Cruising On One Engine

[catalinajack]

My Lehman’s aren’t setting an rpm, so shutting one down and running the other at the the same RPM and allowing the boat to run 1kt slower will save fuel. My last trip I believe I averaged about 4.0 gph at 8.5 kts with the gen running all day and 1 night (plugged in the other two nights). This was a 120 mile trip. We actually averaged 7.3 kts including no wake zones and the locks. That’s 2.1 mpg. If I shut one down, maybe I’d be burning 2.5 gph at 7.5 kts. That’s 3 mpg. Even if I’m burning 3 gph that’s still 2.5 mpg.

Okay, so let's concede that you are saving fuel running on one engine at 1 MPH less in speed. What portion of that savings is due to having slowed down? You can't answer that question. I think most would agree slower speed equals lower fuel consumption. The only way for you to truly know whether running on one engine yields significant savings would be to make a long run on one engine and then another run on both, both at the same speed. Ĺet us know your results.

I am not saying saying that you, on your boat, are not saving fuel. I think you are, just not nearly as much as you think. Here is something for all to consider. The US Navy runs its destroyers on one shaft to save fuel.

 

[Insequent]

To the OP, your fuel saving will be significant by slowing down 1 mph. That is true whether running one or both engines on a twin engine boat. Reducing speed does save a lot of fuel!

Attached is a chart I've posted a couple of times before. It shows my boat running on one or two engines and also "Dreamer" on one or two engines.

Dreamer was out of Oak Harbour, WA. Here is the link to their test results.
Oak Harbor Boat Works

My engines are electronic JD 6068's with fuel flow gauges.

In both cases running on one engine uses slightly more fuel than running both engines. I had freewheeling prop on the engine that was stopped. I needed 7° of rudder angle to go in a straight line, contributing to increased drag.

I would only run on one engine if I wanted to go really slow and needed to keep enough load on the engine to reach a reasonable operating temp.

Now, some boats might get better fuel economy running on just one engine instead of two. You need to do some sea trials to determine if it is the case. Even if it is true, I suspect the fuel saving will be very little if you compare apples with apples - ie run at the same speed during the test.

 

[yamabuki]

Cruising on one engine.

I have used one engine for some long slow trips but not to save fuel but a regular hourly use of switching engines can help with oil changes etc as the engine hours are less.

 

[83GB]

The BW manual says it is ok to free wheel.. page 17 in my manual..

 

[waltini]

Shaft Brake

Have you considered a shaft brake? I work at a ferry company and some vessels have a shaft brake to avoid the free wheeling of the prop not being used.

Not that we are using one engine to save fuel it is a get home feature in the event of a breakdown. And I’m not sure if it’s to save the gearbox or the drip less shaft seal.

 

[IWAB]

lock shaft or no?

I am a fairly new owner of an older GB Europa and have two Ford Lehman 120s. I am not experienced enough to want to try to run on only one engine, but have wondered what should be done if dirty fuel or some other problem required shutting down one or both engines and/or necessitating a tow.

Do I need to lock the shafts, or can I let the transmissions free-wheel until I'm home safe, or at the yard for engine work? My trannies are Borg Warner velvet drives, starboard being reversing and the port not.

I gather from the previous posts that Borg Warner's can be free-wheeled, but preferably at slow speeds (5-6 knots)? I also have Naids and assume I will need to lock them in their neutral position unless the starboard engine is allowed to run which has the hydraulic pump for the stabilizers. Any comment would be appreciated.

 

[boulderdave]

Also keep in mind if you have a crossover on your shaft seal, you risk flooding the engine that isn't running if you don't close the valve.

 

[IWAB]

Hmmm, I think one engine supplies coolant to NAID and the other cooling water to the shaft seals. Might be time to consider adding valves.

 

[ProMaritime]

Why would you want to endure that extra drag from a free-wheeling prop? Aside from the that and the extra noise it is a fact that some transmissions are damaged by this practice. Drag is increased considerably over a locked shaft. Put you transmission in reverse to stop the shaft and the prop when operating on one engine.

 

[Insequent]

Why would you want to endure that extra drag from a free-wheeling prop? Aside from the that and the extra noise it is a fact that some transmissions are damaged by this practice. Drag is increased considerably over a locked shaft. Put you transmission in reverse to stop the shaft and the prop when operating on one engine.

Part in red above is not correct.
The speed trials in the table in the link below show a dogged (locked shaft) versus freewheeling. The freewheeling situation has less drag.

