Running on 1 engine for fuel savings?

[ERTF]

I have 44ft Marine Trader with twin Cummins 210hp. The other day I had a warning light for the port engine, so i turned it off and continued on with just the starboard. I was surprised that I felt no difference and didnt even need to increase rpms to maintain the same 7ish knt speed.

Does anybody here with twins ever intentionally just run on one engine for fuel economy? Are there any mechanical risks from long term use (due to the uncentered nature of forces?)

 

[DavidM]

Usually the fuel savings is minimal running on one engine. Should be better if you remove the dead side's prop .

Also some transmissions can be damaged by the dead side's prop freewheeling.

David

 

[ERTF]

Usually the fuel savings is minimal running on one engine.

David

It sure seems like it'd be nearly a 50% reduction based on my originally posted anecdote.

Isn't it a fact that it only requires a fraction of my 420hp to push the boat at displacement speeds? And after displacement speeds, I'm basically just burning alot more fuel to create a big wake, yet hardly go much faster?

 

[jungpeter]

Hi ERTF,

As DavidM suggests, fuel savings resulting from running on one engine is minimal. In addition, running on one engine can severely damage some free-wheeling transmissions, and (should your boat be so equipped) damage some dripless shaft packing designs.

Given that the cost fraction of boat ownership attributable to fuel cost is very low (typically <<10%), why bother?

You want to save money on fuel? Slow down.

Regards,

Pete

 

[oscar]

It sure seems like it'd be nearly a 50% reduction based on my originally posted anecdote.

You can save 100% by shutting them both down.

Sorry, the laws of physics apply, every day. It takes X-gallons of diesel to move a boat through the water at a given speed. There's a LOT of variables in design, speed, weight etc etc etc that determine X, but that's irrelevant here. It either comes out of one or two engines making Y horsepower and you have to put enough diesel in there to make that happen.

And yes, you better make darn sure your trannies are allowed to turn without cooling and/or lubrication or this could be an expensive experiment.

 

[ERTF]

You can save 100% by shutting them both down.

What's the deal with all the grumpy old man snark??

I asked a simple question.

And it is a fact that most trawlers are overpowered for displacement speeds. Not saying it isn't nice to have the power & the twins for certain situations...but it's not even close to NEEDED for slow speed icw cruising....your calculations will tell you that.

Now whether it's bad for the transmissions (I have borg-warner) is out of my range of knowledge. But that's why I specifically asked about any mechanical risks in the OP...

 

[C lectric]

Fuel saving will be present but not a great deal.

Reason, unless the boat speed is less it takes the SAME horsepower to drive the hull at that speed.

What you may miss is the injection pump will simply add more fuel to the remaining running engine to keep up the rpms at the same level. That works maybe unless the running engine is driven into overload condition which may do some damage if done for long enough or it is bad enough.

Injection pump governors react to your rpm requirements and if the load increases so will the injection pump increase the fueling.

As mentioned what you gain in economy may be lost when you have to pull and rebuild a gearbox. Some gears can be run indefinitely , some indefinitely at less than certain revs, some for a defined number of hours, some may require the stopped engine to be started for a few minutes every so often to distribute the oil, some must be secured so the shaft cannot rotate.

Unless you KNOW what your gearbox requires you could be asking for a gearbox failure.

 

[rslifkin]

With gassers, the fuel savings can be significant due to how badly efficiency drops at light load. With diesels, the difference is typically small, as dragging the dead prop along typically kills most of the gains from loading the other engine a bit more.

 

[psneeld]

You can save 100% by shutting them both down.

Sorry, the laws of physics apply, every day. It takes X-gallons of diesel to move a boat through the water at a given speed. There's a LOT of variables in design, speed, weight etc etc etc that determine X, but that's irrelevant here. It either comes out of one or two engines making Y horsepower and you have to put enough diesel in there to make that happen.

And yes, you better make darn sure your trannies are allowed to turn without cooling and/or lubrication or this could be an expensive experiment.

There's more to it than just hp driving the boat.

Engine efficiencies and drive train losses affect this question too.

Often they are not all that significant, but they can be.

Gaining from shutting down an engine can be very boat and operation dependent.

 

[fgarriso]

Some boats with wide set props will crab walk when running on one engine at 6-7 knots. With that you will burn additional fuel staying on course.

 

[oscar]

What's the deal with all the grumpy old man snark??

I asked a simple question.

Nope, you stated that there would be a 50% fuel savings. Couldn't let the perpetuum mobile track get out of hand. And it was more a funny than a snark. But, limited communication efficiency of the typed word and all that.

 

[psneeld]

Some boats with wide set props will crab walk when running on one engine at 6-7 knots. With that you will burn additional fuel staying on course.

