80s vintage Yanmar 18 only hits 1800rpm

Discussion in 'Diesel Engines' started by DennisRB, Oct 12, 2010.

  1. DennisRB
    Joined: Sep 2004
    Posts: 1,268
    Likes: 25, Points: 48, Legacy Rep: 228
    Location: Brisbane

    DennisRB Senior Member

    My girlfriends 31 foot sailing boat has a Yanmar 18hp and at full throttle the boat hits 5.5K. At this speed the engine is only doing 1800rpm and pumping a lot of diesel smoke. So we reduce throttle so the smoke stops. RPM only goes down a bit and speed is about 5K.

    Do these symptoms sound like an over propped engine? What sort of RPM should this be reaching?
     
  2. gonzo
    Joined: Aug 2002
    Posts: 14,499
    Likes: 659, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    That looks like a classical symptom of overpropping. Are there any identical or similar boats you can check the prop size?
     
  3. DennisRB
    Joined: Sep 2004
    Posts: 1,268
    Likes: 25, Points: 48, Legacy Rep: 228
    Location: Brisbane

    DennisRB Senior Member

    Thanks Gonzo. There are no identical or similar boats around that I can check. I will eventually dive down to scrub the bottom and measure the prop. I will also get exact engine specs.

    Here is a pic of the boat. Not that it helps much but everyone likes pics :) Its unusual in that it has 7" draft on a 31" boat and huge free board for a 31". The boat looks huge next to other boats of similar length. Its strip plank. I think she said it is 5T?

    [​IMG]
     
  4. FAST FRED
    Joined: Oct 2002
    Posts: 4,519
    Likes: 109, Points: 63, Legacy Rep: 1009
    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    Could be a cruise prop, could be a simple lack of fuel.

    IF the smoke is mostly black , that's the sign of too much fuel , Cruise prop.

    A 31 ft boat is probably 25 on the LWL , so 5K is the proper long range cruise speed , and 1800 is nice and quiet.

    If you wish to add 1K at the cost of probably doubbling the fuel burn, some pitch can be removed from the prop.

    Dive and be SURE its clean and not damaged.

    FF
     

  5. Easy Rider
    Joined: Oct 2009
    Posts: 920
    Likes: 46, Points: 0, Legacy Rep: 732
    Location: NW Washington State USA

    Easy Rider Senior Member

    Simple enough. Just look at the specs on the engine. See where it develops max hp and tweak the prop till you achieve max hp rpm. You may want to use some Sea Foam to get rid of some of the black soot like stuff in the engine.

    Easy Rider
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.