Dreamer

 

[Wxx3]

It’s a subtle difference in fuel. Very little if any rpm drop and a loss of 1kt. That engine doesn’t see a difference in load. But i could be wrong....happened once before. Anyone have a tested this with boost gauges or actual fuel flow gauges? I’d be interested to see.

There is no such thing as a free lunch.
When you turn off one engine, the running engine must increase it's work and therefore fuel consumption even though the rpms are the same.

In a simple way to look at it. One of the main differences between a gasoline engine, where power is produced by the throttle setting and a diesel, where power is determined by the load.

I have not read past the first page of comments. So I am sure someone already pointed this out. But I would expect your fuel consumption to remain the same or even go up a bit compared to both engines at the slower speed.

 

[catalinajack]

Hmmm, I think one engine supplies coolant to NAID and the other cooling water to the shaft seals. Might be time to consider adding valves.

Not on my boat. Each of my Lehman 120s supplies cooling water to the shaft seals independently, no crossover line. And, cooling the stabilzer oil when running on one engine is not a concern. One pump on one engine that, if shut down, well, the stabilizer is not operating and is thus not in need of cooling. The fins may be drooping and not centered but I don't see any harm there either.

 

[catalinajack]

Why would you want to endure that extra drag from a free-wheeling prop? Aside from the that and the extra noise it is a fact that some transmissions are damaged by this practice. Drag is increased considerably over a locked shaft. Put you transmission in reverse to stop the shaft and the prop when operating on one engine.

Such a defintive statement. Depends on the boat. See my post earlier in this thread. Briefly, with a shaft not turning, my DeFever 44 requires 20 degrees of rudder correction to maintain a straight course. Free-whelling it requires only five degrees. Seems to me that indicates a LOT more drag with a locked shaft, at least on this particular boat borne out by the fact that speed through the water at an identical RPM is faster with the shaft free-wheeling.

 

[Simi 60]

I have ran several twin engine catamarans on passage.
Mostly been one on one off timed with watch change
After actually speaking with gearbox manufacturer, it was always more of a concern with shaft bearings and possible lack of water lubrication, not gearbox freewheeling issue.
These were ZF boxes.

 

[TDunn]

It takes a certain amount of power to push the hull through the water at a given speed. It doesn't matter where the power comes from (single, twin, triple, sails, diesel, gas, electric, kids hanging onto the swim platform and kicking). With twins it takes added power when running on one engine to compensate for additional rudder drag and the unused prop (freewheeling or fixed). The only other factor is the specific fuel consumption of the engine used to maintain a given speed compared to the specific fuel consumption when both engines are used to maintain speed. Pretty much all diesels have a minimum in their specific fuel consumption versus engine rpms curve (specific fuel consumption is fuel used per hp-hr). So if when running on both engines you are running below the rpm minimum, switching to one engine may cause that engine to consume less fuel to produce the required hp resulting in a small decrease in fuel consumption - IF the decrease in specific fuel consumption is sufficient to make up for the higher power required due to rudder and prop drag . On the other hand, if you are running on both engines at the specific fuel consumption minimum, switching to one engine will require more rpms (i.e., higher specific fuel consumption) and, consequently, more fuel to produce the required power. In that case the fuel consumed to maintain speed will be higher.


In other words. Will you save fuel by running on one engine instead of two? The answer is IT DEPENDS. However, you are more likely to save fuel if running at slow speeds (below the minimum if the specific fuel consumption versus engine rpms curve).

 

[Oldsarge]

Some transmissions are fine freewheeling some are not. Do you have a manual on your transmission? If not try to get one and see what the manufacturer says. I am not sure if yours are good to freewheel or not.

In some of the documentation I got when I bought my sailboat which has a
Perkins diesel and Hurth transmission they recommended that I let the prop
free wheel while sailing, or to put the the transmission in reverse to lock the prop. With the engine off, never lock the prop by putting the transmission in forward.
They seem to think that locking the prop had minimal effect on the sailing of the boat, so stopping its turning while only sailing was OK. It's a cruising sailboat and would be rarely raced, so locking the prop would cause little loss
in speed.

 

[Ski in NC]

Mechanical sailboat gears are different. They have a cam lock system for the clutches so if you leave it in fwd while sailing, the clutches will be partially engaged and will slip. Torque direction is backward sailing compared to motoring. Put in reverse and it will lock.

None of this applies to hydraulic clutch trannies like most trawlers use.