One rule about savings on one engine is that yes you slow down enough things like crab diminish enough to be a lesser factor.

 

[DavidM]

Here is the only objective tests I am aware of while running on two or one engines, prop freewheeling or locked. Be very careful when reading the data. It is somewhat confusing.

Dreamer

David

 

[rslifkin]

Here is the only objective tests I am aware of while running on two or one engines, prop freewheeling or locked. Be very careful when reading the data. It is somewhat confusing.

Dreamer

David

Based on their data, it looks like for that boat, running on one engine burns more fuel in many cases, not less. For example, 6.2 kts through water on both engines burned 2.9 gph total, but on port engine only, it takes 3.1 gph to make 6.2 kts. Interestingly, stbd only shows 2.5 gph at 6.1 kts, but 3 gph at 6.4 kts. That makes me slightly question the data, as one engine vs the other shouldn't show a difference that large. That, or one of their engines isn't right. I would expect a slight difference, but not one this large.

Looking at a few other speeds, 5.7 kts on both engines was 2.4 gph total, 5.7 kts on port only was 2.6 gph total, 5.6 kts on stbd only was 2.2 gph total. At 7.7 kts, both engines was 5 gph total, port only was 5.5 gph, stbd only at 7.6 kts was 5.1 gph.


The claimed nmpg gains and range increase on their charts are bogus, as they're comparing based on RPM, not speed. So they're going slower with 1 engine shut down, which of course helps mpg. But slowing down with both running appears to help them as much or more.

 

[koliver]

I have 44ft Marine Trader with twin Cummins 210hp. The other day I had a warning light for the port engine, so i turned it off and continued on with just the starboard. I was surprised that I felt no difference and didnt even need to increase rpms to maintain the same 7ish knt speed.

Does anybody here with twins ever intentionally just run on one engine for fuel economy? Are there any mechanical risks from long term use (due to the uncentered nature of forces?)

My C&L 44 is on the same hull as your MT 44. I also have twins, Volvo TAMD41s rated at 200hp each.
I had a serious failure at the beginning of my summer cruising a few years ago. My mechanic suggested bringing it in when he was less busy, like nearer to the end of the cruising season. I was OK with that, so ran the rest of the season, about 150 engine hours, on the single. I ran at the same rpm as I would have with both engines running. My speed dropped from 8knots to ~6.5. My fuel consumption remained where it had been for that engine, so I saved 50% over the summer. I learned how to dock an off centered single, in cross winds and tricky currents. Making prop walk my friend was key to that.
I know this doesn't answer your question, but I think pushing from the corner, with corrective rudder, has to use lots more fuel than pushing from the back of the keel.
I have BW Velvet Drive trannies, so freewheeling is no problem. I did put a pipe wrench on the shaft of the dead engine for a while, but I know I needn't have.
I also have a high output alternator on only one engine, so having that engine down meant paying closer attention to battery State of Charge.

I don't think you actually made the same speed without any extra push on the operating engine. For that to be the case, your non-operating engine would have been making no contribution at all to your forward speed. Physics says that wouldn't have been happening.

 

[fgarriso]

PLEASE remember some gear boxes will not stand being driven from the output shaft, such as free wheeling for an extended period. The hydraulic pumps are driven off the input shaft.

 

[sunchaser]

I was surprised that I felt no difference and didnt even need to increase rpms to maintain the same 7ish knt speed.

But, with one engine you likely were burning more fuel in that engine even if at same RPM. Without accurate fuel measurement for each engine it is an impossible question to answer. The old axiom applies, props move boats.

Some years ago we had an engine outage for about 40 hours. The operating engine, even at an intentional lower RPM was really working as evidenced by sound and increased temperature while consuming more fuel "per engine" as measured by calibrated tank draw down.

For those that have electronic engines it would be an easy test with mapped fuel burn available. Any volunteers amongst us?

 

[SteveK]

I have 44ft Marine Trader with twin Cummins 210hp. The other day I had a warning light for the port engine, so i turned it off and continued on with just the starboard. I was surprised that I felt no difference and didnt even need to increase rpms to maintain the same 7ish knt speed.

Does anybody here with twins ever intentionally just run on one engine for fuel economy? Are there any mechanical risks from long term use (due to the uncentered nature of forces?)

After reading all posts I quote your original as no one agreed with you. If I take your post literaĺy then you are either overpowered or most likely operating beyond necessary rpm.
Many posts have said 1800 rpm is ideal. For me ideal rpm is when increased rpm no longer gains proportional speed. Efficient burn is slightly less.
So when you state no change to speed when switching to single to me means the twins were wasting fuel. No wonder 50% savings may be real.
I know I am overpropped. Twins at 700 reach 6 knots. At 1650 8- 8.5 or hull speed with throttle left to reach 12.
So if you burn 50% less on one engine you were wasting fuel with two.

