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Old 06-25-2019, 11:38 PM   #81
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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.
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Old 06-26-2019, 12:26 AM   #82
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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.
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Old 06-26-2019, 05:31 AM   #83
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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.
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Old 06-26-2019, 06:13 AM   #84
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But the engine

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Originally Posted by Steve91T View Post
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."
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Old 06-28-2019, 02:12 PM   #85
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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.
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Old 06-28-2019, 06:54 PM   #86
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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.
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Old 06-28-2019, 07:23 PM   #87
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So, is this all means/confirms that two engines are less efficient than one?
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Old 06-28-2019, 07:37 PM   #88
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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.
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Old 06-28-2019, 08:05 PM   #89
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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.
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Old 06-28-2019, 08:07 PM   #90
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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.
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Old 06-28-2019, 08:10 PM   #91
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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.
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Old 06-28-2019, 08:26 PM   #92
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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.
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Old 06-28-2019, 08:45 PM   #93
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When running on one engine, the other prop will be turning in reverse all the time.
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Old 06-28-2019, 08:47 PM   #94
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When running on one engine, the other prop will be turning in reverse all the time.
Best quote of the thread.
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Old 06-28-2019, 09:42 PM   #95
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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).
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Old 06-29-2019, 01:19 AM   #96
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"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
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Old 06-29-2019, 04:16 AM   #97
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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.
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Old 06-29-2019, 04:42 AM   #98
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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.
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Old 06-29-2019, 04:52 AM   #99
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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.
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Old 06-29-2019, 09:39 AM   #100
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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
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