Straight as she goes!!

Discussion in 'Powerboats' started by grady, Jan 21, 2020 at 10:25 AM.

  1. grady
    Joined: Oct 2006
    Posts: 505
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    Location: Scituate, Ma

    grady Novice

    Hello all, I've got a bit of a dilemma on my hands. Wasn't sure where to post...but here it goes.

    I have a mid 90's ...33' Blackfin combi (express) powered by twin Cat 3208 TA's @ 375 each.

    I have owned her about 8 yrs now.....one of the first things we noticed right away was that if not given any correction.....she will steer to port quite persistently.

    Things to note:
    Noticed the one shaft (I believe the starboard) is longer than the other.....Here's how I know.
    I had shafts out to do some strut work (cutless bearings) and notice one set of prop nuts had
    been machined thinner to allow for both nuts to have enough threads under it to provide good
    service (haven't gotten to the whole story here...shafts could be same lengths but threaded
    differently)????

    Both motors were under performing (now I thought the starboard was the weaker of the 2).
    After breaking down the motors...both had sizable issues not making rated power.
    have had gear inspected and proven straight....all of it...Rudders, struts and shafts.

    So can anyone shine a little light on this perdictament? Was wondering if the weak motor was my port motor and the starboard was doing most of the work....would that make her steer to port??

    Thank you for reading and any input you may have.

    Tony G.
     
  2. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    Do both propellers rotate the same way, or are they counter rotating?
    Re one shaft longer than the other - do you have different spacings between the the struts and the propeller bosses?
    Re both engines not making their rated power, are they achieving it now after you broke them down (and I presume re-built them)?
     
  3. grady
    Joined: Oct 2006
    Posts: 505
    Likes: 11, Points: 28, Legacy Rep: 70
    Location: Scituate, Ma

    grady Novice

    Hi, counter rotating Props, not sure on spacing.... if memory serves me correctly the spacing is the same from back of strut to prop only difference being at the end of the shaft (not 100% certain on this).
    motors are rebuilt awaiting install in spring.

    Thank You

    TonyG.
     
  4. BlueBell
    Joined: May 2017
    Posts: 933
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    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    YES
    What does this mean? Are you constantly having to turn the wheel to keep the boat straight or the wheel simply requires pressure to strd for the boat to go straight?

    I suspect power differences in your engines output but you should have been able to trim this out with throttle adjustments port and strd, or were you RPM matching?
     
  5. kapnD
    Joined: Jan 2003
    Posts: 537
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    Location: hawaii, usa

    kapnD Senior Member

    “Engines not making rated power”
    Dos this mean not making rated rpm?
    If so what is top rpm in neutral?
    It may be overpropped, not allowing engines to reach rated rpm.
    The shaft length disparity is probably due to previous damage, doubt that a small difference in shaft lengths has much to do with problem described. Make sure propeller is properly seated on taper.
    Are the transmissions the same, and the same ratio?
     
  6. Mr Efficiency
    Joined: Oct 2010
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    Location: Australia

    Mr Efficiency Senior Member

    Power output is irrelevant, only shaft RPM is, and if the gearboxes and props are the same, identical engine RPM should give equal thrust both sides. "Weak" engines are not a factor here unless there is a difference in engine RPM. I am assuming this pulling to port occurs even in calm conditions.
     
    Last edited: Jan 21, 2020 at 6:09 PM
  7. DCockey
    Joined: Oct 2009
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    Location: Midcoast Maine

    DCockey Senior Member

    True assuming the props are identical or exact mirror images if counter-rotating.

    grady, are the props exact mirror images of each other? If the props differ then the thrust would be different at identical shaft speeds. Perhaps a good prop shop should check the props.

    Perhaps one prop was damaged and replaced with a prop which was considered "close enough" at the time, but not the same.
     
  8. Mr Efficiency
    Joined: Oct 2010
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    Location: Australia

    Mr Efficiency Senior Member

    Adjusting the throttles a little different one side to the other to compensate might be the answer, depending on how hard it is pulling.
     

  9. kapnD
    Joined: Jan 2003
    Posts: 537
    Likes: 47, Points: 28, Legacy Rep: 40
    Location: hawaii, usa

    kapnD Senior Member

    Rudder trim may also be an issue, read your Dave Gerr on twin rudder setup, or google it.
    Make sure your hydraulic steering is properly setup, bled and filled, problems there can have some weird consequences!
     
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