Running on 1 engine for fuel savings?

[psneeld]

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

No definite answers I know of....

 

[SoWhat]

No definite answers I know of....

This is what you wrote in last thread on subject:

"My experience...... it depends.

Depends on the boat, the speeds used, the size of the engines, etc.

All these posted half theories are just that.

I have seen somewhat scientific studies and the results show sometimes this and sometimes that. So what I have gathered is till you have your boat, set up the way you want it, and are cruising it the way you want it.....it will be difficult to even guess. Much of the time, my guess is the numbers will be so close as to not matter. In extremes though., like where you are willing to crawl to make it across an ocean on limited fuel....then all bets are off.

The people who say it takes the same energy to drive a boat at a given speed. ...often leave out so many details I have to choke reading them. The most basic idea that conflicts is a hull that is driven at a certain speed at one engine idling, keep the same speed (if you even can) and start the other engine. What makes sense now if these are 2 fairly large engines both burning X fuel at idle? The minute you add power to the single to get to the idle speed of 2 (often the case)...then all sorts of other things come into play that are mostly left out of these one or two paragraph theories. This is assuming that the energy available at idle is enough to overcome all the variables in drive train and drive the boat at the same governed rate across the board. So the measurement really needed is the difference of unloaded idle burn versus, in gear at minimum throttle setting. This combination of factors is probably what throws most studies off. If the engines are small and the speed for a single at idle is ridiculously slow or still adds fuel at the minimum throttle setting....then yes the numbers for 1 or 2 engines will probably be close.

So, some rough rules of thumb come to mind, but they really only apply if they fit your boat, engines and very specific cruising speed......

And for the locked shaft theory.....it to seems to be all over the map. The USCG engineer on an icebreaker I was on..... told me locking the shaft of the bad motor would save a ton of fuel over the next several months. That was what his info was, I believe from both engineering school and the US Navy. But I have also seen research on small vessels where that isnt absolutely true."

Sound familiar? Search tool can provide a lot of information. (or you can keep answering the same question every few months) But you are correct. There is no definitive answer to a question with vague parameters. Personally, I don't understand why someone would pay a premium to buy a twin engine boat and then attempt to run it on one engine so the question seems totally academic.

 

[sunchaser]

"Personally, I don't understand why someone would pay a premium to buy a twin engine boat and then attempt to run it on one engine so the question seems totally academic."

Surprisingly, twins cost little if any more. That has been my experience anyway when pricing out vessels (Nordhavn and KK) for new build or used that can often be made either way.

 

[Insequent]

Surprisingly, twins cost little if any more. That has been my experience anyway when pricing out vessels (Nordhavn and KK) for new build or used that can often be made either way.

I can give you a comment from 2013. At the time I had the old engines out, sitting on the ground in order to replace fuel tanks.

I talked to the yard about re-powering. They costed both options: two new engines, or a larger single. The single was more expensive, and by a not insignificant amount either. In part due to cost of engine itself but also due to need for new shaft log & rudder, prop etc.

 

[rgano]

A friend had a trawler, now lost to Hurricane Michael, with two NA 4-cylinder Detroits and ran all the time on one engine with RPM reduced such that he made the wake of a canoe. He didn't care if he ever got anywhere. Says the fuel savings we enormous from his earlier 2-engine practice. He did not lock the off shaft.

 

[SoWhat]

"Surprisingly, twins cost little if any more. That has been my experience anyway when pricing out vessels (Nordhavn and KK) for new build or used that can often be made either way.

I did a totally unscientific uncontrolled study of GB36.
My WAG indicated a premium of 20% or higher for twins, and there were twice as many on the market.

Had the opportunity to run on one engine to get home with rudder cranked over and free wheeling prop. No Thanks.

 

[greatpapabear]

I believe these are the key issues when running on one engine and the asymmetric thrust issues it causes.

