Engine not reaching Rated RPM during gearbox engaged

Discussion in 'Propulsion' started by alby joy, Sep 2, 2021.

  1. alby joy
    Joined: Jul 2017
    Posts: 34
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    Location: india

    alby joy Junior Member


    Recently we had our sea trials in one of our catamaran vessel with 2 Nos of 50 HP Diesel engine. As per our proven hull foam & propeller design, the similar vessel with same displacement was achieved speed of 7.2 knots at 48 Hp. The present vessel is similar to displacements, draft & propeller design.

    But what we found is that engine is rev up only upto 1500 rpm instead of 2200 rated rpm for power output of 50 HP. Why we have rated rpm decrease to 1500 rpm instead of the rated RPM 2200?

    In short do we expect the rated rpm to drop to 1500 instead of 2200 rpm.
  2. Mr Efficiency
    Joined: Oct 2010
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    Location: Australia

    Mr Efficiency Senior Member

    Maybe explain what you mean about the gearbox ? Have you different gearboxes ?
  3. fallguy
    Joined: Dec 2016
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    Location: usa

    fallguy Senior Member

    So many possibile deltas



    Displacement of the vessel not equal weight

    Engine limited somehow. Most likely cause is improper throttle adjustment.

    Report smoke color.
    Report rpms in neutral.
  4. Barry
    Joined: Mar 2002
    Posts: 1,455
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    Barry Senior Member

    What is the max speed of your newer cat?

    Many tachometers can be adjusted to accommodate different numbers of cylinders, perhaps you are achieving the proper rpm but the tach is not calibrated properly

  5. kapnD
    Joined: Jan 2003
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    Location: hawaii, usa

    kapnD Senior Member

    If the motors are operating correctly, and you have favorable results on a similar craft, I would hazard to guess that the transmission ratio is not the same.
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