Kamewa FF450S Problem

Discussion in 'Jet Drives' started by basilisk1985, Sep 3, 2014.

  1. basilisk1985
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    basilisk1985 New Member

    Hi,
    I am working on a SAR boat calls Naji6 equipped with twin Kamewa FF450S waterjets and MAN D2840 engines. the problem is that in high RPMs ( from 1500 to 2300) some of the water exits from the corner of the nozzle of starboard waterjet that causes to loss of thrust.
    There is little corrosion on the stator that does not seem to be a major problem.
    clearance between impeller and impeller housing is about 2 mm.
    thank you in advance for your kind helps.
     
  2. baeckmo
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    baeckmo Hydrodynamics

    I assume the jets are direct drive. What boat speed do you have when this phenomenon sets in? Boat weight laden? What you describe seems to be a sudden change of flow characteristics; the jet becomes a transverse velocity composant added to the axial outflow, is that correct? Does it only occur on one of the jets? Been there from delivery date?

    First, check that the reverse deflector is completely uncovering the nozzle exit area when in fwd position. If that is OK, then you probably have one of two issues, both leading to stalling of stator vanes:

    A/ If there is a disturbance in the fwd part of the hull inlet (Sharp edge, barnacles, piece of wood stuck between grating fingers, damage to hull, wrong position of spray rails, etc.), you will have a boundary layer detachment on the inlet "roof". This increases the velocity gradient in the impeller inlet. The lower half is working with increased inflow velocity and the upper half with reduced.

    The upper half of the impeller is then producing higher tangential velocities at its outlet, which leads to a stall situation on part of the stator vanes. This is observed as a transverse velocity (inreased "swirl") of the exiting jet. It normally happens quite suddenly, and is felt as a thrust loss, often together with a tendency to steer to port.

    It is extremely important that the flow path from the hull into the "roof" of the jet inlet is smooth and without any irregularities. Note that there must be no disturbances (water intakes, instrument fittings etc.) in the hull in front of the inlet in a distance at least 2m forward and two times the width of the hull intake.

    B/ If a piece of rope or a sock (!) or a plastig bag has passed through the impeller, it may "hang" around the leading edge of stator vane(s), which will cause the flow to stall, resulting in a scewed jet outflow. This fault is noticed as a continuous loss of thrust over wider rpm range; not the sudden change as in situation "A".

    The FF450 has a "shoulder" in the inlet, some 10 cm in front of the impeller inlet, in the four o'clock and eight o'clock position, that tends to trig a local, stationary cavity which may increase the problem you mention. Smoothing this shoulder may improve the flow slightly as well.
     
  3. basilisk1985
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    basilisk1985 New Member

    Dear baeckmo
    Thank you for your kind response,
    First: the RPM of engine transfers to the waterjet via a ZF 1:1.286 GB.
    Beside, there is no sign of disturbance in the hull or waterjet. we have docked the vessel recently and there is no sign of obstacles or barnacles in the waterjet.
    -the only issue is that there are some signs of corrosion on the stator blade which have been fixed before. I mean edge of its blades is not that clean. how should we decide that we need to replace the stator or not?
    -in Kamewa Manual, is written that the standard for clearance between impeller and impeller housing is 2mm-2.5mm . I am sure that impeller housing is its original one and has not been replaced or repaired, but , that clearance in our vessel is about 0.65 mm each side right now. is that ok ?
    Thanks indeed
     
  4. baeckmo
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    baeckmo Hydrodynamics

    Ok, then we have to dig a little deeper, so please answer a few questions:

    A/ What size (loa, beam, displacement) is this boat?
    B/ Type of bottom (monohedron, warped etc)?
    C/ What boat speed corresponds to the 1500 - 2300 rpms?
    D/ Is it the 820 or 1050 hp engine?
    E/ Has the phenomenon been observed from delivery or come later?
    F/ Any response from KaMeWa?
    G/ What diameter is the stator outlet nozzle (not the steering nozzle)?

    I am fairly certain you have a cavitation problem here; the "corrosion" marks on the stator are more likely cavitation pittings. In this jet, the impeller material is SS steel (probably austenitic), which has a high resistance to pitting, but the stator is Al. I have seen cavitation damage on stator vanes on several of these jets, still without damage to the impellers. It occurs at varying positions around the stator, depending on the impeller loading. Often only part of the stator inlet is hurt.

