Straight as she goes!!

Discussion in 'Powerboats' started by grady, Jan 21, 2020.

  1. grady
    Joined: Oct 2006
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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
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    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
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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
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    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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    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
  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
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    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
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    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!
     
  10. Barry
    Joined: Mar 2002
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    Barry Senior Member

    A few questions
    1) Are you engaging trim tabs when underway?
    2) Is the boat perfectly level when viewed from the back with the people load on board at rest?
    3) Is the boat perfectly level when viewed from the back with trim tabs up underway?
     
  11. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    Wow, some crazy tangents there.

    Brady, were you able to make the boat go straight by varying the port and strd throttles?
    If so, what kind of RPM readings were you seeing?
     
  12. Mr Efficiency
    Joined: Oct 2010
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    Location: Australia

    Mr Efficiency Senior Member

    Not enough feedback from the OP, some people's idea of a boat pulling to one side, would be different to another's. unless it is strong, in which case it is increasing drag, then hydraulic steering would fix it.
     
  13. kapnD
    Joined: Jan 2003
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    kapnD Senior Member

    IE
    A leaky check valve will cause it to turn in that direction, requiring constant correction, and will make it seem to be “pulling”.
    All responses so far are just shots in the dark, need more information!
     
  14. grady
    Joined: Oct 2006
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    Location: Scituate, Ma

    grady Novice

    Hi Guys, sorry I wasn't expecting this much response to my query.

    I thought I'd give it a couple of days to gather interest.

    But thank you all

    here's some additional info

    yes they made rated RPM 2850 under load, hit the governor at 3050 (I think) in neutral
    props are matched and had been MRI propscaned ...corrected and balanced with report.
    the transmissions are Twin disc 507A's @1.5:1 (both the same)
    no prior damage that I know of....have spoken to both previous owners
    no leaks in helm hydraulics
    to be honest I don't think I ever try to correct this tendency by throttle adjustments
    boat sits level underway, have checked to see if one tab was hanging low, creating drag
    have tried to adjust tabs to correct
    yes I have to correct helm to stay on course in a straight line.
    when I put the running gear back on the boat....I was very careful to get rudder camber right, toe in if I remember correctly

    thank you all again for your replies

    TonyG
     
    Last edited: Jan 24, 2020

  15. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    Tony, what is the nature of your "correcting helm",
    constantly moving the wheel or simply applying static pressure to it?
    Do you see the difference?
    It's a diagnostic question.
    Again, simply static pressure on the wheel or actual motion of the wheel?
     
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