V system. Will require a current of 600A.

    If the motor is 240V induction motor you will not be able to run it directly from 12VDC even if rewound.

    The pump is high pressure for the application. With that pressure at the pump you will have something like 30m/s at the nozzle in open air. The open air thrust will be 24kgf - reasonable. With it being underwater the entrainment loss will be significant but it will still work. I cannot give you a number. If you played around with nozzle size you should find there is an optimum but it is not something I can calculate.

    If you have a swimming pool pump to trial then do some simple tests at a pool. You can measure thrust in open air and compare with under water. You can easily play with nozzle sizes to see what gives the best result.

    Rick W
     

  15. CTSAUTOMATION
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    CTSAUTOMATION Junior Member

    cheers rick i think the 12v to 230v convertor is proberbly best way as the motor at 12v would require too large a current. The basic trial as you said would be to measure the force at the outlet nozzle in the pool and out. A crude sping balance and bucket would give some idea i think at the underwater outlet in the pool. I was thinking of the same spring balance test in pulling the boat at the jetty if you get the idea. Seems a bit rocket science but should give some ideas. i have flow ajustment at the skin fittings on the boat (in the solinoids body) to trim out the best results. I also looked at central heating booster pumps used in open systems. Thanks for the advice. regards TF
     
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