Jet drive disadvantages fixed by new design

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

  1. expedition
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    expedition Thorwald Westmaas

    Two of the disadvantages of jet drives I've read about on forums - regardless of their application and design of the boat they are placed in - are:

    -poor slow speed maneouvring and reverse
    - higher cost of maintainance

    Has this been (partly) solved in newer designs or it it still an issue?

    If so, how much speed is needed for adequate control?

    It seems to me that something with less moving parts should be easier and cheaper to maintain.

    Thorwald
     
  2. baeckmo
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    baeckmo Hydrodynamics

    Don't know what fora you read, but in my experience your statements are wrong. One good reason for selecting jets is just their excellent low speed manouvering performance due to the 360 degree thrusting abilities.

    Older types of reverse buckets often blew aerated water back into the jet inlet, which reduced reverse thrust, but that is long gone. As for maintenance, it is rather a question of knowing how to service.

    The real drawback is instead the inherent low efficiency at low speed, which is mainly the result of losses in the inlet duct, which limit the obtainable flow capacity due to cavitation.
     
  3. Murray Peterson
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    Murray Peterson Structural Engineer

    My comments are similar. Maintenance of jet drives is very minimal compared with every other kind of marine propulsion. Some units require marine grease pumped in to bearings and control joints every so often. My experience is that bearings and seals need to be replaced every 25years or so. They are quite trouble free compared with other propulsion systems. My first boat was a jetboat that was 15 years old when I purchased it and it was fully operational when I sold it 20 years later. I still own my second jetboat - I'm doing some floor repairs at the moment but that is unrelated to the jet unit. This boat is now 30 years old!

    In my experience (I live in Australia) maintenance of a jet is much cheaper and less frequent.

    Maneouvring is easier with a jet - no boat speed is required - you can rotate the boat about itself, or side thrust etc. The way the boat maneouvres is different but better. Some people have an initial hang up because it feels different to prop boats. Don't worry - you will soon realise it is very handy to be able to maneouvre you boat sideways without bow thrusters.

    The only shortcomings of a jet are far outside the capabilities of other propulsion systems. For example a jet drive will operate in very shallow water where no prop can operate. The shortcoming is that the jet can suck in sand or mud that can be pumped through your engine cooling system. A common way of avoiding this is to install a sand trap between the jet's cooling water bleed (jets supply cooling water to the engine - no independant raw cooling water pump is needed.) and the rest of the cooling system.

    Advantages
    - no water thrown radially away from a propeller - the best propellers waste 25% of their input horsepower this way.
    - no gearbox - gearboxes waste about 10% of their input horsepower
    - no projections below the hull line - less drag and no risk of the boat capsizing by a large wave hitting the boat sideways - jetboats can safely operate in mountainous seas and gail force winds (don't recommend you try this.)

    The main reason I seen jet drives perform poorly is because the engine and the jet drive are not well matched. (Same problem as having an propellor of incorrect pitch.) A well set up jet is the best value for money you can get in a marine propulsion system and it is the most fuel efficent. ie it will provide much more thrust for the amount of horsepower you put into it.
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    Agree with the posts above.

    maybe worth going to see a few units being used on some boats, so you can see what is being noted above.
     
  5. Murray Peterson
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    Murray Peterson Structural Engineer

    This is a website of a small manufacturer near me (If you have seen the "Greatest Job in the World" promotion that is based on Hamilton Island - about 40 Nm from my home.) http://www.thebackshed.com/foundryandfibreglass/ click the link to Jet Units.
     
  6. anthony goodson
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    anthony goodson Senior Member

    Link is interesting ,reverse bucket pivots with nozzle, so will reverse steer like a conventional outboard set-up, could make the steering a bit heavy though and put a lot of strain on the steerig pivots in reverse Bit surprised that with their moulding expertise they don't make the jet body out of GRP. Ball bearings are an odd choice for stator bearings ,they dont do much work. unless of course they are forward of the impeller in this application.
     
  7. Murray Peterson
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    Murray Peterson Structural Engineer

    I haven't actually operated a boat with one of these units in it but the comments from the manufacturer when asked regarding steering and reverse it that the unit performs well and is pleasant to "drive". The ball bearings seem excessive but they are sealed bearings with additional external seals and they are very trouble free for many years. My thought on the thrust reverser is that it is quite small and light compared to most thrust reversers on other units - a cleaver design. The thrust reverser on my jet boat for example is quite large and heavy. I have been talking to this manufacturer about making me a new nozzle for my jet unit. It is at idea stage at the moment but I am considering the design of a full gimbal variable orifice nozzle with a thrust reverser like one of those. Full gimbal means trim as well a steering is achieved using the nozzle. The variable orifice will have an effect similar to a variable pitch prop. The mnimum orifice size will be to get maximum thrust with the engine at maximum torque - hence maximum fuel efficency - then with the option of increasing the orifice size to allow the engine to reach maximum power. I have a number of thoughts on the mechanical design to achieve this.
     