1) The rudder will be offset causing the boat to crab; which will
2) Increase drag and therefore increase fuel burn; and
3) Place strain on the rudder stock; and
4) The slower you go, the more rudder displacement you require, placing more and more strain on the rudder stock as well as potentially overworking the autopilot and associated rudder controls whilst limiting the speed of helm response.

Coupled with this is whether the gearbox can withstand sustained windmilling. Your gearbox operating book will detail this.

Much research has been done into whether a windmilling prop creates less drag than a locked prop. The locked prop loses out. Windmilling creates less drag.

Having tried it on Play d'eau, the only tangible benefit was in decreasing the engine hours whilst increasing the service intervals.

Worth it? In my mind, no.

 

[psneeld]

Any links to those studies on drag?

While they may be higher, they may be lower than other factors happening such as drive train losses, engine inefficiencies....
.
And again, the may be significantly different boat to boat, speeds, conditions, etc to say positively across the board.

It seems some boats running on one can result in savings, others not.

 

[mvweebles]

Years ago, Passagemaker Magazine had an article on a guy who took his GB42 from California to Hawaii. I think he single handed it. To conserve fuel and extend range, he removed one prop and ran on one engine.

But here's the crazy part. He took the nut off the shaft of the remaining prop so that he could easily remove it, which he did mid-ocean, and installed the other prop. He wanted balance the engine hours.

Im sure he had actual statistics on fuel burn and savings. Given the rest of the story, wasn't the part that stood out at the time.

Peter

 

[sunchaser]

For arguments sake let's say the OP's vessel burns 5gph at normal cruise. He cruises 200 hours per year resulting in 1,000 gallons consumed. At US prices= $2.50 the cost is $2,500.

Shutting off one engine and resultant fuel savings, if any, then is part of the debate. If all in factors amount to a 10% fuel savings then I've got $250 in my pocket.

But wait. I've run more hours, gone slower, missed 3 current changes, increased cruising hours while saving $250. Those dollars are not noticeable given our annual expenditures of 100X or more of that amount. Leaving the boat at the dock will still not materially result in big savings. So the biggest savings is to sell the boat.

 

[O C Diver]

A friend had a trawler, now lost to Hurricane Michael, with two NA 4-cylinder Detroits and ran all the time on one engine with RPM reduced such that he made the wake of a canoe. He didn't care if he ever got anywhere. Says the fuel savings we enormous from his earlier 2-engine practice. He did not lock the off shaft.

So his reduced fuel consumption was based on a reduction of speed not running on one engine. My boat burns 2 GPH at 7 knots, 1.2 GPH at 6 knots, and .7 GPH at 5 knots. Savings are huge when you slow down.

Ted

 

[moparharn]

Sunchaser- Winner winner chicken dinner! This is an interesting post, but for most of us Sunchaser's comment is spot on. If you are putting big hours or needing to split hairs for a crossing then it is not spot on. I do like when a complex issue can be summed up in one succinct point. Thank you.

For arguments sake let's say the OP's vessel burns 5gph at normal cruise. He cruises 200 hours per year resulting in 1,000 gallons consumed. At US prices= $2.50 the cost is $2,500.

Shutting off one engine and resultant fuel savings, if any, then is part of the debate. If all in factors amount to a 10% fuel savings then I've got $250 in my pocket.

But wait. I've run more hours, gone slower, missed 3 current changes, increased cruising hours while saving $250. Those dollars are not noticeable given our annual expenditures of 100X or more of that amount. Leaving the boat at the dock will still not materially result in big savings. So the biggest savings is to sell the boat.

 

[catalinajack]

I have had to run on one Lehman 120 twice, DeFever 44. It required having to input 18 degrees of rudder correction. The autopilot was rendered useless. Turning one way was a very sensitive and minimal rudder adjustment. The other way required a significant change. It was a constant battle to keep the boat going straight. No way would I ever choose to run on one engine just to get a bit more fuel mileage.

As to an earlier comment that "most" transmissions will be damaged by freewheeling, perhaps "many" is more accurate. B-W Velvet Drive transmissions can be freewheeling. It is in the manual. Many, but not most, boats use B-Ws.

