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42 post prop size

Discussion in 'Post Yacht' started by Gambetica, Oct 4, 2016.

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  1. Gambetica

    Gambetica Member

    Joined:
    Aug 3, 2013
    Messages:
    45
    Location:
    Venezuela
    Hi, i have a post 1978 42", not doing so well in cruise speed, what size you recommend on props? 3 or 4 blades?

    Right now i have 24/24, 3 blades, and at maximun rpm uits ginig 2500 RPM, @1900 cruise speed, 11 knots, 2300 14 knots, 2500 17 knots.


    Our injector n-65.

    regards Juan
  2. baltimore bob

    baltimore bob Member

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    Sep 22, 2013
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    Location:
    Baltimore, Maryland
    Hmmm, that don't sound right. If they are J&T 6-71s they should have n 80 injectors. I'm running 14X28 four blade props and get 23 knots @2350 on my 1975 42.
  3. Gambetica

    Gambetica Member

    Joined:
    Aug 3, 2013
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    45
    Location:
    Venezuela
    Yes they are 671-N, but I saw the original specs and recommend N-65
  4. OutPost

    OutPost Member

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    Jul 26, 2016
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    Location:
    Long Island, New York
    I assume hull is clean - when was last time the existing props were scanned and conditioned? Do you have any historical reference points (old owner statements, old surveys, etc) that may show if this is a recent development or how long it's been running at these speeds?

    My new to me 42 is a gasser so hard to compare, but it's a little slower than some other gassers I've managed to locate and find info on (not too many). She also doesn't turn up to top RPMs as she should - going to have props reconned over winter and possibly a little pitch reduced.

    Good luck - you should definitely be cruising at least 16 if not more. (In neutral current).
  5. baltimore bob

    baltimore bob Member

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    Sep 22, 2013
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    Location:
    Baltimore, Maryland
    I need a picture of those engines. To my knowledge the only 6 71 n package was the 310 HP from Johnson & Towers. They would have N08s, I'm looking at the J&T spec sheet.
  6. RT46

    RT46 Senior Member

    Joined:
    Feb 22, 2011
    Messages:
    1,058
    Location:
    Long Island, NY
    Sometimes boats accumulate so much stuff over time that all the stuff adds substantial weight.

    I read a good suggestion (on this site I think):
    Go through the boat once a year, if its not safety equipment and you haven't used it in a year, get the extra stuff off the boat!
  7. My Affair

    My Affair Member

    Joined:
    May 23, 2006
    Messages:
    115
    Location:
    Mystic, CT
    The 1975 42', hull #11- I just sold had 6-71 naturals. She has nibral 24x25 with medium cup Federal 4 blades on her. At sea trial she did 22 knots at 2550, at 2350 cruise 17-18.