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Old 10-25-2015, 08:12 PM   #1
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Running a twin on just one - on purpose?

I've just read two very long threads on twins vs. single - and this is NOT that again!

Among the boats we're looking at, the vast majority are twins, which I'm fine with. But a lot of them have much bigger, more powerful (thirsty) engines than we really want. I don't want to get a boat with two 500 hp engines, and then run them practically at idle to get the fuel economy we're interested in - that just can't be good for the engines.

So, how about shutting one down on purpose, and running the other one at a higher RPM, somewhere in the range suggested by the engine maker for that engine, as a way to save fuel? Assume, of course, that the gearbox on each engine is just fine with being "free wheeled" like this.

We recently went out for a few hours on just one and lost only about 25% of our speed from our normal cruise, and we also recently spoke with a captain who does a lot of long distance deliveries, who does the same. So: better to run two at slower than their intended speed almost all the time, or run only one at the intended speed, and leave the other shut down?
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Old 10-25-2015, 08:20 PM   #2
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I asked our very reliable mechanic this same question. His exact response was, "you will be dragging you non-turning prop and hurting efficiency. We don't recommend it."
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Old 10-25-2015, 08:53 PM   #3
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Having just experienced issues with one of my DD 6-71s. I was forced to move the boat 30 - 40 miles on one engine. My fuel consumption did not go down. It went up. As compared to running 8knts on 2 engines.
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Old 10-25-2015, 09:00 PM   #4
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I suggest one operate the boat as it was designed. If that doesn't conform to your needs, get a proper boat.
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Old 10-25-2015, 09:47 PM   #5
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Quote:
Originally Posted by BrianSmith View Post

So, how about shutting one down on purpose, and running the other one at a higher RPM, somewhere in the range suggested by the engine maker for that engine, as a way to save fuel? Assume, of course, that the gearbox on each engine is just fine with being "free wheeled" like this.
This has also been discussed in several threads over the years. I believe Timjet gathered some real world numbers and I've played around with it myself. In my case it was a 30,000 pound, semi-planing 44' OA hull with twin turbo 250s. Normal "slow" cruise is 8.4 knots (max is 18). Placed one in neutral and allowed the prop to free wheel (less drag than locked). Then pushed remaining engine up to achieve 8.4 knots. Using prop curves for estimating fuel consumption, the single operating engine showed about 5-7% improvement in overall fuel consumption over twin operation. Coolant temp climbed from a too cool 165-170 to the "normal" 185. Keep in mind you're looking at 65-70 HP (total) to push the hull at that slow cruise speed. So the single operating engine jumps from about 35 HP to about 70. Still very low power on a boat equipped with 375's, for example. (Keep in mind that some stuffing boxes are cooled with pressurized water from the engine cooling system, so that must be addressed). Anyway, it's not worth the trouble in my estimation....boat crabbing through the water, etc.... Maybe when fuel gets back o $5-6/gallon....which should be coming fairly soon.
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Old 10-26-2015, 02:29 AM   #6
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Well it may be boat specific, but both Dreamer and myself found a slight increase in fuel consumption when running just one engine at the same speed. Data for Dreamer is here.

My sea trial results are in the pdf. I needed 7 rudder to keep a straight heading on just one engine. So that plus the drag of free wheeling prop, strut and rudder used more fuel than consumed by the parasitic losses in the second engine.

I would only run on one (by choice) if I needed much more range, by slowing down of course, and running both engines meant they weren't able to stay at a reasonable operating temp. Modern engines wont be harmed by operating at the lower end of the power curve in the manufacturers graphs, assuming the engine was run-in correctly.
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Old 10-26-2015, 03:35 AM   #7
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I tend to agree with Brian from Insequent, we often cruise on one engine , ours are twin Cat 3406 540 hP a side, but the engines are big approx 12 litres so plenty of grunt. My fuel consumption is almost halved using only one engine and free wheeling the other.
Obviously there is a lot more load on the single and therefore rpm needs to increase slightly , and so the fuel consumption does not quite halve. I can sit on 9 knots all day on one engine. It is interesting once the hull is at a comfortable hull speed, how closing one engine makes a slight difference but you certainly don't half the speed, you may loose a up to 1.75 knots.
We run Twin Disc , and they have advised and it is in the manuals that you can free wheel the other prop up to about 9 or 10 knots up to 8 hours . The main determining factor to free wheeling is the temp of the oil in the gear box that is freewheeling, what you can do is start the unused motor for 5 mins every two hours and this will circulate the oil thru the cooler.
Our Auto pilot handles it quite well , and as Brian has said you need to have about 7 degree's oversteer to keep a straight course.
Obviously we only do this when conditions are suitable, if there is a sea on then we always run the twin engines.
It also has a bearing on maintenance costs , as the running hours are reduced also
Being 57 foot and weighing in 77,000 lbs once momentum is up it is relatively easy to keep it going at displacement speeds.