 

[DavidM]

Based on their data, it looks like for that boat, running on one engine burns more fuel in many cases, not less. For example, 6.2 kts through water on both engines burned 2.9 gph total, but on port engine only, it takes 3.1 gph to make 6.2 kts. Interestingly, stbd only shows 2.5 gph at 6.1 kts, but 3 gph at 6.4 kts. That makes me slightly question the data, as one engine vs the other shouldn't show a difference that large. That, or one of their engines isn't right. I would expect a slight difference, but not one this large.

Looking at a few other speeds, 5.7 kts on both engines was 2.4 gph total, 5.7 kts on port only was 2.6 gph total, 5.6 kts on stbd only was 2.2 gph total. At 7.7 kts, both engines was 5 gph total, port only was 5.5 gph, stbd only at 7.6 kts was 5.1 gph.


The claimed nmpg gains and range increase on their charts are bogus, as they're comparing based on RPM, not speed. So they're going slower with 1 engine shut down, which of course helps mpg. But slowing down with both running appears to help them as much or more.

Your observations are correct and you saw your way through the confusing way that the data was presented. Congratulations, others have gotten trapped by that data. Surprising that Bob ? (can't remember his last name) presented the data in such a confusing way.

David

 

[Insequent]

But, with one engine you likely were burning more fuel in that engine even if at same RPM. Without accurate fuel measurement for each engine it is an impossible question to answer. The old axiom applies, props move boats.

Some years ago we had an engine outage for about 40 hours. The operating engine, even at an intentional lower RPM was really working as evidenced by sound and increased temperature while consuming more fuel "per engine" as measured by calibrated tank draw down.

For those that have electronic engines it would be an easy test with mapped fuel burn available. Any volunteers amongst us?

Ok, I'll bite. I've posted this graph before as this is something of an old chestnut that re-appears fairly frequently. It has Dreamer's data, from Bob Lowe, and I believe I read it right! For my own data, I chose conditions where there was minimal wind & current, but in any case ran E, W, N & S and averaged the 4 runs at each data point to remove external influences as much as possible. GPS speed, and electronic engines with digital tach and fuel readings.

With diesel's, running on one engine you can leave the rpm the same, but you will go a bit slower and you will use a bit more fuel in that engine at that rpm. Overall, for me it was the 7° of rudder and free-wheeling prop the negated any gains.

So, slightly higher fuel consumption (at the same speed!) on just engine. I suspect my (and Dreamer's) results are typical for semi-displacement trawler hulls. Different boats, and in particular different hull shapes, may yield a different outcome. If you have such a boat, and the capacity to get reasonably accurate data, do some tests and post the results!

 

[sunchaser]

Thanks Brian. The rudder offset seems indeed a factor. Much better data than I was able to get, but largely corroborative of our unplanned escapade..

I'm awaiting available data on a vessel that recently did a Seattle to Los Angeles trip. At the 400 mile stage they lost an engine and finished the next 800 on one engine.

 

[oscar]

One rule about savings on one engine is that yes you slow down enough things like crab diminish enough to be a lesser factor.

You have some engineering data to back that theory up? As speed decreases the rudder inputs will need to increase, thus increasing drag. Crab will INCREASE with lower speeds. (34 years of flying large airplanes dealing with rudder and asymmetrical thrust.......)

And then you lose control. My Hatt does 5 knots at idle on two engines. It makes a small wake, but a wake nonetheless. In very tight passages through areas with lots of moorings/docks I pull one in neutral so as to slow down and create absolute minimum wake. I need to do at least 3.5 knots or she starts a turn whether I like it or not, at that point I need to put the other one back in gear. Crab is at a maximum at that point.

 

[psneeld]

You have some engineering data to back that theory up? As speed decreases the rudder inputs will need to increase, thus increasing drag. Crab will INCREASE with lower speeds.

And then you lose control. My Hatt does 5 knots at idle on two engines. It makes a small wake, but a wake nonetheless. In very tight passages through areas with lots of moorings/docks I pull one in neutral so as to slow down and create absolute minimum wake. I need to do at least 3.5 knots or she starts a turn whether I like it or not. Crab is at a maximum at that point.

If the crab is because differential thrust, reducing power may require less force to stay straight, drag is complicated with many variables. Loss of rudder control may happen at some point and a bit more speed necessary, but not so much that it overcomes other variables.

Exactly how much? No one knows as a generalization, thats why I posted before about how it is very boat and operations dependant.

Some twin boats can be more efficient on one, some may not.

The key is the right speed, not trying to maintain the most efficient 2 engine speed.

The best answer is run your own tests on your own boat, all other data is useless or nearly so.

