Meridian 411 – QSB 380 HO – ZF 85A

[tonycav]

I am lookin for suggestions.  My friend purchased a 2007 Meridian 411 with Cummins QSB 380 HP HO engines.  It originally came with ZF 80 gears with a 2.0:1 ratio.  The transmissions were upgraded due to an issue with the coupler which caused torsional vibration.  The new gears were a ZF-85A.  The boat topped out at 30 knots at 3100 RPMs.  Last year he had a situation where the starboard prop hit bottom.  The props were sent out for reconditioning and were scanned at the same time.  The yard identified an issue with the starboard gear and replaced it with a new ZF-85A.  The boat was sea trialed and is hitting 3060 RPMs, but the speed has dropped to 22.6 miles per hour.  The yard that repaired the boat denies that the boat ever did 30 knots at WOT and says that everything is working correctly.  The survey report was shown to the yard and the manager still won’t accept responsibility for the issue.  ZF was contacted next.  They have recommended getting an optical tachometer and checking the output shaft RPMs at the shaft coupler.  They believe that the new transmission gear ratio may have been marked incorrectly.  I will do the recommended test once I find my optical tach.  If one gear is at 2.0:1 and the other is a different ratio, would the RPMs make 3060 RPMs?  Would the boat be running extremely well but at a lower speed?  How is this possible?  It doesn’t make sense to me.  Any suggestions?

 

Topic

[tonycav]

Knots vs MPH

Very observant!
The original survey I have from 2016 was in knots. 30 knots at WOT. I have a hard time believing that it could possibly hit 30 knots.
The new speed is in MPH.
Took it out again today and tried to check the propeller shaft RPMs at WOT. Was unsuccessful. the tachometer wasn’t working properly. We will try again in a couple days.

[Rob Schepis]

Okay report back after the new sea trial and fuel burn readings and clarification of MPH vs Knots.

[Loulouton]

Miles or nautical miles

Can you clarify the speed units?

In your first post you wrote 30 knots = 30 nautical miles per hour.

In your sea trial data I see 30 mph = 30 miles per hour

1 knot = 1.151  mph

30 mph= 26 knots

Is the new speed 22.6 knots or 22.6 mph?  

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Rob Schepis

[tonycav]

Meridian

Rob, Thank you for your reply.
The performance numbers were confirmed with 2016 & 2018 surveys/sea trial data.
Boat was hauled today, bottom pressure washed, hull sounded by surveyor, re-launched for sea trial. 
Props are 24 inch x 24 inch.  Confirmed with 2016 survey.
Fuel flow comparison was not done yet.  Will do that tomorrow.
Going to check RPMs at the shaft coupling with optical tachometer tomorrow.

SEATRIAL DATA: (2018 Survey and Sea Trial)
RPM              OIL          TEMP       SOG(mph)
1000             40.5/48.6   150/155    6.2
1500             52.7/53.9   155/155    9.8
2000             52.7/54.4   180/188   13.6
2500             46.3/49.2   180/180   21.7
3000              45.2/48.1   182/185   29.3
3100/3100(wot)   45.2/48.1   182/185   30.1

[Rob Schepis]

Have you compared the running data on the displays between the two engines, ie. fuel burn and load ?

Do you have historical fuel burn vs. rpm data that you can compare to current fuel burn vs. rpm readings ?

Are you 100% sure the prop specs now match the historical prop specs?

You’ve ruled out the obvious such as bottom condition?

How confident are you in the accuracy of the statement of  “she used to do 30 knots at 3,100” ?  I ask this because those engines are governed at less than 3,100 rpms …..

[Author]

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