Jet Drive problem

Discussion in 'Jet Drives' started by Orie, Dec 20, 2017.

  1. Ben Landgren
    Joined: Jun 2017
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    Location: Newcastle Upon Tyne

    Ben Landgren Junior Member

    Have to agree with baeckmo with this one. Probably it is a cause of: first cavitation damaging the stator coating which will expose the aluminum cast to sea water. I base this on the fact that the stator suction side coating seems to be more damaged than pressure side and this would suggest me that coating was not damaged by debris etc, but by back cavitation instead.

    Then the aluminum casting begins to corrode. There are different forms of corrosion that might be present at the same time. And based on some experiments that I have done previously, the aluminum corrosion process will spread out under even a slightly damaged coating quite rapidly in sea water. Also these stator vane edges are problematic when it comes to coating as they need to be sharp and hence the coating is difficult to be applied around the edge properly.

    Are the waterjets coated also with antifouling? Antifouling paints can have high copper content and therefore may speed up the corrosion process of cast aluminum.

    Cheerio
     
  2. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    If the cavitation removed the anodized or other finish, corrosion will be next.
     
  3. anthony goodson
    Joined: Mar 2007
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    Location: Dorset UK & Murcia Spain

    anthony goodson Senior Member

    Good morning baeckmo,yes I am still about,I hope you are well. You seem to have hit the nail on the head again with this one ,not that you ever miss. My own experience with Kamewa is limited to an occasional inspection at places like Seawork, and the experiences of a family member,who until recently operated a CTruk 18m with twin Cummins QSM11 and Kamewa jets,as a wind farm support vessel. Cavitation was not a major problem with this boat being mainly confined to air entrainment in heavyish seas, particularly following seas as you would expect.The bollard pull/push was quite impressive when pushing on to the transition piece it was quite happy at 2100 rpm. I realise that these comments do not directly apply to the monohull that the op is asking about but it would seem to be a good example of the matching of hulls engines and Kamewa jets.Since the stator is probably hard anodised LM 25 or similar and is protected by anodes it would seem a lot of damage to occur by corrosion alone,which is where your explanation makes perfect sense. I will follow this thread with interest and perhaps learn a thing or two more.
     
  4. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    I would love to get my hands on a sample to analyze.
     
  5. baeckmo
    Joined: Jun 2009
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    Location: Sweden

    baeckmo Hydrodynamics

    So, finally we can agree that there must be cavitation to start with....! Corrosion may be next, but not yet here, since there are intact (although partly consumed) anodes remaining in the stators (round "doughnut" around cutlass bearing carrier). If you look at the pitted bare metal you see very clean, circular craters typical of cavitation. Once the anodes were consumed, the combined effect of cavitation and corrosion would accelerate the material loss
     
  6. Ben Landgren
    Joined: Jun 2017
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    Location: Newcastle Upon Tyne

    Ben Landgren Junior Member

    I have understood that corrosion can occur despite that there are zinc anodes left. This could be caused e.g. by insufficient area of zinc anodes or the lack of regular brushing/cleaning of corrosion products from zinc surfaces . Please correct if I am mistaken.

    If the red paint on the stator is high copper content antifouling wouldn't it then require the stator anode area to be notably larger than with just standard coating?

    Ben
     

  7. Barry
    Joined: Mar 2002
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    Barry Senior Member

    highly unlikely a copper base anti fouling for an aluminum stator,
     
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