Doesn't rev out!!!

Discussion in 'Inboards' started by 202_ski, Apr 3, 2008.

  1. 202_ski
    Joined: Mar 2008
    Posts: 12
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Australia

    202_ski Junior Member

    I converted a 15ft fiberglass outboard runabout to mid mount inboard using a 202ci 6 cylinder holden (GM) motor with the prop shaft connected to the balancer end of the crank with a universal joint as straight as possible. My shaft angle is approx 18 degrees from the line of the keel. Prop is a 12 by 11. Motor has roughly 100HP. I cannot get the boat to rev past 2400rpm, I have run out of ideas. This motor should rev too 5000.
     
  2. waikikin
    Joined: Jan 2006
    Posts: 2,383
    Likes: 151, Points: 73, Legacy Rep: 871
    Location: Australia

    waikikin Senior Member

    202, google up, Porters props or Austral or Lundberg for some repropping advice. All the best from jeff.
     
  3. Jango
    Joined: Aug 2005
    Posts: 519
    Likes: 7, Points: 0, Legacy Rep: 63
    Location: Mid Atlantic

    Jango Senior Enthusiast

    If you have 100HP, 11" Pitch is correct. Assuming displacement of 1600 lb, you should get about 37 mph @ 4000 rpm with a 11 x 11 prop. 12 dia prop -probably 8 - 10% less rpm and speed. Problem has to be with motor and/or perhaps CG location - too far forward causing plowing.
     
  4. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 488, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Check to see you're getting ignition advance at the proper RPM's.
     
  5. 202_ski
    Joined: Mar 2008
    Posts: 12
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Australia

    202_ski Junior Member

    I am allready getting 30 mph at 2400rpm. Does that mean i should get about 50 mph at 4000 rpm? The motor is quite far foward but it feels like it gets up on a plane well. Could it be riding up to high? Would that load the motor up too much?
     
  6. Jango
    Joined: Aug 2005
    Posts: 519
    Likes: 7, Points: 0, Legacy Rep: 63
    Location: Mid Atlantic

    Jango Senior Enthusiast

    You might want to check the accuracy of your speedometer and/or Tach. With a 11" pitch @ 2400 rpm, 11 - 12% slip, your speed would be about 22mph.
    Also check your Prop. If it was repitched and is alot more than 11" pitch, it could also account for the lack of revs. Prop has an effective pitch of about 15" @ 30 mph and 2400 rpm with 12% slip. This could be your problem assuming your motor is putting out 100hp with no issues.
    Riding higher or more out of the water, should "unload" the motor and increase speed with increased revs and lower slippage.
     
  7. lazeyjack

    lazeyjack Guest

    that is so strange, she is well up on plane, direct drive, how long does it take to come onto plane and at that speed does she trim flat or bow well up?
     
  8. 202_ski
    Joined: Mar 2008
    Posts: 12
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Australia

    202_ski Junior Member

    It gets up on a plane easily and fast. Probably 2 seconds from idle. It revs up fast enough but just cuts at 2400.
     
  9. 202_ski
    Joined: Mar 2008
    Posts: 12
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Australia

    202_ski Junior Member

    I have read that a traditional pitot speedo cannot give a speed reading HIGHER than actual, but only LOWER. The prop is brand new, and i had the pitch measured because I also doubted it. I first tried a 10 and 3/4 by 14 which gave me 25 mph at 1900 rpm.
     
  10. Jango
    Joined: Aug 2005
    Posts: 519
    Likes: 7, Points: 0, Legacy Rep: 63
    Location: Mid Atlantic

    Jango Senior Enthusiast

    Most Tachs, have a 4,6 or 8 Cyl. switch. If so, is yours set correctly? Also if the motor "cuts out" abruptly, it sounds like an ignition advance problem.
    An 8 cyl Tach connected to a 6 cyl motor indicating 2400 rpm is actually turning 3200 rpm. Prop with 11"P turning 3200 rpm and 12% slip equals 30 mph.
     
  11. lazeyjack

    lazeyjack Guest

    jango, these are very very basic engines, but the tach advice sound spot on, try reading revs at high idle, thats a diesel term, but means full rpm with no load
     
  12. Jango
    Joined: Aug 2005
    Posts: 519
    Likes: 7, Points: 0, Legacy Rep: 63
    Location: Mid Atlantic

    Jango Senior Enthusiast

    WOW, Full rpm, with NO load. Must be some Engine? Is it a Diesel? I,m assuming Gas. Don't like to run gas engines at Full Throttle with NO load.
     
  13. lazeyjack

    lazeyjack Guest

    i just mean blip the throttle, not hold the pedal down!!!
     
  14. Jango
    Joined: Aug 2005
    Posts: 519
    Likes: 7, Points: 0, Legacy Rep: 63
    Location: Mid Atlantic

    Jango Senior Enthusiast

    Thanks for the clarification. Yes, bliping the throttle might tell something about rev capabilities. Good idea.
     

  15. 202_ski
    Joined: Mar 2008
    Posts: 12
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Australia

    202_ski Junior Member

    It revs to 4000 easy with no load. I had a friend watch the way the boat rides from outside the boat and it doesn't get up on a plane properly, it just pushes through the water. Im gonna try trim tabs/ cavitation plates.
     
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.