 

[Oldsarge]

Mechanical sailboat gears are different. They have a cam lock system for the clutches so if you leave it in fwd while sailing, the clutches will be partially engaged and will slip. Torque direction is backward sailing compared to motoring. Put in reverse and it will lock.

None of this applies to hydraulic clutch trannies like most trawlers use.

Thanks for the info. I didn't know that.

 

[sunchaser]

To the OP, your fuel saving will be significant by slowing down 1 mph. That is true whether running one or both engines on a twin engine boat. Reducing speed does save a lot of fuel!

Attached is a chart I've posted a couple of times before. It shows my boat running on one or two engines and also "Dreamer" on one or two engines.

Dreamer was out of Oak Harbour, WA. Here is the link to their test results.
Oak Harbor Boat Works

My engines are electronic JD 6068's with fuel flow gauges.

In both cases running on one engine uses slightly more fuel than running both engines. I had freewheeling prop on the engine that was stopped. I needed 7° of rudder angle to go in a straight line, contributing to increased drag.

I would only run on one engine if I wanted to go really slow and needed to keep enough load on the engine to reach a reasonable operating temp.

Now, some boats might get better fuel economy running on just one engine instead of two. You need to do some sea trials to determine if it is the case. Even if it is true, I suspect the fuel saving will be very little if you compare apples with apples - ie run at the same speed during the test.

We have a winner. Two vessels and modern diesels showing data.

 

[CapnCrunch]

It’s a subtle difference in fuel. Very little if any rpm drop and a loss of 1kt. That engine doesn’t see a difference in load. But i could be wrong....happened once before. Anyone have a tested this with boost gauges or actual fuel flow gauges? I’d be interested to see.

I've actually tested the difference 3 times - twice on gas engines and once on diesels (two were my boats and one was a friends). All three times the results were almost exactly identical. Running two engines at 1500 RPM gets you an additional 2mph at twice the fuel consumption. I have also run many river miles on one engine in different boats with no damage to Velvet Drive trannys.

 

[Steve91T]

I think what some people are forgetting is these boats are way overpowered at displacement speeds. So running on one engine doesn’t meant that engine is working twice as hard.

 

[tiltrider1]

If you want to move a boat at a given speed it takes a given amount of HP. Weather you have one engine or two makes no difference. If you were making the given speed with two engines and you turn off one engine, the other engine will have to approximately double it’s HP to maintain the given speed. I say approximately because there will be some additional drag from the non powered prop and the offset rudders. Now a certain amount of HP is absorbed by an engine just to make revolutions. The closer you are to idle the bigger a % of available HP is consumed by friction from all the moving parts. If your given speed is made by two engines at idle then shutting one down might require the other engine to increase its HP by less than double.

 

[psneeld]

Exactly....this forum discussion really never gets specific enough to "prove" anything.


Ask a question with enough specific parameters included and an answer is more viable...yet still no good for so many other boats under so many different conditions/op parameters.

 

[long-cours.62]

But the engine

I disagree. When I’m at 1650 and I pull one engine back to idle and shut it down. Don’t have to advance the throttle to maintain rpm. It’s not using any more fuel. Now this is true if I want to maintain 8.5 kts single engine. Then I’d have to push that engine up.

I believe if one runs the same rpm on one engine as two, it is half the fuel burn. But it’s not twice the fuel economy since you’re going slower.

who continue to work is more loaded, but for us clearly up to around 8.5 kts it is more economical on one engine but from 8/8.3 kts the engine don't stay on his better ratio hp/liter and the drag of the second propeller become too big
Here : Motorisation - Trawler long-cours

"-sur un moteur à 1400 t/mn : 6,6 nds et consommation de 0,61 lt au mille.
- sur un moteur à 1500 t/mn : 7 nds et consommation de 0,81 lt au mille.
- sur un moteur à 1600 t/mn : 7,4 nds et consommation de 0,92 lt au mille.
- sur un moteur à 1800 t/mn : 8,3 nds et consommation de 1,46 lt au mille.
- sur deux moteurs à 1780 t/mn : 8 nds et consommation de 1,14 lt au mille.
- sur deux moteurs à 1850 t/mn : 9,80 nds et consommation de 1,78 lt au mille.

Il est intéressant de comparer les consos à 8,3 nds sur un moteur et celles à 8 nds sur les deux. Dés que le régime sort du couple maxi, nous ne sommes plus dans la zone ‘économique’ et il vaut mieux avoir les deux moteurs qui tournent."

