Cruising On One Engine

[Steve91T]

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.

I don’t have meters. Unfortunately I can’t provide the data I’d love to. For my own personal knowledge.

Ok so here’s what I’ll do. Run 1600 rpm with both engines and note my speed. Then bring one to neutral and note the speed. It should be in the 7-7.5 kt range. Then I’ll run both engines and whatever rpm gives me the single engine speed (7-7.5kts) and again pull one back to neutral and see what happens.

 

[BruceK]

The 2 times I ran on one engine for reason other than economy,I had to apply some rudder to maintain course. Rudder not straight ahead acts as a brake. Those with sailboat experience will know fastest sailing comes from sails well balanced with no rudder applied.It has to be a factor when running on one of 2 engines.

 

[DrissZougari]

When running on one engine, the other prop will be turning in reverse all the time.

 

[Steve91T]

When running on one engine, the other prop will be turning in reverse all the time.

Best quote of the thread.

 

[CapnCrunch]

When my friend and I did our tests, we weren't interested in maintaining a specific speed. We ran the engines at 1500 rpm for the entire tests on the theory that what mattered in the economy tests was that the engines were working at the same hp output all the time, regardless of SOL. Again, we consistently found that two engines at 1500 rpm used almost twice the fuel as one ant only moved the boat @2 mph faster than one running at 1500 rpm.

Conversely, it stands to reason that trying to maintain a specific speed running only one engine will be less fuel efficient than with two because 1) the engine will have to run at a higher rpm to deliver the same horsepower to the prop and 2) to overcome the drag produced by the idling prop and the rudder offset needed to maintain a heading.

I dont know about y'all but, when I'm on a long cruise, I run on one engine at what I call its "comfortable" cruising rpm and simply accept whatever speed that rpm produces (usually @ 6 mph in slack water).

 

[long-cours.62]

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

Not necessary as I explain we had " day tanks", this day tanks could be filled by electric or manual pump;
For the manual pump it is easy to know how much diesel you fill at each move, for us it was 0.26 lt, we fill up the days tanks to the top (it is easy to know when they are full the diesel come back to the main tank ;-)) we do 30,40 or more at constant speed, we fill up again the tanks, and count how much time we move the pump and we know exactly how much liter we use to do x nm at y speed. After 15 year doing that frequently with the same boat and engine, on one or two engine,at different speed we had a good idea of our consumption
On the actual one it is more complicated because no day tanks but two "uper" tanks of 2000 lt each, ok the first 80 cm from the top are square, we check the surface and found each 18 mm is 20 liter, we check if it is thru by filling the tank by 20 liter :it is rignt , now for check approximatively we mesure on the visual gauge how much mm change after 30, 40 etc nm done ...and:lol:

 

[Xlantic]

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.

You are right. One needs to do the comparisons at a given speed and not a given RPM.

In the graph below one can compare one versus two engines at different speeds. (For one engine I averaged the speeds and consumptions of the port and starboard results.)

The data is not perfect but I think it shows measurable savings for a single engine running. According to best-fit line, the fuel savings are in the order of 10%.

This is with the other prop free-wheeling. When dogged, the savings dissapear.

 

[Insequent]

You are right. One needs to do the comparisons at a given speed and not a given RPM.

In the graph below one can compare one versus two engines at different speeds. (For one engine I averaged the speeds and consumptions of the port and starboard results.)

The data is not perfect but I think it shows measurable savings for a single engine running. According to best-fit line, at 6 knots the fuel savings are around 8% and at 6.5 knots they are around 16%.

The way I read your graph, your conclusions are the wrong way around. With two engines running there is greater nm per gallon at a given speed. That means lower fuel consumption for two engines running.

 

[Xlantic]

The way I read your graph, your conclusions are the wrong way around. With two engines running there is greater nm per gallon at a given speed. That means lower fuel consumption for two engines running.

I stand corrected. I did not see the forest for the trees, being convinced that running on one engine was more efficient.

So the Dreamer data actually confirms it is more efficient to run two engines. This is probably because of the factors discussed in the thread such as the drag of the prop on the engine not running and the added drag of the required rudder correction.

 

[long-cours.62]

For us

I stand corrected. I did not see the forest for the trees, being convinced that running on one engine was more efficient.

So the Dreamer data actually confirms it is more efficient to run two engines. This is probably because of the factors discussed in the thread such as the drag of the prop on the engine not running and the added drag of the required rudder correction.

the drag of the propeller and the (small or nil if we put down the right side board) ) angle of the ruder became to big above 8kts.
Also you could put the right engine for the side you receive the wind at this moment, the wind could partially counter balance the engine and you will need less ruder angle

 

[cafesport]

You probably won’t save anything on fuel but you can save on maintenance cost per mile and your engine will probably be happier and thank you for it down the road. The consumables such as fuel, oil and air filters all cost less than half the cost of the same for the bigger engines used in our single engine sisterships. When we run in single engine mode, we run with shafts locked and four blade props positioned so that only one blade is in the water flow. No difference in fuel use, locked or freewheeling, but the shafts and transmissions don’t heat up, so we lock them. Also less than 2 degrees of rudder needed and we can turn either direction when running on one engine. Engine room temp drops around 10 degrees and the boat is much quieter. Here is a photo of our underbody to show how the wheels can be oriented. And no the props don’t turn backwards, they are counter rotating, ie they produce water flow on the outside of each of the rudders, instead of only one side of a single rudder. I had a a single engine owner aboard for a five day trip last week. His next boat will have twins. This is the underbody of a single engine sister ship although an extended version of the next size up from ours.

 

[garrobito]

Interesting...

Years ago I had a customer with a huge 55' with twins.
One of the engines was non-functional (387 big blocks) and the other work ok.
I was hired to move the boat from the bay to the delta (70 miles)
I removed the unused propeller, check the good engine all the way, we move 6/7 knots on a long day and reach the marina without any problem.
If consumption is not a problem or if you have a problem with one engine, move a trawler or cruiser with one engine would involve two things:
1) have a clean hull (call the diver!!)
2) remove the unused propeller
Just in case, have your boat insurance at hand to call the tow.
In my experience, this is the best way.
Of course, you will be forced to compensate with the rudder and any long passage would include a careful preparation to avoid bad weather. For this particular passage, I start at 4;00 am and finish around 3 pm using the high tide.