I did a totally unscientific uncontrolled study of GB36.
My WAG indicated a premium of 20% or higher for twins, and there were twice as many on the market.

Had the opportunity to run on one engine to get home with rudder cranked over and free wheeling prop. No Thanks.

 

[bayview]

On my 44' boat trying to go too fast on one engine resulted in rudders not being able to steer easily. Single engine steering torque was too much for the rudders Slowing to minimum wake speeds allowed very easy steering. It ran well at about 6 kts. I never tried very slow speed.

I spoke to ZF the maker of my IM 301 transmissions and was told that free wheeling was not a problem unless the temperature got too high above 140 F or so. They also suggested alternating engines to for temperature control. My longest trip on one engine was about 150 miles so I think I got decent data.

It was good practice but fuel savings were not significantly different than going the same slower speed on two engines.

 

[Ski in NC]

When discussing this, you need to understand how a diesel engine governor works. You are not controlling a "throttle", the lever is a "speed control" lever. You set it to the rpm you want, and internally (whether mechanical or electronic) it will adjust the fuel rate as needed to keep THAT rpm (with some variation).

So if you are running 1600 on two engines, and shut one down and leave the other at 1600, the load will go up on that one and governor will increase fuel rate. RPM's might drop 50, but fuel rate might go up 25%.

On my Cummins 450C, if I am running hull speed at 950 and simply pull it to neutral, rpm goes up about 50 and engine just sits there at that new rpm. Much less combustion noise, as there is much less fuel being injected.

One way to look at this is that most 6-8liter diesels take about 1/3-1/2gph just to run at lower rpm. Pumping air, pumping fluids, things rubbing on each other through oil film, all that takes energy. At higher rpm (like say 1800, this parasitic loss is more like 1gph.

So if you only need 50hp to go through the water, it makes way more sense to use a single 120hp motor to do it.

BUUUTTTT... That crabbing and prop drag and rudder ineffectiveness can completely offset the 1/2gph saving. Most boats, it is not worth it. It depends on the boat particulars.

 

[ghost]

I’d agree with the overwhelming voices that it is complicated and rarely measured anywhere close to objectively.

I experimented in years past and arrived at the conclusion that while there were savings they were not significant, say maybe 15%ish +- 5% at my 8 knot neighborhood.

I spent the better part of a day unintentionally testing with a transmission lever not fully engaged. I was about a knot and a half slower and my autopilot constantly had my rudder offset by about 6 degrees. That’s a bit of drag, not crazy, but worth noting.

Also spend a season running mostly on one engine as I had developed an undiagnosed slight knock and did not want to run it until diagnosed. So over quite a few more miles than I had experimented with previously I discovered that the fuel savings were nearly a third. That is very significant. Of course that’s complete apples to oranges, because like many others who report distorted results, we also found that on one engine the “sweet” spot of running was naturally slower. We just got used to 6.5 knots or so and had a great time. So most of the savings, while real, came from running slower, not more efficiently. I wouldn’t really want to run that slow on two engines, the beat frequencies produced are just not as happy.

So all this boils down to a few basic principles and a ton of factors that are subjective and seemingly boat specific. Slowing down looks to be the biggest helpful factor, increased drag being one of the biggest detractors. The rest, YMMV.

 

[North Baltic sea]

Hi,

Articel this topic https://www.passagemaker.com/lifestyle/strategies-for-long-range-cruising-with-twin-engines-2

NBs

 

[riverfrontauthor]

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?)

I have a gulfstar 43 mki with twin 135 Perkins. On one screw I burn 4gph at 1500 making about seven kts. On two screws at 1500 I make about 8.5 kts and burn 2.5 gph total.

With the full keel and locked prop (to prevent tranny damage) I burn almost twice the diesel on one screw as I do two.

 

[Packer fan]

Running on 1

I'm planning on running on one engine for this whole summer. That is, except for when my generator is on.