 

[Xlantic]

Here is the data from the Dreamer sea trials that has been referred to before in this thread:

Dreamer

Dreamer is a 1970s Alaskan 45 with twin Ammarine 404 engines which are John Deere 6404D engines and Velvet Drive transmissions 2.1:1.

The tests were carried out by Bob Lowe, the owner of Dreamer and the lead guru of the Grand Banks Forum. Here is some discussion on the trials from the Grand Banks Forum:

Speed Trials on Dreamer - International Association of Grand Banks Owners

Here is what I got out of the reported data:

- freewheeling was, on average, 10% more efficient, in terms of gallons per nautical mile, than dogging the shaft of the off engine.

- running on one engine (with the other engine's shaft freewheeling) was, on average, 44% more efficient, again in terms of gallons per nautical mile, than running the two engines. This was pretty independent of speed (the single engine trials were between 4 and a bit over 7 knots).

If so, the savings can be significant, particularly on longer cruises.

Unfortunately fuel tends to be, in any case, a small percent of the expenses associated with a trawler.

 

[tiltrider1]

Here is the data from the Dreamer sea trials that has been referred to before in this thread:

Dreamer

Dreamer is a 1970s Alaskan 45 with twin Ammarine 404 engines which are John Deere 6404D engines and Velvet Drive transmissions 2.1:1.

The tests were carried out by Bob Lowe, the owner of Dreamer and the lead guru of the Grand Banks Forum. Here is some discussion on the trials from the Grand Banks Forum:

Speed Trials on Dreamer - International Association of Grand Banks Owners

Here is what I got out of the reported data:

- freewheeling was, on average, 10% more efficient, in terms of gallons per nautical mile, than dogging the shaft of the off engine.

- running on one engine (with the other engine's shaft freewheeling) was, on average, 44% more efficient, again in terms of gallons per nautical mile, than running the two engines. This was pretty independent of speed (the single engine trials were between 4 and a bit over 7 knots).

If so, the savings can be significant, particularly on longer cruises.

Unfortunately fuel tends to be, in any case, a small percent of the expenses associated with a trawler.

After reviewing the data I find you have miss interpreted the findings. He clearly shows that at 5.7 knots he burns 2.4 gallons per hour using both engines and 2.6 gallons per hour using just the port engine. He did not match his speed when he did the starboard engine so there is no accurate comparison. He did get a 44% savings in fuel when he slowed the boat down.

 

[markpierce]

So, is this all means/confirms that two engines are less efficient than one?

 

[Steve91T]

Listen guys...common sense. It’s takes fuel just to make an engine run. The more engines, the more fuel. Period. You will never see a 4 engine boat more efficient than a 2 engine boat. And a 2 engine boat will never be more efficient than a single engine boat. So a twin doing 7 kts will burn more than a single at 7 kts. Now a twin running on one engine won’t as efficient as a single because of the added drag.

There are so many variables and it’s so difficult to accurately measure the difference that this thread will most likely never be concluded.

The next time I’m on my boat I will have both engines at 1600 and pull one back to neutral and see what happens. I know I lose 1 knot, but I’ll pay extra attention to the operating engine. My Lehman’s do not set RPM, so I should get an idea of how much extra load it’s under.

 

[tiltrider1]

Listen guys...common sense. It’s takes fuel just to make an engine run. The more engines, the more fuel. Period. You will never see a 4 engine boat more efficient than a 2 engine boat. And a 2 engine boat will never be more efficient than a single engine boat. So a twin doing 7 kts will burn more than a single at 7 kts. Now a twin running on one engine won’t as efficient as a single because of the added drag.

There are so many variables and it’s so difficult to accurately measure the difference that this thread will most likely never be concluded.

The next time I’m on my boat I will have both engines at 1600 and pull one back to neutral and see what happens. I know I lose 1 knot, but I’ll pay extra attention to the operating engine. My Lehman’s do not set RPM, so I should get an idea of how much extra load it’s under.

Steve91T, you keep changing the parameters. Two identical boats, one with one engine and the other with two engines, both engines are identical. The single will burn less fuel at 4 kts than the twin at 4 kts.

On your boat, pick a speed, see what your flow meters say. Put one engine to neutral, increase the throttle on the other engine until you have returned to your picked speed, then look at your flow meters.

If you don’t have flow meters you won’t have any usable data.

If all you do is put one engine to neutral and your boat slows down 1 kt then yes you will save fuel but not because you went to one engine but because you slowed down.

 

[tiltrider1]

So, is this all means/confirms that two engines are less efficient than one?

Depends on your parameters. If time is your measurement instead of fuel then twins are more efficient.