Perplexed! Performance/Propping my 6CTA’s

[Brian Kelly]

Hey guys, I purchased my Formula 41PC with twin 6CTA’s a little over 2 years ago now and I have been absolutely struggling to get my RPM’s and props dialed in. The boat had a set of 5 blade’s on her that were only reaching 2400 RPM’s and after reading all of Tony’s Tips related to propping and revs I knew this was nowhere even close. My spare set of Michigan 4 blades got me a bit closer at 2500 rpms and after giving my specs/numbers to ACME propellers, they made me a set to achieve 2700 rpms and I am still only getting about 2600 (with full tanks). I am baffled as to what’s going on or what steps I should take next. The motors are in very good working order, pressures/temps are great and they run strong with no obvious issues. The cooling/aftercooler systems have been maintained very well and are flushed and inspected annually. I am just at a loss at this point on where to go next. Additionally, I have a sister boat in the same bay as me here in Rhode Island with essentially the exact same specs sans different exhaust setups and his seems to be performing where I would expect to be at. We even have the same prop tuning shop so we know our shared prop measurements/data is accurate/precise. I charted out a propeller data sheet below for my boat with the 3 sets of props I have tried and his “sister” boat for comparison. All data is listed below. Any help is welcomed!

<u>1999 Formula 41PC “Water-Boarded” Propeller reference sheet. </u>

Powered by 1998 6CTA 8.3M3 rated 450hp @ 2600rpm (2172 cpl) Port motor is original and has roughly 2500 hours, Starboard motor is rebuilt and has approximately 100 hours. 

Captain Brian Kelly – 508-269-04642

Notes:

1. ALL SPEEDS ARE IN KNOTS
2. N/A = Not Achieved, blank = Unknown/no data
3. All tests performed in similar conditions, similar loadout, engine sync’s OFF
4. All speeds listed are averages of multiple tests to ensure accuracy
5. Engines are properly maintained and have NO performance difference between time of tests. Turbos spooling at 1600rpms and appear to be making good power, recent oil samples from last year normal
6. Ralph Vickery “sister boat” is exact same hull, model, layout/outfit, engines, transmission and gear ratio, for reference. ONLY difference in spec is Ralph’s sister boat has underwater exhaust exits bottom of keel. Mine has lifted mufflers with side exhaust.

RPMs	Michigan Dyna 4 blade   23×22.5	ACME 4 blade   22×22	Teignbridge 5 blade   22×24	Ralph Vickery 4 blade S&S Katapult 22×25.5
1600	14 kts	10 kts
1800	18 kts	13 kts	15 kts
1900	20 kts	15 kts	17 kts	23 kts
2000	21 kts	16 kts	18 kts
2100	22 kts	19 kts	20 kts
2200	24 kts	22 kts	23 kts	27 kts
2300	26 kts	23 kts	24 kts
2400	27 kts	24 kts	25 kts
2500	28 kts	26 kts	N/A
2590	N/A	27 kts	N/A
2650	N/A	N/A	N/A	32 kts
2700	N/A	N/A	N/A

 

Some of the questions I am left with after reviewing this data…

1. Could timing a contributing problem?
2. Should throttle levers hit throttle stop on fuel pump?
3. Maximum revs in neutral – I’m getting Port 2800 and Starboard 2700 – could this be contributing?
4. Could this be due to aneroid adjustment?
5.  Am I not making full boost? Are the turbos just tired even though they’re spooling at 1600revs?

Topic

[bluebyu]

From this site, boost port: https://www.sbmar.com/articles/boost-fitting-locations-for-b-c-series-marine-diesels/ I see 28 to 31 PSI on our boost gauges (6CTA8.3-M3)

1 user thanked author for this post.

Tony Athens

[Brian Kelly]

Tony, 

Of course I know no two boats are ever the same, the stark differences in speeds and RPM’s just really stood out to me.

 

I lasered the motors to make sure the tachs are accurate and they are, my starboard tach is dead on the port tach is about 25-40 rpms off at times but never any worse than that. 

 

Boost gauge is a great idea, is there an easy spot to install one without having to drill into any manifolds? and how much PSI should I be looking for?

[Tony Athens]

Three  quickies:

 

Not ever are two boats the same

 

Throttle levers MUST hit the stop on the P7100 pump  solid,  and NO-load should be right at 2950-3050 RPM–Tachs are accurate?

 

  Let’s get good WOT and 2100- 2200 RPM boost numbers–Use an accurate – 0-30 /0-60 PSI gauge

[JimmyK]

I see Gene’s point and it is valid.  The engines have nothing to do with speed at a given rpm.  That is a function of the gear ratio, props, and boat (bottom design, weight, etc), but not engine (aside from engine weight).  You have to understand and accept that before going any further.  The engine can dictate what rpm can be reached but not the speed achieved at that rpm.

With that said, it seems clear you have two problems compared to your friend’s boat. 

One is what Gene describes–there is at least one significant difference between your boat and your friend’s boat totally outside the engines.  Props are different, but are yours that much less efficient?  Doubtful IMO.  Weight would be number 1 suspect.  Some boats are like some people and have a habit of gaining weight.

On the other hand, it seems clear to me that you also have something going on with the motors.  They are likely not making full power.  I would start by trying to figure out why they will not reach rated RPM in neutral.  This has nothing to do with the aneroid or turbo.  As said above, number one suspect would be something going on with the injection pump.

[Gene Fuller]

I’ll try again.

If the tachs and speeds are accurate the differences in the following excerpt cannot be related to engine performance in any way. It is possible your “corrections” are not quite right or there are real “non-engine” differences.

 

[Brian Kelly]

Gene, thanks for your response. If you read in my original post, it says we corrected for conditions, boat setup, loadout, etc. All speeds were done at similar conditions (between 1/2- Full tank) and the boats are spec’d out identical aside from style of exhaust. I cannot ascertain peak RPM’s which could be due to propping but also could be due to engine performance which in my case seems more likely. 

[Gene Fuller]

You may be chasing the wrong issue.

At a given RPM, such as 2200, any differences in speed are NOT due to the engines. Your friend’s boat may be lighter, better trimmed, cleaner, more efficient props, etc. etc.

There could be engine problems, of course, in addition to the boat differences.

[Brian Kelly]

How were your problems similar? What exactly was wrong with your injection pumps? What was done to “freshen them up”? What improvement did you see after this work was done? Do you have any numbers

 

[Brian Kelly]

How were your problems similar? What exactly was wrong with your injection pumps? What was done to “freshen them up”? What improvement did you see after this work was done? Do you have any numbers

[Nathan Brady]

Both engines are coming up short on rpm in neutral. Being the same age probably have lazy gov springs or whatnot in the injection pumps. I chased my tail with the same issue before, find a good shop that can overhaul and injection pump and let them basically freshen them io and tune them in. You should see at least 2900 unloaded from what I’ve researched and personally i see just over 3000 rpm on my 6cta. 

[Author]

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