Anyway these are my thoughts
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Old 10-26-2015, 03:45 AM   #8
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I am surprised that you see that much benefit on one engine. I guess it means your Cats are not as fuel efficient at low rpm as my John Deere's. I think JD's farming heritage made them work very hard on fuel efficiency. Mine are Tier 2 engines, not the full-blown Common Rail electronics, but still have an ECU. The JD's are much more fuel efficient at low rpm than the Cummins V8's I removed also. At higher power levels there isn't much difference.
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Old 10-26-2015, 03:55 AM   #9
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Brian

On our last trip of 2500 miles we averaged 38 litres per hour for the trip, mostly running on both engines, but probably did about 300 miles on one engines. Our average speed for the trip was about 9.1 knots.

If I run on one engine we consume about 24 litres per hour for the same speed.

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Old 10-26-2015, 06:02 AM   #10
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I did not see in a quick read the primary reason NOT to run the twin engine boat on one engine. Match your cruising speed with 75% engine load. Search Steve DiAntonio's web site for a very good article on this.
Runnings engines at low loads does long term lube harm.
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Old 10-26-2015, 06:27 AM   #11
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I did not see in a quick read the primary reason NOT to run the twin engine boat on one engine. Match your cruising speed with 75% engine load. Search Steve DiAntonio's web site for a very good article on this.
Runnings engines at low loads does long term lube harm.
I have not experienced any issues with this vessel or our previous vessel , both a displacement vessel and both considered to be over powered , if you use the rule of thumb of the 75 % loading. We run a lot less than this for the majority but always run them up for a short period of time on most runs to blow out the cob webs.

I do hear of more machinery/ part / turbo failures out of engines being stressed overworked than any underworked. Our 3406 diesels were originally designed for trucks and a lot of idling , they are heavy displacement and slow RPM so all the research and people I have spoken to , tell me no harm will be done using them this way. Our vessel has covered over 50,000 miles during its life and the motor's just keep purring away. They also are reasonably quiet , and not whizzing their heads off.

Maybe it depends on the type and brand of Engine? I don't believe anyone knows the true answer or solution. I am just enjoying the use of the boat and it still performs extremely well at sea, I don't think a smaller engine would change my enjoyment or the boats performance.
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Old 10-26-2015, 06:41 AM   #12
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This topic is full of comparing apples to oranges.

Even the discussion of just freewheeling or locked props.

Unless you choose an identical sistership for data.....no one else's boat matters.

The only universal item on the tip of my tongue is if you run the boat a tad slower than normal on the single, often you can save fuel into the 10 to 15 percent range. Try and run just a bit higher and consumption will surpass running both at low rpms.
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Old 10-26-2015, 06:56 AM   #13
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This topic is full of comparing apples to oranges.

Even the discussion of just freewheeling or locked props.

Unless you choose an identical sistership for data.....no one else's boat matters.

The only universal item on the tip of my tongue is if you run the boat a tad slower than normal on the single, often you can save fuel into the 10 to 15 percent range. Try and run just a bit higher and consumption will surpass running both at low rpms.
Psneeld,

I think you are correct all vessels are different.

In relation to your last comment , it may have more to do with the hull speed , once you reach that whether you use both or one engine the fuel consumption is going to climb fairly fast. In theory a hull should only need a certain amount of Hp to drive it to hull speed whether single or twin, it would be a matter of where the engine , engines are on the power curve at that point, and yes the comment on the characteristic of the hull will have a bearing on this.

It is a bit of a puzzle !!!

Cheers Chris D Liberty
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Old 10-26-2015, 07:16 AM   #14
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Psneeld,

I think you are correct all vessels are different.

In relation to your last comment , it may have more to do with the hull speed , once you reach that whether you use both or one engine the fuel consumption is going to climb fairly fast. In theory a hull should only need a certain amount of Hp to drive it to hull speed whether single or twin, it would be a matter of where the engine , engines are on the power curve at that point, and yes the comment on the characteristic of the hull will have a bearing on this.

It is a bit of a puzzle !!!

Cheers Chris D Liberty
Absolutely....just my point even though there could be much more going on....

People often discuss the theory in such a total vacuum....no wonder results and guesses are all over the place.

There is some truth in a lot of everyone's posts....but not enough often to paint a clear picture of what is more universal over what works for them and them only.
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Old 10-26-2015, 09:00 AM   #15
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<snip>

There is some truth in a lot of everyone's posts....but not enough often to paint a clear picture of what is more universal over what works for them and them only.
That seems to be the long and short of it. We will continue to look for NA engines, and if we find a boat we love with more power than we need, I guess we'll just play around with speeds and engines and such and do the best we can.

