jet drives reconsidered.

Discussion in 'Jet Drives' started by Frosty, Aug 12, 2012.

  1. Ad Hoc
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    Ad Hoc Naval Architect

    We made some many years ago out of composite. You need them thin and strong. Kevlar become the only viable option. GRP is too thick and too much deflection and fatigue strength wasn't so good either.
     
  2. Frosty

    Frosty Previous Member

    Ad hoc did you work out or could you hazzard a guess at the Hp required to lift 1 foot or % of loss of performance --however you like . This is something I could never do.
     
  3. CDK
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    CDK retired engineer

    The flap would be hopelessly in the way for the reverse bucket. A powerful deck wash pump connected to the bowl in the direction of the impeller may also work to seal the gap between the impeller and the wear ring. The jet pump will sputter a bit on the water/air mixture until there is enough water in the bowl.

    Boat owners in your wake will have a memorable day when you take off.
     
  4. Frosty

    Frosty Previous Member

    You think it would be that impressive? The maths of it all are beyond me but its just the weight of it being shot backwards that will make the boat go forwards, I wouldnt have thought it be that much.

    If it were a solid lump of water say 1 foot in diameter my uhm Ahh computer would think ---say----15 feet!!!

    The flap seal for the prime would intangle with the bucket as we know it now but a few things would have to be changed around.
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    No I didn’t, had other things to do.

    BUT, it can be done, trouble is, it’s a simple question but the solution requires far more data than you have provided. Assuming a WJ type application rather than a simple (rho).g.h change of potential energy type.

    If you go for the simple solution it is simply flow rate times the head. Pd = Q.H

    Trouble is it is more complex that that since what flow rate, what head, and then losses of the inlet and the outlet, then the pumps efficiency and so on.

    However knowing the answer, what will it make you do knowing the final answer?...change your installation?
     
  6. baeckmo
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    baeckmo Hydrodynamics

    The increase in pump s.c. geodetic head (physical lift from inlet status to outlet) is not a big issue, except for jet propulsion in foilers. Here an example (I don't dwell on the subject of flowrate selection here, this is a reasonable figure for this size of unit):

    Jet power input 100 hp = 73600 W
    Pump efficiency ~80 %
    Flowrate ~0.3 m3/s
    Velocity ~30 kn (¨15 m/s)
    Inlet efficiency ~65 %
    Wake 10 %

    Power = pressure x flowrate/pump efficiency:

    Pressure(nominal)=73600 x 0.8/0.3 = 196267 Pa ~196267/9810 ~20 m
    Add dynamic head from inlet: 0.65 x (0.9 x 15)^2 x 500 ~59231 Pa (~6 m)

    Deducing 2 % of this for nozzle losses, and the total driving head over the nozzle is 25.5 m . If you raise the shaft level 10", you have to supply one percent more power, which is far less than what you lose in a "barnacle farm". So in this perspective, it would certainly be worth wile to have a "dry" pump.

    The real problem is thus not the increased head, but:

    A/ Priming
    B/ Marine growth in the inlet parts that still remain submerged.

    Waterjets are basically SIMPLE, and anything that introduces a complication is avoided. Jets that see daily use are never a problem, but then there are the "docksidesailors"........and they have trouble with propellers as well!

    We actually had a very good supplyer of GRP jets; the PP, which was bought by Vospower, and then killed as a result of incompetent marketing efforts. They had units with very good efficiencies up to some multi1000 hp.
     
  7. DCockey
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    DCockey Senior Member

    Any concerns about increased suceptability to cavitation due to lower pressure on the suction side of the impellor if the unit is raised?
     
  8. Frosty

    Frosty Previous Member

    Good point, this is looking possible and to be honest doable. Thing is it would be easy and thats making me nervous. Why has it not been done.

    The 1 % increase in power absorbtion would be hardly noticable.

    If priming can be cured then the second problem of cleaning the inlet could be solved.

    How is the intake cleaned on conventional jets?
     
  9. Frosty

    Frosty Previous Member

    Baeckmo --do you think this is possible?

    Why do you think its not been done already.

    This would be a breakthrough in boating. A boat without a drive in the water--at all.

    The sea Im sitting in now is 30 degrees its salty and hot. Growth starts in 3 days.

    Fishermen use outboards or long tails removing everything from the water.

    My props are bagged, they are now 3 months and I cant see the twine holding them on. If I can find the twine I slice it with a knife and drop the bags . They are so heavy and fouled.

    I could not possible fit conventional jets, they would be unrecognizable as jets in 3 months needing hauling of the boat.

    This simple manipulation of the unit as surface props did for conventional will be a huge simplification of boating.
     
  10. baeckmo
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    baeckmo Hydrodynamics

    A good point, but it is not a major concern. It will boil down (hehe...) to a rpm reduction of roughly 2 % in the example above; say from 3000 rpm to 2945 at the critical load (~hump speed). Use the Suction specific speed for comparison: nss=rpm x (flowrate)^0.5/NPSH^0.75.

    And again: a lump of barnacles at the intake grid or lip is a worse disturbance.

    With some good thinking and free beer access it is certainly doable. Is it commercially interesting then? Very few boat byers (or ship operators for that matter) are taking real world service costs into their budget considerations at the investment time. In most cases you find that other qualities, like "sex appeal" (locally named the Viag*a factor....) are far more important, at least in the leasure market.
     
  11. Frosty

    Frosty Previous Member

    I cant think of anything more sexy with Viag*a factor than having a boat with jets sticking out the back with gushes of water on take off.

    Comparable to Arnesons.

    It would seem pointless to have a gearbox on this set up as the water will drain from the pump if the shaft was stopped so a constant prime would be necessary.

    Once stopped the unit can be washed and rinsed.

    In the 40 50 foot pleasure vessel jets are just not acceptable for maintenance
    reasons.

    I find it difficult to accept and consider it careless of boat operators to not take maintainence into financial considerations.

    With a jet it will be high. I really don't know how you could possible clean a jet internally without a full dismantle.
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    I think your taring many with the same brush there!

    Its all about preventative maintenance and good design.

    One of the GRP duct vessels i referred to above operates in the tropics, or did for 10years. Sea water 30c nominally, not clean water in the harbour either.

    Not one bit of corrosion anywhere.
     
  13. Frosty

    Frosty Previous Member

    Barnacles --millions of them seemingly attached by there own epoxy like glue --incredible things. The nozzles and operating rams and or cables, a night mare to keep going.

    A jet will not run with barnacles. Do you antifoul inside a jet?

    Would not make any difference any way, it is ripped off in minutes. Ok if you are the uk Ireland ferry running 24/7.

    If your not you dont have a choice but to refuse jets,--and only for one reason.
     
  14. CDK
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    CDK retired engineer

    This picture I plucked from the Place Diverter website. The device lifts the nozzle to make a rooster tail, to some people that is their ultimate goal.
     

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  15. baeckmo
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    baeckmo Hydrodynamics

    I agree with Frosty on the barnacle/weed problem; we do give the innards of the jets an antifouling "protection". If done properly it works.....sort of. There is never weed inside, only barnacles. So a device that makes the swimming barnacle-larvae uncomfortable in the environment is what we search (magnetic fields, noise, galvanic potential, bubbles, flexing surfaces- whatever).

    The corrosion and external growth are more about good design and understanding of corrosion in sea-water. In these areas, I have seen really stupid mistakes beeing done.
     
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