Boat using Waterjet troubles

Discussion in 'Boat Design' started by ephraimjosuah, Dec 26, 2015.

  1. ephraimjosuah
    Joined: Dec 2015
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    ephraimjosuah Junior Member

    Good day, hi I am a newcomer in this group, my name is Jusak and want to ask something about the boat which we build now. The condition is like this. We just build boat using 2 waterjet castoldy 340 HC and 2 Cummins Engine @ 500 hp Rpm: 2600, design speed: 28 knots ( particular, Loa:15 m ,Lwl:13.3 m, B:4m, H:2.13 m,draft: 0.76 m ). We performed dock trial & were having troubles that boat can't reach the design speed, It is just 13 knots. The 1st assumption is overweight, the draft is now on 0.87 m ( weight: 19 tons). We can't bring out equipments from the boat again. Can the top speed higher than 13 knots, How can we increase the boat reach 24-25 knots?. Thankyou very much
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    It was designed to be 0.76m draft, but is actually 0.87m ? Perhaps you can post the lines of the boat so you can get some answers.
     
  3. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    ephraimjosuah

    Well come to the forum.

    Firstly, from the numbers you posted, the displacement, length power etc, suggests, that whether a round bilge or hard chine, you should get in the region 22-25knots.

    However,

    Is this the weight from hydrostatics of reading drafts (assuming the marks are correct)....can you physically pick it up and weight it with a load cell(s) to confirm the 19 tonne (not Ton).

    Thus it is either a mechanical issue or a hydrodynamic.

    If a hydrodynamic issue, this would suggest you have something that is affecting the flow and thus pressure on the bottom hull surface affecting the hull and/or waterjet. So, check that you haven't got hull penetrations that should not be there, or are too close to the inlets or simply too large or just poorly made with tubing that extends past the hull into the oncoming flow..

    Also check the inlet to the duct, this is also a big region of pressure loss, so double check that the inlet radius/lip is correct...and also that there are no blockages. Debris is a good example....plastic bags, leaves, old rope etc.

    Failing that check the engine. Is it reaching the max rpm...does it spin/over speed/race for example?..or does it not get the rpm that it should.

    Do you have a gear box?

    However, what may be the problem, which is usually over looked. Is that when a vessels grows in weight, the resistance at the hump speed is too much for the engine to power through. Everyone tends to look at the top/max speed only. And thus forget if the vessels weight has grown, then the resistance at the hump speed has increased. Often the increase in resistance is such that the engine cannot deliver the power required (at the given rpm) to get over the hump and plane.
     
    Last edited: Dec 27, 2015
  4. Tad
    Joined: Mar 2002
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    Tad Boat Designer

    As Adhoc says above, it may well be a lack of thrust at hump speed.

    You need to ascertain the real weight of the boat. One draft measurement isn't enough because trim has probably changed (from design) as well. You also need to know distance of LCG from the transom. That is found by floating a computer model of your hull at her current flotation.

    Then calculate some Savitsky planing resistance points and overlay those on the thrust map from the jet manufacturer. A planing resistance spreadsheet can be found in the download section of this website.

    The rough penned in points and lines on this are the resistance at various displacements and LCG's.

    Graythrustmap.jpg
     
  5. gonzo
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    gonzo Senior Member

    Another common problem with jets is an impeller and/or housing worn or damaged. If the clearance is too large, it won't produce enough thrust.
     
  6. FAST FRED
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    FAST FRED Senior Member

    I assume its a simple lack of thrust at hump speed.

    Most diesel engines can be operated at higher than rated RPM for short periods of time.

    I would have the engine supplier mechanic come out and see if another 10% in RPM gets you up on the plane , where the RPM could be reduced to factory specs.

    It could also be the wrong sized units are installed , which would be a horror.
     