BTW - looked at a Hatteras 58 LRC on Friday, and fell madly, deeply in love! Great flybridge, salon, aft deck ("coffee deck", as we call it), cockpit for easy diving... and a couple of NA DD 6-71's. Hope we can make the finances work to get one!
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Old 10-26-2015, 09:22 AM   #16
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Running a twin engine boat on one engine means having the rudders over 40 degrees to compensate. That is a head scratcher.
You may run your engines at light load and not have issues like some cigarette smokers live to 80. Reality is we can compression test, oil sample the 75% load engine against the run cold engine and the loaded motor will score much higher.
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Old 10-26-2015, 09:42 AM   #17
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How far offset the rudder need be depends on how fast you are trying to go. Most people think they can cruise at the same speed on one engine. In my experience you need to slow down until the boat holds course easily and will turn against the running engine without much difficulty. That is still within the "trawler" speeds people seem to prefer.


Rudder position is also a function of prop size ( drag) and separation between engines ( turning torque).
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Old 10-26-2015, 09:44 AM   #18
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[QUOTE=semi-planing;382485]
Quote:
Originally Posted by BrianSmith View Post

So, how about shutting one down on purpose, and running the other one at a higher RPM, somewhere in the range suggested by the engine maker for that engine, as a way to save fuel? Assume, of course, that the gearbox on each engine is just fine with being "free wheeled" like this.

/QUOTE]

This has also been discussed in several threads over the years. I believe Timjet gathered some real world numbers and I've played around with it myself. In my case it was a 30,000 pound, semi-planing 44' OA hull with twin turbo 250s. Normal "slow" cruise is 8.4 knots (max is 18). Placed one in neutral and allowed the prop to free wheel (less drag than locked). Then pushed remaining engine up to achieve 8.4 knots. Using prop curves for estimating fuel consumption, the single operating engine showed about 5-7% improvement in overall fuel consumption over twin operation. Coolant temp climbed from a too cool 165-170 to the "normal" 185. Keep in mind you're looking at 65-70 HP (total) to push the hull at that slow cruise speed. So the single operating engine jumps from about 35 HP to about 70. Still very low power on a boat equipped with 375's, for example. (Keep in mind that some stuffing boxes are cooled with pressurized water from the engine cooling system, so that must be addressed). Anyway, it's not worth the trouble in my estimation....boat crabbing through the water, etc.... Maybe when fuel gets back o $5-6/gallon....which should be coming fairly soon.
"Then pushed remaining engine up to achieve 8.4 knots."

Therein lies the difference between running on one engine to conserve fuel and running on twins. The $$ fuel savings (actual per nautical mile fuel use reduction) that can be accomplished by using single instead of twins only comes into play of a notable amount when the speed is considerably reduced. If instead of pushing the single to reach the twins' speed of 8.4 knots you had rather reduced speed to say 6 knots then the nmpg fuel usage will notably diminish... albeit your miles per day accomplished surely diminish too!

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Old 10-26-2015, 09:56 AM   #19
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The answer is it depends, and as markpierce mentioned it's all about design. For any given hull there is a fixed amount of horsepower that is needed to achieve the designer's hull speed under specific conditions, oh and here's the rub, boats don't live in that world. That being the case, you'll find various compromises made. Some favor extreme efficiency, others sea-kindliness, still others speed, and then you have the extreme outliers of the envelope. The case you specially mention brings up the issue of running a boat designed for life at planning speed, run at waterline speed, which though done by many, is I believe in the end less than optimal. Running this kind of boat on one engine probably won't deliver the results I think you are looking for. Fwiw I've spent some time on identical displacement boats designed to be run as singles and as twins. They both work remarkably well and the twin version run as a single suffers none of the problems many have encountered running other boats on single engines mainly because of the boats design. A boat originally designed to run as a single and redesigned for twins is much different than a boat originally designed to run as a twin run on one engine. Having essentially 3 keels two big rudders and a 4 blade prop properly locked with only two blades outside the aperture yields at most 2 degrees of helm and remarkable stability and tracking even in heavy following seas on a single engine. I had a 38' blackfin with 1100hp and she was a great heavy weather boat, but she really sucked running on one engine mainly because she needed 700 hp to get her on plane where her tiny rudders were designed to work best.


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Old 10-26-2015, 10:15 AM   #20
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This topic is full of comparing apples to oranges.

Even the discussion of just freewheeling or locked props.

Unless you choose an identical sistership for data.....no one else's boat matters.

The only universal item on the tip of my tongue is if you run the boat a tad slower than normal on the single, often you can save fuel into the 10 to 15 percent range. Try and run just a bit higher and consumption will surpass running both at low rpms.

I just went back to my old notes. Per the prop charts (admittedly a fairly crude methodology), at 9.4 MPH with both engines @ 1550 rpm the prop charts show 4gph (total). At 9.4 MPH with one engine @ 1875 rpm the chart shows single engine fuel burn of 3.1 gph. With one engine at 1550, speed was 7.9 mph and fuel burn was 2.0 gph. So the numbers are considerably better than I had recalled off the top of my head in previous post. Regarding my previous post, I also mentioned the boat crabbing through the water....in fact the autopilot was engaged (per my notes) and all data was steady heading. The OA 44 has relatively large rudders and the autopilot had no trouble at all.
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