RPM question: Engine Survey - Sea trial

[nautibake]

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

[MikeP]

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.

[Tim Powell]

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.....

[Finalee]

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.

[nautibake]

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!!

[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.

[rwappleton]

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

[Milacron]

My accept / reject date is October 1.
Jerry

Well ?

[nautibake]

Well ?

We worked it out, closing Monday or Tuesday!! Very excited, and anxious, and LOTS to learn. I've been poring over the manuals for the various systems. We're heading to Pensacola Saturday to take "delivery" early next week. Will spend a month getting to know her, and hitting the punch list from the surveys plus bottom paint, already have the yard scheduled. Hope to get her down to Marco Island before Thanksgiving, weather (and everything else) permitting... thanks for asking!!

[Milacron]

We worked it out, closing Monday or Tuesday!! Very excited, and anxious, and LOTS to learn. I've been poring over the manuals for the various systems. We're heading to Pensacola Saturday to take "delivery" early next week. Will spend a month getting to know her, and hitting the punch list from the surveys plus bottom paint, already have the yard scheduled. Hope to get her down to Marco Island before Thanksgiving, weather (and everything else) permitting... thanks for asking!!

Very cool....congrats !