  7. ephraimjosuah
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    ephraimjosuah Junior Member

    Dear all,

    I have upload the linesplan Mr.Efficiency, in this linesplan the draft 0.72 m but the true one is 0.76 m.

    the weight is from hydrostatic and can't still pick it up with load cells, i upload hydrostatic too (Ignore the yellow mark) . The engine reach only 2500 rpm at latest dock trial and can't give more RPM due to stopping automatically for engine failure or because something.
    The gearbox's ratio is 43/53, it is clearly attached on Castoldy name plate. The boat has no hull penetration around of ducts and now i don't have idea to check the inlet radius. Do it influence much for the significant speed decreasing?

    From Inclining Test, we estimate the LCG is 4.575 m from AP Mr.Tad.
     

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  8. ephraimjosuah
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    ephraimjosuah Junior Member

    And for some infos that Brochure put that Castoldy 340 HC needs input power UP TO 750 HP-850 HP besides Our Cummins is 500 HP. We have talked to the Engine technician and said it doesn't matter.
     
  9. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    Hi, take a look at the thread "Castoldi TD340 HC Problem", where a similar situation is dealt with. In comparison, your weight/Power ratio is 19 kg/hp, which means your trouble at the hump speed is going to be far worse. Please note that Castoldi recommend a maximum all up weight of 15 to 18 tonnes with double 340-jets! And that is irrespective of the power used, it is a function of the cavitation performance of this jet unit. So, unless the jet supplier has approved of the overweight, the blame for your problems here lie with the boat designer/builder.

    In order to transmit 500 hp, the jet shaft must operate at ~2180 rpm. With 2500 rpm on the engine, the jet will transmit roughly 450 hp; it may well be the closest ratio available.

    The problem is not lack of power, but lack of thrust due to cavitation. There are several observations regarding this problem with Castoldi jets in particular, and the supplier is reluctant (unable?) to offer any solution to their customers.

    In order to check the jet performance, it would be helpful if you could measure the jet nozzle diameter (Note: not the steering nozzle, but the stator outlet inside the steering nozzle). That makes it possible to calculate a reasonable figure for the flow, and from that the cavitation limited thrust.

    BTW, Tad; what jet is it you are referring to in your diagram?
     
  10. Tad
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    Tad Boat Designer

    http://wmcjet.com/

    Whitewater Marine jets, built in Canada in composites......
     
  11. baeckmo
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    baeckmo Hydrodynamics

    Interesting, seems that the former PP-jets (alias Vospower) have been reborn and just slightly reshaped. Except for the use of inch dimensions in bearings and seals, they were very good pieces of machinery; they should do fine in Canadian colors too!
     
  12. ephraimjosuah
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    ephraimjosuah Junior Member

    Good day, happy new year..
    Are there any solution for the boat's problem?
    Are there things to modify ? like waterjet or the hull?
    because we have been stucked in this condition.

    thankyou very much.
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    Maybe too heavy in the stern ? What is the trim at rest ?
     
  14. baeckmo
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    baeckmo Hydrodynamics

    Happy New Year to you as well! In a previous note I asked for the nozzle dimension in order to see what can be done to the jet; that info is still missing.

    What do the Castoldi people say about it?
     

  15. ephraimjosuah
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    ephraimjosuah Junior Member

    Dear sir,

    The nozzle dimension is 250 mm (the discharge nozzle )

    trim at rest
    aft port = 0.92 m
    aft starboard = 0.99 m

    mid port = 0.82
    mid starboard = 0.89

    fore port = 0.74
    fore starboard = 0.81

    the trim at rest is stern trim 0.2 m

    Castoldy suggest to reduce the weights up to their maximum 15-18 tonnes. We have just 19 tonnes from hydrostatic and attention to the installation of waterjet and trim tab or lifting strakes. We have performed docktrial at 18.8 tonnes weight and have put 300 mm x 400 mm trim tab, at the 20 degree and 70 degree experiment but we get no result, the speed is still max.around 11-13 knots.

    Is it normal for the speed falling to much to the speeds ?

    additional info. Castoldy maximum input power up to 750 Hp Continuous duty engine and 850 Hp Intermittent duty engine. we have twin engine 500 hp
    i am aware that something problem at the impellers but can't still get what is that.
     

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