  8. anthony goodson
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    anthony goodson Senior Member

    I shall be interested to know how things evolve, please post your progress
    Nozzle trim was an option on PP jets as far back as the early eighties this was just a gimbal mount ,as regards the variable size nozzle ,this link might give you some food for thought if you haven't seen it.
    www.marinejettech.com. If you are looking for even more substantial losses ,look to your intake there is a lot of scope here too. To me the real strength of a jet unit is its simplicity and while it remains simple it will always be a compromise and excel only in specific applications. However it is always nice to see someone trying to broaden that spectrum.
     
  9. Murray Peterson
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    Murray Peterson Structural Engineer

    Interesting ideas. Very complex design though - I assume it uses pneumatic "diff-lock" actuator technology.

    My idea for a variable orifice is two concentric stainless steel cones with silts to form about 12 "fingers" on each cone and the slits on one cone are aligned in the centre of the fingers of the other. The fingers willl be flexible like reed valve fingers. The design will allow them to flex under nozzle pressure. At low engine rpm the orifice will be smaller to increase thrust at low speed and at high engine rpm (above rpm for engine maximum torque) the nozzle will open up to level out the input torque demand of the jet unit and enable the engine to reach full power. This near circular orifice opens and closes as a simple spring action.
     
  10. baeckmo
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    baeckmo Hydrodynamics

    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).

    When you study the factors governing the momentum efficiency, you can see that the higher the nozzle velocity, the lower the efficiency. The energy is added to the fluid via the pump, with its hydromechanical efficiency, which is varying with the flow. So the total efficiency is basically the product of those two efficiencies.

    Now, for a given engine power, thrust may be produced by high-flow, low velocity increase or from low-flow high velocity increase. Provided similar pump efficiencies, the former will give a higher thrust. The pump impeller and stator is optimized for a certain design flow and rotational speed. If the flow is changed, the pump efficiency is reduced.

    The really critical part of the jet is the inlet throat area, which has to match the flow over a range of boat speeds. The losses in the inlet are also increasing when you divert from the design flow.

    Here an example:

    Say you have a 100 kW engine available, the pump efficiency is 80 %, the inlet losses are 35 % at a design advance speed of 15 m/s (~30 knots). The design flow is 0.3 m3/sec. and nozzle losses are ~4%.

    This setup will result in a net thrust of 3163 N (not considering effects of bottom boundary layers et c.). The nozzle velocity will be 25.5 m/s, the nozzle diameter 125 mm, and the optimum inlet throat area 0.02 m2.

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

    Now, if you reduce the nozzle diameter to, say 100 mm, the following happens:
    The pump efficiency at non-design flow will go down to ~60 %, nozzle losses increase to ~7 %, inlet losses increase to ~50 % and the resulting flow is down to 0.2 m3/sec with a nozzle velocity of 25.6 m/s. Total thrust is down to 2121 N, giving an overall efficiency of 31.8 %! You are now spending 68.2 kW in heating the ocean.......... clever??

    So, what if I goofed with the efficiencies here? Suppose the design flow was actually 0.2 m3/s, with max component efficiencies! Result: Thrust 3027 N, nozzle speed 30.1 m/s and nozzle dia 94 mm and power to heat the pool is 54.6 kW. Still no improvement over the high-flow, low jet velocity approach. Throttling from 0.2 m3/s gives effects as above.

    The part of the jet unit that really is in need for improvement is the flush inlet and ducting up to the impeller inlet; forget about adjustable outlet nozzles. If you are still interested in jet development, go study basic hydrodynamics and pump theory first!
     
    Last edited: Oct 20, 2009
  11. Murray Peterson
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    Murray Peterson Structural Engineer

    Thankyou for you comments. You may be correct in thinking that variable nozzles have little to offer. I agree that there is much room for improvement in intake design, but i am retrofitting a 30 year old boat and modifying the intake is outside the scope of my renovations at the moment. Maybe it is easier to maintain design flow while varying pressure increase. I do not know the pump efficencies and how varying the pressure increase will effect the pump efficencies. My current nozzle is effectively 140mm inside diameter but is has significant stator wear/corrosion which is my motivation for a new nozzle.

