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Old 07-01-2017, 11:38 PM   #1
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Default Can't reach max RPMs underway in gear.

1999 4100 SCA 330 Cummins

No issues last time out, 1 May.

Pre-underway engine start up, warm up, and increase throttle (in neutral) easily reaches max RPMs. Normal temps and oil pressure.

Underway idle/low RPM (10k) run out of no/low wake zones - no issues.

After ~20 min clear of low wake area and throttle up but can't push past ~14k. Temps and oil pressure are normal.

Thought it might be a trim tab issue and I was just not driving out of the hole and getting up on plane. Can't actually see trim tabs, and no remote indication. Checked trim tab accumulator and listened to motor cycle as helm switches are pushed for up and down. All seem ok.

Underway, when transmission is in neutral, still no issue getting to max RPMs.

No black/blue exhaust even at full throttle (in neutral) so seems turbos are ok.

Any thoughts would be appreciated. I have two more days to trouble shoot before this vacation is over and I'll call my maintenance guys to have them take a look on our next time back at the boat.

Thanks in advance.
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Old 07-02-2017, 03:05 AM   #2
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First reving an engine in neutral is not good for the engine and it tells you nothing.

When was the last time they achieved proper WOT? Hit anything? Marine growth?
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Old 07-02-2017, 03:08 AM   #3
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Sorry, noticed a typo in my original post ... missed putting in the decimal point in my RPM reports.

In neutral, idle ~200 RMP, ~3/4 throttle 2,400 RPM easily reached then I back off.

In gear, for low/no wake, about 5 knots ~1/4 throttle ~1,000 RPM. Full throttle 1,400 RPM, only about 7 knots.

Starting to get a feeling that the bottom/props/shafts might just need a good cleaning.
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Old 08-09-2017, 02:44 PM   #4
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Problem found & corrected. An hour of scrapping made all the troubles go away.

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Old 08-09-2017, 03:22 PM   #5
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Wow... when was the last time you had it cleaned?
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Old 08-09-2017, 03:56 PM   #6
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@Phillbo ... Did a scrape in early March during survey.
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Old 08-09-2017, 10:39 PM   #7
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That growth will do it. When last cleaned did you paint the running gear?
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Old 08-10-2017, 01:48 AM   #8
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That's a candidate for Eelsnot for sure! The gear snot stuff keeps the little critters off like you wouldn't believe.
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Old 08-10-2017, 02:58 AM   #9
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@mmwjr & jmq26 - no, haven't begun periodic preventative maintenance, but I've read about the effectiveness of eel snot on the boards. Will read up on application and work it into next short hall or next time on the hard.
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