Traveling at lower speeds with C 28

[CaptT]

The Admiral and I are planning to do the Great Loop next year using our C 28. We were taken aback when we read the statements made in the book " Looper's Companion Guide" claiming the newer high speed diesel engines are not a good choice to accomplish this trip as they are not designed to run at lower speeds. The author goes on to say these engines will require "detuning" and cooler system changes, as well as reduced injector size, to protect against carbonization problems.
Is this true? I have read the Volvo D4 260 engine does like to be worked and opened up on a regular basis but does it require the changes stated above to cruise at a slower more economical rate?
Has any done prolonged cruising at slower speeds that caused carbonization issues? Any Looper's out there that used a similar boat/engine?
Thanks in advance for any insight you can provide!
CaptT

 

[pleeson]

We have over 500 hrs on our D4 - 260, the majority of which have been between 1100 - 1400 rpm. Many days we get 3 hrs per gallon at about 6 knots. I try to open her up for a few minutes at about 16 knots/2500 rpm once in a while but not a hint of an issue in eight years. Tough to imagine too many issues. The engine runs at the same 185 degrees no matter the rpm, and there is no question the fuel metering system is very stingy in supplying fuel at these lower RPM's. I have done two weeks of cruising on under 40 gallons. We enjoy the journey not just the destination.

 

[dgiles]

We have over 500 hrs on our D4 - 260, the majority of which have been between 1100 - 1400 rpm. Many days we get 3 hrs per gallon at about 6 knots. I try to open her up for a few minutes at about 16 knots/2500 rpm once in a while but not a hint of an issue in eight years. Tough to imagine too many issues. The engine runs at the same 185 degrees no matter the rpm, and there is no question the fuel metering system is very stingy in supplying fuel at these lower RPM's. I have done two weeks of cruising on under 40 gallons. We enjoy the journey not just the destination.

Just to confirm the math, you are burning 1 gal of fuel every 3 hours or .33 gal/hr at 6 knots? That is 18 NM/gal. Seems a little on the high side.

 

[FlyMeAway]

I know this is heavily discussed on power boating and diesel engine forums, because I researched it a ton before making my purchase decision.

The jury is really still out on this. Of note, the kinds of "problems" that people identify really wouldn't be an issue until you get into the 1000-8000 hours on the engine range (depending in who you listen to). It also seemed like the commentary was very engine-model-specific, which makes trying to do reliable research on this somewhat difficult.

The research was confusing, and ultimately inconclusive. I don't think anybody has a good handle on the impact of constant low RPMs (or constant high RPMs) on these engines, including the mechanics themselves. When considering these boats, I called four different Volvo Penta dealers and got four differently answers, two of which were highly inconsistent. The only answer which *did* seem consistent across a few non-Volvo mechanics (I was looking at some Yanmar-based R29 classics as well) is that the old-school marine diesels which are essentially modified tractor or car/truck engines are designed to operator at lower RPMs for long periods of time (though, again, opinions differed wildly on whether almost always running at 80-90% WOT would reduce engine life or not matter).

If anybody has what they feel is *conclusive* data on this, I would be very interested to hear it.

 

[NewMoon]

Not conclusive, but here's my experience:

Our 1998 Bounty 257, a heavy planing-hull 26-footer, was powered by a Volvo KAD44P, one of the earliest electronically-controlled diesels, predecessor to the D4 and D6. 24 valves, 3.6 liters, 260 hp at 3900 RPM. After 3+ summers and our first 2000 hours of cruising at 3000-3300 RPM and 16-18 knots, we switched to 1300-1400 RPM and 6-6.5 knots. This made the engine work just hard enough to keep the temp up to 175-180. At 6.5 knots we got 4 nmpg. At first, after a day of slow cruising, we would run up onto plane for a few minutes before anchoring for the night. In later years we did this less and less often, most often after drifting at idle to fish for halibut. We never had to open up the engine to work on anything internal, only a few external things like belts and seawater pump. When we sold the boat after 18 years we had put 6502 hours on the engine. It was still in fine shape.

