Is there such thing as tuning for Detroit Diesel engines?

[brico]

I have twin DD 6-71 Naturally Aspirated engines on my 1980 Hatteras 37 with roughly 2500 hours on each and I believe Starboard engine had an overhaul done in the past bit I do not have records and cannot guarantee that. Engines are running well. Start easily and in winter if I warm them up with a block heater for few hours they start instantaneously as they do in the summer time without warming them. No black, blue, white, grey or any smoke when starting or running. Few small specks of diesel are visible in the exhaust water at starting. I've had the boat since March 2014 and I usually run it at 1250-1300 rpm to maintain 8-9 knots of speed with very little if any wake. I am happy with that speed and consumption. Anything between 1300 and WOT (2400RPM) or close to it is pretty much a mountain behind the boat and struggle to get the boat on the plane. One of my trim tabs is shot so I do not use them even though I believe I would get her on the plane at lower RPM (perhaps 2100RPM). I've hauled out the boat in July 2020 and painted the bottom and applied PropSpeed on both props. The props are sparkling clean after 2 years. Well worth the $500 for probe Speed application.

What I did observe in the past two years is that the consumption seem to have gone up despite the clean hull and clean props. I have not done any work on my engines except three oil changes, coolant change and heat exchanger cleaning since 2014 and roughly 800 hours run on them. The engines are running at right temperature, oil analysis is good. What would be the reason for increased fuel consumption? is there anything I could do to the injectors? Any maintenance, adjustments etc?

[ian w]

I changed my injectors after 15 years and
1500 trouble free hours I too cruised at hull speed .l had my mechanic change the injectors and I am using twenty five per cent less fuel

[cww]

Your problem sounds like there’s something wrong with your trim tabs more so than the engines. Have you verified they’re working properly? These boats really struggle to plane without them.

[Captain Ralph]

How much more weight have you added to the boat?
Once the spares, tools, BBQ grill, clothes, galley equipment and recliner are on board, ALL MPG ideas sink quickly,, even at hull speed.

The exhaust tells most of the story, if it is clear and the exhaust does not burn your eyes; all fuel is getting burnt.

Now, exhaust will stay clear if the engine is not getting enough fuel. But you kind of state you are making WOT RPM.

The beauty and design of 6-71s goes back before WWII. Bomb proof and most engines parts are still interchangeable with the engines from the 40s. They usually do not require any work past oil, oil filter and fuel filter changes.
Oil & fuel labs are important if you are really want to stay on top of your engines. These reports can help guide you if anything needs attention.

If you really have to get a finger in somewhere and some oil on you, Grab a flat feeler gauge set and check a few cold exhaust valve to rocker clearance. Anywhere close to spec (0.012" on a 2 valve head, 0.016" on a 4 valve head), button it back up and don't touch any adjustments.

IMO, get rid of the dead weight, fix the trim tabs, start pulling oil & fuel labs.
Yearly (or more) oil changes including the correct CF-2 oil.

Remember; Never kick a sleeping dog..

[Captain Ralph]

You also mentioned you are making 2400 RPM. Has this been verified?
What is your no-load WOT?
You may not have enough prop. You may have a faster boat than you think.

[brico]

Thanks for the feedback guys. The weight on the boat is pretty much the same since I got it. I can plane on WOT at 21kts of speed and believe that I can go faster if my trim tabs are working, but one is not so I do not use them and have no intention to repair it as I seldom if ever go WOT, maybe to avoid ferry or commercial vessel. I check my tachometer occasionally ands still reading correctly. My idle RPM is 550 and WOT is 2400 which I can achieve with clean props. It is just that I've noticed increased consumption. I've spoken to a marine engineer I know and he suggests removing injectors for checkout and cleaning. They may have clogged up a bit and not dispersing/spraying the fuel properly.

[cww]

Thanks for the feedback guys. The weight on the boat is pretty much the same since I got it. I can plane on WOT at 21kts of speed and believe that I can go faster if my trim tabs are working, but one is not so I do not use them and have no intention to repair it as I seldom if ever go WOT, maybe to avoid ferry or commercial vessel. I check my tachometer occasionally ands still reading correctly. My idle RPM is 550 and WOT is 2400 which I can achieve with clean props. It is just that I've noticed increased consumption. I've spoken to a marine engineer I know and he suggests removing injectors for checkout and cleaning. They may have clogged up a bit and not dispersing/spraying the fuel properly.

If your tabs aren't working that's your problem 100%. You don't really know how the boat is performing without the tabs working, they're intended to be used on plane. If you can reliably (not just downhill with a tailwind and empty fuel and water) plane the boat without tabs, your engines are healthy. That's especially true if you're getting 21kts out of naturals in a boat that heavy. I'm somewhat confused by your focus on fixing what's not broken while saying you have no interest in fixing what is. Leave the engines alone and fix the tabs.

[cww]

Here just so you don’t think my comments are off the mark, this is the original write up on these with data from factory testing. The test boat with naturals had difficulty breaking 20 knots without tabs. With tabs almost 25kts. https://www.hatterasowners.com/Broch.../37CA-0777.pdf

[Scott Mather]

Had this Same boat and engine package (310 hp. 671’s) for 14 years.
Never was this boat a struggle to get on plane. Never had to use the tabs either.
Would jump on plane easily at 14 knots and was easy cruise 16 to 18 knots. Had a top end speed of 21 knots depending on load. You could use the tabs at cruise to bring the bow down but then made boat hard to keep in a straight line. Maybe gain 1 knot using tabs. We always ran at 2300 rpm. And had fuel burn on 25 gph. Total.
Had the boat on scales and was right at 30,000 lbs. and had 4 blade props.

Scott

[brettportzer]

How much more fuel are you using now?

What I did observe in the past two years is that the consumption seem to have gone up