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Prop Size Question

Discussion in 'Ocean Yacht' started by Rob Wardlow, Aug 5, 2018.

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  1. Rob Wardlow

    Rob Wardlow New Member

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    Right now turning 28”x32” and coming up a little short on rpm. I know going down in pitch would help that issue, but I am looking for a little more stern lift to get the boat running up on top a little better. Any suggestions from the experts? Thanks
  2. Capt Ralph

    Capt Ralph Senior Member

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    Could use ANY data. Whats a little short?
    ie; ship size, power, ratios, rpm rated, desired, turning now?
    Clean bottom, mother in law on board with grill, charcoal, extra chain and anchor? Angle of bow, or ever on plane?
    My ESP got fried on wife #2. Been off line since.
  3. Capt J

    Capt J Senior Member

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    Switching from a 3 blade to 4 blade propellers will give you more lift. Generally you will need to lose 1" of pitch when going from a 3 blade to 4 blade just to stay the same. On a diesel inboard, reducing 1" of pitch on the same propeller will give you 50 more rpms. Where are you in RPM's, what props do you have, and what boat do you have, engines? I'm guessing you have an ocean?
  4. Rob Wardlow

    Rob Wardlow New Member

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    I guess I was a little short on information sorry about that. 1992 Ocean 53 8V92’s 300 hrs since overhaul. Props are 3 blade 28”x32”.
    1100 rpm's 11mph
    1500 rpm's 17.9mph
    1700 rpm's 20.9mph
    1850 rpm's 22.7mph
    1950 rpm's 23.4mph
    max speed at 2200 rpm's was 28.8mph

    Thanks for the help!
  5. Capt Ralph

    Capt Ralph Senior Member

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    Ah, I remember.
    Thought it was just the port engine, both 92s not making rpm?
    That would be the J&Ts 8V92TI? Single turbo in the back center?
    First, have the tachs been verified?
    300 hours after big rebuild? Did they make rpm before the rebuild?
    What color is the exhaust smoke?
    Assume bottom clean?
    Last fuel filter change?

    What concerns me did the motors ever make rpm after the build.
    If the exhaust smoke is not puking dark and you have fresh filters (fresh put on this morning, not last month) and still not making rpm, I'd have the governor and rack looked over.
  6. Rob Wardlow

    Rob Wardlow New Member

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    Both engines are not but the port is off more than the starboard engine. J&T 8V92TA with dual turbos (pulling that from survey). No smoke at all, bottom was clean, and not sure on last fuel filter change. They did not make rpm before or after the rebuild. Same surveyor did the survey in 2009 and boat ran exactly the same and had the same props. Boat just seemed to be heavy in the stern and I was thinking maybe a 4 blade would give it the lift it needed and get RPM up where it should be.
  7. Capt J

    Capt J Senior Member

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    I managed a 63' Ocean 1987 for decades. The owner bought it in 1988. It did 30 knots at cruise the first year he owned it and started slowing down after that year after year till it settled at 24 knots at 1950. He did majors on both (the 12v92's made it 5500 hours overloaded and towing a 35' marlago most of the time LOL). We ended up having to cut the props.

    If the fuel filters are clean, bottom and props are clean and engines in good condition. 4 blades would probably help with stern lift, but you might lose speed. 28x29" would probably be the right size.
  8. Capt Ralph

    Capt Ralph Senior Member

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    No smoke at all
    Good indicator the engine is not straining, just needs more fuel.
    PM coming.
  9. PacBlue

    PacBlue Senior Member

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    With mechanical Detroit's you should make an effort to verify full throttle travel / correct fuel rack settings.

    At the dock, in neutral, to need to confirm that the no load (neutral) rpm on each engine is close to 2500 rpm:

    upload_2018-8-8_8-22-47.png

    With regards to propellers - Ocean was a budget conscious builder and they could get the speeds they needed with a 3-bladed propeller for a lower cost. The good news is that propeller design has advanced considerably since 1992 and you have good options in the new Michigan Wheel product as well as ZF, Acme and a few others. After verifying the engine makes power (has the right compression), the fuel rack is correct, you have the right throttle travel, etc., you can approach a good prop shop and they will be able to help you.

    There are ways to get additional stern lift through cupping, but this takes some trial and error and can get expensive if they are not close on the first try. Some of the better shops like General Propeller and Frank & Jimmies know the cupping tricks, make sure you get a cup spec by the number, instead of vague words such as "light, medium, heavy". Get cups done with gages, they are usually numbered #1 thru #10 (lightest to heaviest).

    You can easily take out 2" of Pitch on your current props and then add some cup back into the equation to get to a correct spec full throttle rpm of 2300 - 2350 at full load.

    And as always, remove any UNNECCESSARY GEAR onboard, as excess weight is your greatest reduction of performance on any powerboat. You would be surprised at how fast your boat is without owner's gear, just like the factory's original sea trials :)

    Finally, August is usually the most difficult benchmark month when dialing in performance - high ambient temps, high sea water temps, high humidity. It is usually the most conservative approach by boatbuilders to prop for August conditions, knowing that they have margin for cooler months. This means that you could be satisfied with 2300 rpms in August conditions, knowing you will get a "boost" in cooler months up to the 2350 rpm ideal target.
  10. Capt Ralph

    Capt Ralph Senior Member

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    PacBlue is correct.
    Making sure the racks roll wide open without binding and the governors are set correctly.
    No-load, WOT for a few seconds will determine if there is a governor problem. No-load, high idle should be around 2500-2550 rpm.
  11. Rob Wardlow

    Rob Wardlow New Member

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    Those are all good points. No load on starboard was 2550 but port only turned 2400. When I get her back over to St Pete I will get her tuned to make sure I have her making full power before making any other changes. Thanks for sharing all of your expertise!
  12. PacBlue

    PacBlue Senior Member

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    Sometimes your throttle cable/connection may be off a bit and need an adjustment to get full throttle travel.
  13. Capt Ralph

    Capt Ralph Senior Member

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    Port was slow under load? You just found that problem. It takes both engines to find loaded max rpm. Full load of fuel, bunch of folk and excess stuff can hurt also.
  14. Rob Wardlow

    Rob Wardlow New Member

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    Yes under load it only turned up to 2126 and Starboard turned up to 2258 under load which props would certainly fix starboard but port obviously needs some work.
  15. Capt J

    Capt J Senior Member

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    If port is not making the power or rpm it is supposed to, it will also bring down the rpm's on the starboard as it's carrying more load than usual. Sort out the port engine first and why it is not achieving unloaded rpm's, then sea trial again.
  16. Capt Ralph

    Capt Ralph Senior Member

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    No load needs to be at 2500 - 2550rpm. Loaded should be around 2300 - 2350. I'm sure that is a part of your issue.
  17. Rob Wardlow

    Rob Wardlow New Member

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    Thanks guys. I have been an outboard guy and have a lot to learn about these boats!

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