    Do you have pictures of the stator (before repair) or of the boat?
     
  5. basilisk1985
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    basilisk1985 New Member

    A/ What size (loa, beam, displacement) is this boat?
    Loa:20m - breadth: 4.7 - Light:20tons - heavy:26 tons
    B/ Type of bottom (monohedron, warped etc)?
    V shape
    C/ What boat speed corresponds to the 1500 - 2300 rpms?
    I guess 18-30 knots but the speed in delivery time was 35
    D/ Is it the 820 or 1050 hp engine?
    820hp=~603KW
    E/ Has the phenomenon been observed from delivery or come later?
    owner says that it came later
    F/ Any response from KaMeWa?
    unfortunately nothing
    G/ What diameter is the stator outlet nozzle (not the steering nozzle)?
    26.5 cm

    thank you for your kind attention, I have plenty photos of jet and the boat. if its possible would you please gimme your email so I can send them directly to you ?
    besides there are some small issues on impeller racing that may be noticeable for you.
    thanks indeed
     
  6. baeckmo
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    baeckmo Hydrodynamics

    It is not possible to send a message to you via the Boatdesign "User" page. But you can send me a message with your mail adress and we take it from there.

    For personal reasons I do not want to publish my adress in the forum threads.
     
  7. baeckmo
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    baeckmo Hydrodynamics

    Come to think of it; about the tip gap: FF jets have been delivered with two different impeller housing linings, SS steel and polyurethane. With PU, the tip clearance can be smaller than with the SS.

    Which type is yours?

    You are welcome to contact me by "Personal message"; I'm interested in your pic's and the general problem with loss of thrust.
     
  8. basilisk1985
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    basilisk1985 New Member

    Dear baechmo,
    First of all, I wanted to thank you indeed for your kind help.
    our water jet is equipped with PU. but in its manual has been written that standard gap is 2~2.5 mm. is this correct or you think that this clearance is for SS housing ?
    unfortunately I dont have any picture showing the leakage of water from sides of nozzle in full speed. but there are some pics illustrating the scratches on impeller housing and defects on stator vanes.
    [​IMG]
     

  9. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    A few observations from your pics:

    Top left: This shows a few scratches where impeller tips have touched the liner. Obviously the PU is either not smooth from the beginning, or it has started to detach locally from the Al housing. There have been some cases where the PU has split from the substrate, so check if you can spot any signs of corrosion in the edges, between PU and Al.

    The PU has a high resistance against "shearing erosion", which is why it is commonly used as lining for "gravel pumps". The backside is that it is less resistant to vertical (90 degree to the surface) impacts, as you find in cavitating flow. This is a good reason to find a solution to the cavitation issues that are manifest in the impeller pic.

    Top right: Sorry, can't see where the photo is taken. The surface closest to the lens seems to have the sandblasted texture you often see where light cavitation has played for some time.

    Bottom left: This is an important view. The smooth, dull/matte areas on the impeller pressure side, starting from the leading edge at about 75% radius shows that there is a serious mismatch in impeller pitch. This is support of my previous note, that you have a flow detachment in the inlet duct, leading to a "free jet" following the duct bottom into the impeller. Its velocity is so high, that the lower part of the impeller is acting like a turbine, hence the cavitation marks sts on the wrong side of the vane (the pressure side).

    I would like to see a pic of the impeller from the inlet side as well, since there may be a slightly different cavitation pattern there.

    All the energy added to the flow is imparted in the upper half of the impeller, where the inflow velocity is low. According to the classical laws for turbomachines (see Euler), this leads to a high tangential composant of the leaving flow. The absolute velocity is hitting the stator vanes with a too high angle of attack, causing the stator overload. This is what you observe as a tangential flow after the nozzle.

    Bottom right: I believe this shows the stator exit. With a flow disturbance like we have here, there will probably be cavitation damage to the stator as well, but it will be most obvious at the stator inlet side. The irregularity seen at the vane tip may be a bad casting, covered by a thick coating, or a lousy repair (of a cavitation damage?), likewise covered. It is not per se a sign of malfunction; only in the combination of inlet side damage.

    So, there is a flow detachment problem, leading to loss of performance and physical damage, and a possible lining detachment (although in a very early stage).
     
    Last edited: Sep 24, 2014
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