De-Rate, De-Prop or De-Throttle Detroit

More discussion will just make your head spin more. My question would be how much faith you have in your mechanic. He's the one who'll face you if he gives bad advice. Since it has no cost to you I'd go with his recommendations.

You omitted the most important element which is how will you use the boat.

if you abasilutly plan on only running at hull speed (9kts) then smaller injectors would be an option. The question is why would you want to restrict the boat that bad

otherwise, leave it alone. Make sure the engines can turn rated RPMs And if you want to extend the longevity of your engines run the boat at a lower power setting. As long as the boat is fully on plane it will be easier on the engines

personally I don’t buy the Often heard “diesel are meant to be tun hard” mantra. There is a reason why many diesel manufacturers set service intervals in hours or fuel used, whichever comes first. The more fuel used, the more heat and the more revolutions therefore the more wear

over propping a diesel is a no no. A BIG one, even if run at slower speed. Lousy idea.

"Diesels are meant to run hard" means long hours and pushing heavy loads. It definitely doesn't mean constantly at WOT or below rated cruise RPMs, and definitely doesn't mean skipping maintenance.

When your life and your livelihood depend on your motors you tend to take better care than when you have more money than sense and your priority is making it to your lunch reservation on time and your landlubing guests think 1100 departure time means sometime before 1300. Recommendations are only as good as their source.

You don't hear it from the pleasure guys because the engines outlast them.

My initial question was "WHY?"

Why do you want to take a well designed, dependable setup and start trying to change it? Why do you think all these others and now you know more about it than the manufacturers? Why are you trying to really accomplish?

Now, you're tossing around numbers like 2000, 3000, and 10000 hours. I'll say your longevity is far more related to the quality of the overhaul and to how you maintain the engines plus proper propping and no overpropping than all these other things. Perhaps you can find one of the other options that somehow increases your life of the engine 10% but you'll never know as you'll never have a placebo engine to compare it against and if the overhaul was done well, then engines are very likely to outlast you.

You have a good combination. Don't start messing with it. The solutions being pushed remind me a bit of some of the cancer treatments that when you look at extensive clinical trials you find increased average life 3 months at a cost of hundreds of thousands of dollars and with a huge increase in side effects.

You bought a nice boat, accept and love it, make sure it's properly propped. You are very likely to run it easier than the previous owners and easier than commercial fishermen. You are very unlikely to ever know the life of the overhauls, whatever that word means as done to your engines. I'd want to know exactly what was done and with the people doing it now suggesting detuning and other that makes me a bit concerned. What did they find and do?

But regardless, I'd take it and run it. And if it fails in x000 hours, you'll never know if it would have gone longer or not as long with these other ideas.

Depending how old your boat and props are you could see some benefit from tuning your props as there's been a lot of improvement in that over the last couple decades, but when it comes to changing your propping or tuning of your motors I'd be very hesitant to say I know more than those who built you motors or designed your boat. Many will mess with things to get an extra 2 kts or an extra mpg but at what cost both in the price of the job and longevity of the motors. People go through years of schooling and gaining experience to become diesel mechanics and naval architects. I'd trust them over the guy down the dock 10 out of 10.

The after market spec is more use specific. So you're comparing apples to oranges. The diesel manufacturer makes a motor that might be used in a truck, heavy equipment, a fast boat or a slow powerful one. Then it's adjusted for specific use.

You are just way overthinking all this and getting way too many opinions when you should be just enjoying your boat.

The DD is a good engine. Well designed. Long life.

The DD and the Hatteras were paired effectively by those who knew the engine and the boat.

If it ain't broke, don't fix it.

I'm in the same camp as olderboater and NYCAP123, Keep the tune that you have and run at lower RPMs. The commercial guys typically run their engines at 1500 RPM and get the 10000 hour life. On my 6-71 I got her with 1400 hrs, now have 2400 hrs, most of this time is at 900 rpm and never cruise above 1850 and the engines are strong.

DD naturals go decades with reasonable care. Hours almost don't matter. Turbo versions go from 1500 to 12,000 hours depending on the use and operator. I been running DD 2cycles since 1960 and started rebuilding them 10 years later. I had a yard that repaired and rebuilt Detroits for commercial and party boats. The people that kept their oil clean and ran under 80% of full power got extended hours. The hot rod captains got less than half the hours. If they didn't like changing the oil, got as low as 1000 hours.
Going slower has the same effect as smaller injectors. At a given rpm/load the fuel required is constant, controlled by the governor. If you put in smaller injectors, the governor opens them more. It can become a control issue if the injector size is too far off. I believe it pays to run finer fuel filters on Detroits because their injectors have many more internal parts. Debris in a liquid flow causes wear. My current injectors are 10 years old, none replaced, some years I do 500 hours. I use 2 micron filters. Once your tanks are clean and you buy clean fuel, it doesn't matter how fine your filters are, they don't plug. I change my filters by vacuum gauge reading, but around 500 hours.
Many commercial fishermen have way over propped DDs to balance their fishing speed/thrust and manage engine pto rpm. With a good captain that understands his engines, over propping has no effect on the hours between overhauls.

Ok, I know this is an old thread, but just thought I'd ask. I have been chasing smoke (black) for the last couple years. 1989 Bertram 37 - within the last 2 months - new turbos, injectors, aftercoolers rebuilt, blowers, etc. you name it and it's either been rebuilt or brand new. Forgot to mention, 6v92's @550 hp mechanical w/1750 hours on rebuilds. Injector timing is set at 1508 - smoked less when set at 1484. Compression is like new in both engines - oh yea, bottom and running gear cleaned 2x per month. Make 2650/2550 no load. Loaded I am at 2200/2100. That said, I smoke like a Santa Fe freight train from 800 rpm up to 2150 WOT to the point where it takes 1/2 a day to clean the soot off the transom and bulkhead. Speeds are good - at 1950 we are turning about 23 kts.

I have had multiple mechanics on board over the last year, some who are legendary DD guys in the area with 40+ years in the business. All seem to just scratch their head about the smoke. So my thoughts are that I am over propped. Stock from Bertram were 3 blade 25x28. The previous owner tuned them to 24x24, but caused cavitation, so I had them tuned to 24x27/28 (unfortunately I could not add diameter back in). But, I am thinking I might be over propped - mechanic say's no, but to be fair, there is not a whole lot left we can address if anything - I mean black smoke is lack of air or fuel, and we know we are getting both.

I'll grab some popcorn!

Many thanks in advance!

Isn’t rated RPM on those engines 2350 at WOT? Are both engines smoking badly? Or is the lower rpm side smoking worse?

They are rated 2350 full load. Smoking more on the port side vs stbd side, but it's from 800 rpm up we get the black smoke. No smoke at start up or @ WOT no load in the slip.

I’m confused. You are nowhere near where you should be at full load WOT rpm and you have multiple mechanics telling you you don’t have a problem? What am I missing?
Have you tried running with engine room access open to make sure you don’t have air restriction?

We’ve run the engine hatches open and no difference.

No load I am on par with factory specs and we easily make 2550 to 2650 rpm. Loaded I am not making full rpm which is 2350.

I'm fighting a similar issue on 671, 300 rpm short but do not really smoke. Having the props tweaked as I write this.
Have you had the engine instrumented is see what is going on, turbo press, press drop across intercooler, air box pressure and back pressure. Have you run with air breather off? Since you have spec compression, my guess is you have an air restriction. Has this problem presisted since the rebuild?