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  1. #1

    RPM question: Engine Survey - Sea trial

    During sea trial yesterday the engines would not come close to rated RPM of 2350 at WOT. Engine surveyor says there should only be a ~200 - 250 RPM drop when comparing WOT with no load (neutral) and when WOT underway. It may be that the reasons include over-squared economy prop, some slight bottom drag (really not too much growth, maybe 7 out of 10 on the clean side), and slightly bent props (on leading edge). Vessel info is:

    1977 Hatt 58 MY (galley up), extensive refit in 2013
    DD 8V71TI’s (port engine 1000 hours SMOH / stbd engine 45 hours SMOH)
    28x32 wheels

    Direct from ER tach readings:

    WOT underway

    Port: 1855 Stbd: 1892

    WOT No load (in neutral)

    Port: 2510 Stbd: 2480

    So there is a 600+ RPM “droop” instead of the supposed target 200-250 RPM difference.

    The engines performed flawlessly in all other respects from start-up to shut-down and all phases of sea trial. Oil analysis (for what it is worth) came back on the low end of the ranges for wear metals and was overall a very good report.

    I would greatly appreciate any perspective on what others may have observed or insight in general as whether or not this is something to be concerned about or not. My accept / reject date is October 1.

    Many thanks in advance…
    Jerry

  2. #2

    Re: RPM question: Engine Survey - Sea trial

    The fact that both engines show about the same WOT RPM would initially lead me toward thinking that it's not engine related. Props, marine growth/barnacles - something like that. Those RPM figures are almost exactly what the 8V71Tis on our boat do with fouled running gear. In that case, when I clean the running gear the WOT RPM (and boat speed) immediately goes up to spec.
    Last edited by MikeP; 09-26-2015 at 03:10 PM.
    Mike P
    San Miguel de Allende, Mexico; Kent Island MD; San Antonio TX
    1980 53MY "Brigadoon"

  3. #3

    Re: RPM question: Engine Survey - Sea trial

    For what it is worth i agree with Mike. Did you pull the boat during or before engine survey. If not and you are going to pull it clean the running gear and give the old girl another world.....

  4. #4

    Re: RPM question: Engine Survey - Sea trial

    I suggest you join boat diesel. lot's of info for $25 annually. The tach readings are not reliable until verified with a photo tach. I recently did mine , and there was a 100 rpm difference port, 225 stbd.
    Fred
    31 Tiara Open
    "Escalation"

  5. #5

    Re: RPM question: Engine Survey - Sea trial

    Quote Originally Posted by tIM pOWELL View Post
    For what it is worth i agree with Mike. Did you pull the boat during or before engine survey. If not and you are going to pull it clean the running gear and give the old girl another world.....
    We pulled the boat before the trial and the seller suggested a pressure wash. As soon as they started the first few swaths with the sprayer the ablative bottom paint was coming right off and we all agreed that would be a waste. The growth was very minimal, so we just went ahead with the inspection and then the sea trial. Maybe the slightly bent ear in addition to the other drag items contributed to the wider spread between no-load WOT and underway WOT...

    The fact that both engines show about the same WOT RPM would initially lead me toward thinking that it's not engine related. Props, marine growth/barnacles - something like that. Those RPM figures are almost exactly what the 8V71Tis on our boat do with fouled running gear. In that case, when I clean the running gear the WOT RPM (and boat speed) immediately goes up to spec.
    MikeP: This is very good to know that this is so close to your experience, perhaps we are not too far off what is expected, especially with the over-squared props...

    I suggest you join boat diesel. lot's of info for $25 annually. The tach readings are not reliable until verified with a photo tach. I recently did mine , and there was a 100 rpm difference port, 225 stbd.
    Finale: Thanks for the tip on boat diesel, I was not familiar with them and will look into it.

    Edited to add: I personally took the tech readings with photo tach in the ER's. The PH and FB techs have all been replaced within the last 3 years and they were almost useless (200 - 500 off). Also, it is VERY impressive when these engines are running wide open only a few inches away from you...

    Thanks to all for the thoughts, really appreciate it!!
    Last edited by nautibake; 09-26-2015 at 04:35 PM.

  6. #6

    Re: RPM question: Engine Survey - Sea trial

    I wouldn't do anything until you get the bottom paint in good shape. If a pressure washer is blowing the paint off I would question the surface prep. prior to painting. You're not likely to pick up 600 rpm though. More than likely you're going to have to take a couple of inches of pitch out of the wheels to get the rpm's where they need to be.

  7. #7

    Re: RPM question: Engine Survey - Sea trial

    I am piling on!!!!! Too much prop.

  8. #8

    Re: RPM question: Engine Survey - Sea trial

    I suppose it is safe to assume the boat never got up on plane?

  9. #9

    Re: RPM question: Engine Survey - Sea trial

    Sounds like way too much pitch. Sam's or other owners should be able to tell you what OEM was, if the owner doesn't have the original papers.
    George
    Former Owner: "Incentive" 1981 56MY
    2007-2014

  10. #10

    Re: RPM question: Engine Survey - Sea trial

    Quote Originally Posted by oldawg View Post
    I suppose it is safe to assume the boat never got up on plane?
    That is correct, I don't believe we were even close to plane.

    GJH, rwappleton:
    Sounds like way too much pitch.
    You are correct, way over-propped. Apparently done deliberately by PO that wanted better "economy cruise" and didn't care about the upper end of the power curve. I've seen from researching other threads that the consensus is to stick with OEM specs on the wheels.

    dsharp:
    I wouldn't do anything until you get the bottom paint in good shape. If a pressure washer is blowing the paint off I would question the surface prep. prior to painting. You're not likely to pick up 600 rpm though. More than likely you're going to have to take a couple of inches of pitch out of the wheels to get the rpm's where they need to be.
    Good point about the bottom paint prep or lack there-of. I suppose at the moment we were reluctant to take off what was at least an apparent layer of paint. It was one heck of a HIGH pressure washer.

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