Our current boat has a Cummins 6BTA 5.9 M3, 330hp at 2800 RPM. We cruise at 1300-1400 RPM, get 3.8 nmpg, have put 3400 hours on it in 5 summers, and all is well.

My understandng is this: make sure your engine is broken in before you spend a lot of time at very low power levels, and especially avoid extensive periods of idling, with little or no load on the engine. Slow cruise at just enough power to keep engine temps up - maybe just fast enough that you can see your bow beginning to rise. All other things being equal, this will do your engine no harm.

Here's a good article from a real authority on marine diesels, Tony Athens of Seaboard Marine:

https://www.sbmar.com/articles/low-spee ... e-diesels/

 

[pleeson]

My figures are correct and confirmed at the pump. We routinely do long cruises lasting multiple days with just a few gallons of diesel. 0.3 - 0.4 gal/hr at the low end, 0.5 gal at the high end if you stick to around 6 knots! We've never used the bottom half of the fuel tank. This is a very efficient hull and engine at displacement speeds. Oh, we are also loaded up with two weeks of remote anchorage supplies like full water, extra drinking water, full fuel, two sea kayaks, RIB and outboard, spare anchor and rode, all our own groceries for full self sufficiency, five lead acid batteries, etc.

 

[dgiles]

My figures are correct and confirmed at the pump. We routinely do long cruises lasting multiple days with just a few gallons of diesel. 0.3 - 0.4 gal/hr at the low end, 0.5 gal at the high end if you stick to around 6 knots! We've never used the bottom half of the fuel tank. This is a very efficient hull and engine at displacement speeds. Oh, we are also loaded up with two weeks of remote anchorage supplies like full water, extra drinking water, full fuel, two sea kayaks, RIB and outboard, spare anchor and rode, all our own groceries for full self sufficiency, five lead acid batteries, etc.

That is awesome but I would never sell it because you have a unicorn. Typical burn rates on the D4 - 260 is 2.5L (.66Gal) at 1000rpm and 8L (2.1GAL) at 1500 RPM. You are getting 12 NM/gal which is almost 3 times better fuel economy than most other boats. I probably need to be educated, but I am not aware of any boat of substantial size that gets over 10NM/gal.

 

[FlyMeAway]

My figures are correct and confirmed at the pump. We routinely do long cruises lasting multiple days with just a few gallons of diesel. 0.3 - 0.4 gal/hr at the low end, 0.5 gal at the high end if you stick to around 6 knots! We've never used the bottom half of the fuel tank. This is a very efficient hull and engine at displacement speeds. Oh, we are also loaded up with two weeks of remote anchorage supplies like full water, extra drinking water, full fuel, two sea kayaks, RIB and outboard, spare anchor and rode, all our own groceries for full self sufficiency, five lead acid batteries, etc.

This is way off the manufacturer spec for this engine, which is interesting -- Volvo says ~7.5 l/hr (~2 gallons/hr) at 1600 RPM, and the fuel curve of a diesel typically flattens below that to idle. Even at no load, in neutral, this engine would use something like an eighth to a quarter of a gallon an hour or thereabouts (higher end if the alternator is running).

Something is weird about these numbers. I wonder if other owners with the D4-260 can corroborate? If so, then the Volvo spec is way off, which is itself interesting, and this engine is substantially more fuel efficient both under low load and at idle than would be expected.

For comparison, Passagemaker reviewed the R-29 with a D4-260 as getting 0.5gph at 1000 RPM and 1.3 GPH at 1500 RPM. The GPH at a given RPM should be comparable for the C28, unless the prop is way out of whack; the only thing that would be different is the speed... Right?

 

[FlyMeAway]

That is awesome but I would never sell it because you have a unicorn. Typical burn rates on the D4 - 260 is 2.5L (.66Gal) at 1000rpm and 8L (2.1GAL) at 1500 RPM. You are getting 12 NM/gal which is almost 3 times better fuel economy than most other boats. I probably need to be educated, but I am not aware of any boat of substantial size that gets over 10NM/gal.