 

[DennisB1]

Single Engine operation on twin diesel

I have run my boat a Grand banks 49 classic with one engine on several occasions. The props are 3 blade 30 inch diameter. First noticeable problem is that you have to crank in about 7 to 10 degrees rudder to maintain a straight course. Typically I run at about 1400 rpm on Cat 3208s which is about 9 knots. Same rpm on a single was about 8 knots. If I had to run for extended period of time for an engine failure it would be desirable to remove the non working prop. There was no noticeable sound change or temperature change on the engines.
This would lessen the amount of required rudder angle and reduce the drag. I did not run dragging one non working prop long enough to obtain fuel burn. With both engines at the 9 knot speed its about 1.5 nm/gal. The only reasonable way (but expensive) would be to do the sailboat approach with feathering propellers on each shaft. Then you could get the fuel savings. I would never in our use of the boat have any financial payback. I was told that the featuring props were in the 10K range each.

 

[Mike Lowthian]

Running on 1 engine for fuel saving

I am no expert but a couple of years ago I dinged one prop on a reef in Cuba. No repair facilities there so I returned to Florida (90 miles) on one engine. On arrival the transmission of the engine that I was not running was damaged - needed a rebuild. Cause? Not sure but I suspect running without lubrication?

 

[lelievre12]

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...

What you are missing is that your engine will increase fuel flow to compensate for the dead engine. Just because you dont move the throttle doesn't mean your governor is not putting in bagfuls of fuel to maintain RPM as the load it sees increases.

The reason two engines are often as good as one (sorry Nordhavn) is that the BSFC (specific fuel efficiency) usually decreases for engines as power levels go up. So your one engine running at a higher BSFC wont be much different to two engines running at optimum BSFC. Particularly when that one engine is now trying to maintain the same prop RPM with a much higher disc loading most likely now with cavitation and a draggy rudder half over to compensate for the asymmetric thrust.

So sometimes old folks get cranky when new owners ask questions that don't really understand even the basics of the engines they are 'operating'. The inference that one engine might use 'half the fuel' of two is not a great question for any captain for so many reasons.

 

[Trawlerman]

Moving a boat through water requires energy, and moving a boat through water at a given speed requires a given amount of energy. We like to call energy "horsepower" for antiquated reasons, but regardless of what you call it, it is energy. There is a rough rule of thumb that every gallon-per-hour you burn is equivalent to producing 20 horsepower constant during the hour. My 50' Marine Trader Walk-around burns 5 gallons per hour at 9 mph, so despite having two ostensibly 160 HP rated engines, cruising at this speed, I am using a total of 100 HP and thus only 50 HP from each engine, or said another way out of the total available of 320 HP. That is the power my boat needs to overcome the resistance the hull presents when you push it at 9 mph. If I shut down one engine, and want to still go that speed, I will still need a total of 100 HP, and while one of my engines can easily do that, it is not efficient since my now dragging propeller adds more drag, and the prop itself is moving into a region of more "slip" than normal (i.e. losing efficiency) and as well the counter-rudder needed adds more drag. I will say that there is one time that I have used the single engine cruise. Doing the Great Loop, in some of the Trent-Severn canal route in Canada, I had to go at only about 5 MPH for long periods, and it could barely bring my engines up to temperature, so I shut one down, and I ran single just so as to not be idling both engines for a long period. My transmissions allow free-wheeling to not exceed 700 RPM and that was easily obeyed.

 

[tiltrider1]

Lelievre12 is correct. It is really annoying every time this comes up. You get some one who says, “Hey, I turned off 1 of my engines and my boat only slowed down .75 knots, look at all the fuel I’m saving”

First, they never actually measured fuel usage. Second they are ignorant to the fact that the governor just increased fuel flow to maintain RPM. Third they are ignorant or discount the fact that the lower speed on its own will increase fuel savings. Fourth the concept of rudder drag and prop drag are never considered.