    Maybe you would like to clarify these questions:-
    1) Your calculation for jet momentum efficiency (eta) tends towards a limit of zero with a zero advance velocity and can have values well in excess of 1. Is this equation correct? This equation implies tha that jet momentum efficincy is very poor for a larger heavier boat but good for a small light boat.
    2) It appears to me that the energy you have calculated to be "heating the ocean" contains the hydrodynamic losses from moving the boat through the water which of course are present for every boat with any means of propulsion and therefore are not specific to the jet unit. Could you provide your calculations including the definition of terms used and the basis of you assumptions made?
     
  12. baeckmo
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    baeckmo Hydrodynamics

    Your interpretation of the "eta" equation is basically correct, but must be understood in connection with the requirement that a change in fluid velocity is necessary to produce thrust. And yes, there was a typo fault in the equation, now corrected, thanks for beeing observant!

    When Va (advance speed) tends to zero, the work done, ie T (thrust) * Va tends to zero as well, and the propulsion efficiency > zero.

    At the other end of the scale, Vj (nozzle velocity) can not be less than Va (no thrust produced). If Va>Vj, you have a turbine. For short moments, when you throttle down from top speed, this is actually occurring, causing a torque reversal in your driveshaft.

    The conclusion on heavy versus light is correct once you relate it to power/weight ratio. If you keep the ratio Vj/Va constant, the "eta" stays constant. The success of jet propulsion for large catamarans is due to a number of optimizing processes. Vj/Va is one, pump efficiency increasing with physical size (Reynolds number) another. One field, where there is a marked difference is the inlet.

    Big size installations often have their inlets individually optimized for the vessel operating envelope. In slender catamarans, the jet is ingesting a major part of the bottom boundary layer, which holds some energy otherwise lost behind the hull.

    And, of course, all the EFFECTIVE power (T*Va) spent to propel your vessel is heating the water in the end. The difference is the additional power wasted with the different levels of propulsion efficiency. So, the 68.2 kW mentioned are the extra kW's eaten by the propulsion system itself, to be compared with 52.6 kW for the original configuration!

    You could regard the momentum losses described by the "eta" as the hydraulic variety of the unavoidable Carnot heat losses in thermodynamics.
     
    Last edited: Oct 20, 2009
  13. anthony goodson
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    anthony goodson Senior Member

    Good morning Murray[well it is here anyway ]
    I was pleased to see that Baekmo's comments havent discouraged you, [he only does it to annoy and because he knows it teases ] you clearly have an enthusiasm for jets and this is shared here by all of us . However Baeckmo is of course right ,but if you put the theory fully into practice and make everything ajustable you will end up with a horrendously complicated machine which will not only be very expensive but will not survive the operating conditions for very long. Some of the most succesful commercial jets are in high speed ferries and with good reason, the operating parameters are fixed and the jets are designed to work within them, move outside these parameters and the jets become as innefficient as any other.I will confess that a many years ago I embarked on the same journey that I think you are on ,to revolutionarise the water jet ,I failed of course and ended up accepting that cleverer people had already got most of it right.There were several good things that came out of this though,I got to meet some very interesting people ,it kept me out of the pub ,and I realised that I had probably accrued enough information to build my own jet.
    So about ten years ago I built four for my own use. I derived a great deal of satisfaction from this, two of these are still running ,one was cut up to experiment with intake design and the other is in my garage waiting for my grandsons to grow up,if I should be lucky enough to live that long .So don't be discouraged ,nothing is lost in the pursuit of knowledge Just one final point ,if you are tempted to put you bucket on the nozzle,you may end up with the following problem. You cannot direct the water directly under the boat as the entrained air in the water will cause cavitation in the jet, so you vector the flow outwards and under the transom beyond the chines. The problem with the bucket on the nozzle is that when turning to port or starboard one of the jetstreams will always be towards the intake.The whaletail fixed bucket may look heavy and ugly but it works
     

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  14. apex1

    apex1 Guest

    A pretty bold statement, hähh?
    More so, when it is made by one who promotes his own product...............:(
     

  15. anthony goodson
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    anthony goodson Senior Member

    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?] I introduced it in an attempt to lighten the tone of an interesting post posted by a newcomer,the post was getting a little fraught I thought and when this happens communication can break down The friendly and helpful exchange of views is what attracts me here, although I do accept the need for honesty ,it is sometimes neccessary to temper this a little.The quote is from Alice in Wonderland and seemed apt to me at the time. With reference to the rest of my remarks ,If you can show me a jet of the size that we are discussing that features an adjustable inlet an adjustable nozzle and a variable pitched impeller ,at a reasonable price and which will spend its working life in a soup of salt water stones ,sand ,fishing lines not forgetting condoms ,in the Swedish version.then I stand corrected. As for promoting my products I made it quite clear that I only built these units for my own use and satisfaction but rest assured if I ever feel the need to promote myself or my products here I will aquire a large motor bike and a pair of leather trousers first
     
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