 

[Insequent]

snip

I'm awaiting available data on a vessel that recently did a Seattle to Los Angeles trip. At the 400 mile stage they lost an engine and finished the next 800 on one engine.

Tom, by all means post the info when you get it. But it won't be all that useful IMO. There will have been different sea states, wind strength & direction, water currents, and decreasing fuel load. For good data all of those variables need to be controlled and minimised.

 

[Lepke]

I suspect you were in a favorable current. It doesn't make sense that stopping one engine doesn't change the speed. My boat at 2.5k below hull speed, freewheeling one prop, looses over half of its speed. About 200hp twins.
Most transmissions made in the last 50 years can be damaged by freewheeling. Most are hydraulic and need the input shaft turning so oil circulates. The input shaft runs the pump. If the transmission has an oil cooler, oil circulation is necessary. And the engine needs to be running for raw water to cool the oil cooler. Most mechanical transmissions can freewheel, but need the temperature checked to be safe. Damage is caused by heat and lack of oil in bearings that aren't submerged in the oil. I've seen people melt their seals from the heat.

 

[ERTF]

After reading all posts I quote your original as no one agreed with you. If I take your post literaĺy then you are either overpowered or most likely operating beyond necessary rpm.
Many posts have said 1800 rpm is ideal. For me ideal rpm is when increased rpm no longer gains proportional speed. Efficient burn is slightly less.
So when you state no change to speed when switching to single to me means the twins were wasting fuel. No wonder 50% savings may be real.
I know I am overpropped. Twins at 700 reach 6 knots. At 1650 8- 8.5 or hull speed with throttle left to reach 12.
So if you burn 50% less on one engine you were wasting fuel with two.

Yeah I was approximating when I suggested 50% less fuel (simply based on 1 less engine). I don't remember the exact rpms & exact speed. But I surprisingly took note at the time because I was still cruising at about the same speed & at the same rpms with 1 engine, as I was just cruising with 2. It was only for about 6 miles, but thats long enough to make me curious because it was plain as day and the boat didn't feel any different. So would it be closer to 35-40%...sure, but thats significant savings? If the conditions allow it.

I'm certain I've read somewhere about some trawler cruisers running on 1 engine for economy. And I know some sport boats troll on 1. Just was curious if anybody here did it, since I was surprised at the outcome after I had to go to 1 temporarily.

 

[ERTF]

My C&L 44 is on the same hull as your MT 44. I also have twins, Volvo TAMD41s rated at 200hp each.
I had a serious failure at the beginning of my summer cruising a few years ago. My mechanic suggested bringing it in when he was less busy, like nearer to the end of the cruising season. I was OK with that, so ran the rest of the season, about 150 engine hours, on the single. I ran at the same rpm as I would have with both engines running. My speed dropped from 8knots to ~6.5. My fuel consumption remained where it had been for that engine, so I saved 50% over the summer. I learned how to dock an off centered single, in cross winds and tricky currents. Making prop walk my friend was key to that.
I know this doesn't answer your question, but I think pushing from the corner, with corrective rudder, has to use lots more fuel than pushing from the back of the keel.
I have BW Velvet Drive trannies, so freewheeling is no problem. I did put a pipe wrench on the shaft of the dead engine for a while, but I know I needn't have.
I also have a high output alternator on only one engine, so having that engine down meant paying closer attention to battery State of Charge.

I don't think you actually made the same speed without any extra push on the operating engine. For that to be the case, your non-operating engine would have been making no contribution at all to your forward speed. Physics says that wouldn't have been happening.

Yeah I wasn't being literal when i said 50% less. But my point was the savings definitely seemed to be closer to 50% than 0% in the situation I experienced (which surprised me).

I agree I had a favorable current, and typically operate at an easy cruise speed, so those were factors. However, your personal experience seems to support mine?

The other thing it made me wonder was if my starboard was just doing most the work the whole time (before I put the port in neutral).

 

[ERTF]

Just ran this topic past my professional captain / surveyor friend. He lives on a Hatteras thats about 70ft. He says his boat does 6.5 knts at same rpms, whether 1 or 2 screws. Past 6.5 knts is where it starts to differentiate for him.

 

[rslifkin]

Just ran this topic past my professional captain / surveyor friend. He lives on a Hatteras thats about 70ft. He says his boat does 6.5 knts at same rpms, whether 1 or 2 screws. Past 6.5 knts is where it starts to differentiate for him.

At lower speeds prop slip is probably about the same with 1 or 2 running, so the one just powers up a little, holds rpm and you maintain speed. At higher speeds where more power is needed, prop slip increases with one shut down, so some speed is lost at a given rpm.

 

[SoWhat]

What's the deal with all the grumpy old man snark??

I asked a simple question.

You could have done a simple search. Your question has been asked and answered before. Free wheeling a prop has also been addressed.