Finally you end up in an apples and oranges discussion because they are convinced that they are saving fuel but they have no means of even measuring fuel usage but they state their estimated fuel savings as a fact. Never mind Physics, never mind that it takes a given amount of HP to move the boat, never mind that a gallon of diesel can only produce approximately 20 hp. Nope, none of this matters because when I shut down one engine my boat only slowed a little and I know that I am now using half the fuel. Of course, they really don’t even know if both engines are using the same amount of fuel. It has never dawned on them that it’s possible that one engine could be doing 65% of the work when both engines are in use. How do they know both engines are turning the same RPM,s? Have they ever calibrated their tachometers, have they ever had their props mapped out?

There is so much more to this conversation but what’s the point, we never get past the fact that there is no accurate fuel consumption measurement being used.

 

[bayview]

I have a gulfstar 43 mki with twin 135 Perkins. On one screw I burn 4gph at 1500 making about seven kts. On two screws at 1500 I make about 8.5 kts and burn 2.5 gph total.

With the full keel and locked prop (to prevent tranny damage) I burn almost twice the diesel on one screw as I do two.

Try running both at the less than 7 knots speed. Any comparison at different speeds is useless.

 

[rolomart3]

You got excellent answers to your question. But what we are really dealing with in here is with an obtuse design of twin engine boats. To come up with a twin engine single transmision single propeller mechanical design is not rocket science. Such an arrangement would have less components and would provide the increased reliability of two engines without drastically doubling the propeller, seal/bearing and transmission ownership and maintenace costs. But you see, usually boat manufacturers were not concerned with maintenance costs of their boats. There is even anecdotal information about a boat builder that did not provide an access panel to the fresh water pump. And we all know how "easy" it is to service diesel tanks in most trawlers. It is a crying shame for the huge amount of money they get paid for their lousy designs.

 

[FF]

"To come up with a twin engine single transmision single propeller mechanical design is not rocket science. Such an arrangement would have less components and would provide the increased reliability of two engines without drastically doubling the propeller, seal/bearing and transmission ownership and maintenace costs."

These exist and are OTS since WWII. Used on landing craft and other applications.

Another plus for the system is the shaft is oversize to carry the HP of both engines and there is no problem with 2 different sized engines.

A modest unit for economy . or start up the defuler to make big waves.

 

[rslifkin]

If you could make it efficient enough to nearly match a mechanical drive, a diesel electric setup with twin screws and 2 or 3 gensets would be a very attractive option. Just bring more genset power online when you want to go faster. Might still be a little less efficient at high speeds, but at low speeds you could almost certainly do better than anything other than a setup with engines optimized for that specific slower speed (which wouldn't have the ability to go much faster).



For those talking about extra maintenance costs for shafts, props, etc. on a twin, don't forget this: a single with the same total horsepower will have much bigger shafts, props, etc. so the overall maintenance costs will be a lot closer than you'd think, just with less individual parts involved.

 

[koliver]

If you could make it efficient enough to nearly match a mechanical drive, a diesel electric setup with twin screws and 2 or 3 gensets would be a very attractive option. Just bring more genset power online when you want to go faster. Might still be a little less efficient at high speeds, but at low speeds you could almost certainly do better than anything other than a setup with engines optimized for that specific slower speed (which wouldn't have the ability to go much faster).



For those talking about extra maintenance costs for shafts, props, etc. on a twin, don't forget this: a single with the same total horsepower will have much bigger shafts, props, etc. so the overall maintenance costs will be a lot closer than you'd think, just with less individual parts involved.

Comparing maintenance cost of a single 400hp Cummings ISL v a pair of 200 hp Volvo TAMD41, I have the former in a Motorhome and the latter in my boat. Both units weigh 44,000#.
I bought my oil at the same place, so /l cost is the same.
Each 200hp takes 10l, the 400 takes 20l
Each oil filter for the 200s cost less than 1/2 the cost of the single, larger oil filter for the 400. Both were bought at the same time, from the same place.

Fuel filters do the same math.

 

[Mako]

On arrival the transmission of the engine that I was not running was damaged - needed a rebuild. Cause? Not sure but I suspect running without lubrication?

Did you lock your marine gear or let it freewheel - probably culprit!