Agreed. If your boat defines the laws of physics and the manufacturer's spec, never sell it!

 

[BB marine]

We are Looping in segments with a D3. D4 is 3.7 L, D3 2.4. 260Hp D4, 220 D3. The D3 being smaller displacement but higher Hp per liter. Both engines would be considered High output small displacement engines. The D4 being a more substantial higher hour longevity if operated with in its perimeters. The D3 and D4 are common rail fully electronic controlled engines. Operating them at low Rpm will not cause harm as long as the engine temperature is at operating range. 170F to 190F. The ECM or ECU will control fuel and monitor temperatures to provide a even controlled fuel burn. Operating your D4 at 1500 to 1800rpm all day will not hurt anything. It will actually provide you high engine hour longevity. All that being said the Turbo used with the D4 has a tendency to foul. I don't think that a continual use day in day out operating at 1500 rpm to 1800 rpm will cause fouling but an occasional throttle up to 3000 or WOT won't hurt a thing. It is not a bad idea to do this periodically just to check that the engine is turning recommended WOT rpm (D4 3500rpm +/-100 rpm) (D3 4000rpm +/-100 rpm) (I like to see +100 rpm to confirm the engine is loaded properly)

Our experience operating our C26 is we are cruising at 2000 to 2200rpm which nets 7 kts. 2.5 gph average 3.2mpg. ( C26 is not nearly as fuel efficient as the C28 ) We have cruised for days at this speed but there will be the occasional its getting dark or the sea's start to pick up and C26 and C28 do not like rough water when traveling slow they roll a lot because of the narrow beam especially in a following sea. In these instances we find running at higher rpm helps. When we are cruising in open water we tend to push the throttle. There is nothing to see so why not? You will find there will be plenty of times that you exercise the throttle so don't worry about cruising at lower RPM's. Enjoy the ride.

We just found our way to Marathon Key yesterday. We cruise slow most of the time. Our recent trip from Charlotte harbor to Goodland Florida (south of Marco) was all at 1800 to 2200 rpm. When we left Goodland to Shark River with 2' to 3' following sea's. Not enjoyable at 7 kts. We kicked up the rpm to 3400 cruising at 12kts and life was good! The following day flat water and we cruised part of the way at 12kts then backed down to 7kts beautiful ride. I'm sure you will find many opportunities to exercise the throttle during your Loop.

 

[pleeson]

There is no magic here. Start to push a bow wave and your fuel burn goes exponential. On our 28 the turning point is 6 knots. I can routinely do that under 1400 rpm at the numbers I supplied. Above that and you need to recognize the fuel burn numbers provided by the manufacturer. Below that and this hull with the D4 - 260 just sips. Maybe I got a winner in my fuel control. I'll have to make sure it never fails... I've always been amazed at how a half knot can often double your fuel consumption at displacement speeds.

 

[mferguson]

My 2018 C28 will do 6 kts @ 1400 RPM and burn 1.3 gph, for 4.6 MPG. It is either in fresh water or on the trailer, so the hull is real clean. I think pleeson might have mixed up his terms, saying hours / gallon, but meant gallons / hour. My 7612 doesn't read hours per gallon, it reads gallons per hour. That would be 2 MPG for him, a bit high for that RPM, but perhaps justified by the load on the boat and maybe some scuzz on the hull.

But if I'm wrong and you're right, I want to buy your engine.

I only have a couple of hundred hours on mine, a good percentage in this 1250 -1400 RPM RPM range, and no issues so far.

FYI, my high cruise sweet spot is 2900 RPM, where I burn 8 gph @ 18 kts, and get 2.3 mpg with a light load.

 

[mferguson]

Oh, I forgot to add, I always keep a time / RPM / fuel burn log based on the GPH indication on the 7612. The fuel quantity indications on the Volvo display always show more fuel remaining than there really is, are a waste of time, and could be dangerous.