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Old 09-19-2019, 09:26 AM   #1
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Fuel efficiency

I may be opening myself up for some ridicule here,but looking for input as to using one engine out of two while underway in reasonable conditions.After helping a friend return to dock with only starboard engine available,and no need for wheel usage more than normal,I received a message from another friend that he did not want twins as too expensive to run the loop.This combination makes me ask thoughts about alternating engine use during travel.Thanks.
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Old 09-19-2019, 10:03 AM   #2
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No ridicule...but there is no right answer that I knw of...someone might have it..but in decades of listening, reading and laughing at many posts here...I doubt you will get anything but tidbits that you will have to decide what is real and piece it together to fit your particular situation.


Some boats may be the extreme on one end, others at the other end.


How you run your boat and/or set it up will matter.


Unless you give a specific example with very specific setup and running parameters...all you will get is the spectrum of what some one else thinks or has experienced.
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Old 09-19-2019, 10:05 AM   #3
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Transmission and drag issues aside, as long as you are comfortable with the resulting reduction in speed and some minor handing issues, go for it as long as you balance out the hours. I have heard that the savings in GPM is somewhat minimal. A boat designed for a single is probably going to be more efficient, a boat designed for twins and run as a single might not be its equal. I would be interested in the reduction in wear for the engine not run as long as it is run at no greater load than when run as a twin, thus the resulting loss of hull speed.
I have never owned a diesel engine, or for that matter a trawler. Most of what I am regurgitating here is from what I have read on this forum. To say my words are suspect would be an understatement. My mother tells me I am very smart though. Good luck.
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Old 09-19-2019, 10:08 AM   #4
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There is no such thing as fuel efficiency in boatig.

running single at 6 knots fuel use in my 38,000 # boat was a bit better than with twins at the same speed. But it was silly because the numbers went from about 2.5 NMPG to maybe 3 NMPG. At 8 -9 Kts with twins is got about 2NMPG and was much happier.
Fuel is not the big expense anyway.
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Old 09-19-2019, 10:52 AM   #5
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If you need to consider it you’ve got the wrong boat.
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Old 09-19-2019, 10:53 AM   #6
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If you think about it for a minute, you will realize that a boat designed for twins will be less efficient being pushed through the water by only one of its two engines:
1 dragging the non-running gear through the water
2 pushing from a propeller location that is offset to one side, so steering at an angle to compensate
3 unable to get up to a normal cruising speed

Though #3 may get you the illusion of greater efficiency, due to reducing the size of the waves that you are making by going slower.

If all you want is to reduce the gallons burned per mile, pull back on the throttles. All of the energy put into making waves can be eliminated by slowing down.
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Old 09-19-2019, 11:36 AM   #7
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Thanks to all for responses.Rather what I anticipated,that final result is minimal difference,but thought worth exploring.Actually reinforces that two engines are preferable(as mine is),especially given that the safety factor in event of engine breakdown plus greater docking abilities far outweighs small fuel savings.
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Old 09-19-2019, 11:48 AM   #8
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I am purposely looking for a single. If you want to run at hull speed or close to it, why have two engines? More fuel, oil changes, general maintenance, shaft seals, anodes, etc. One good engine well maintained should work fine I hope. I also argued that you could run twins at hull speed just fine, but I learned from my snowmobiling that you cannot use what is not there. By this I mean if I had twins capable of planing I would likely being planing more than I thought and using much more fuel. I am trying to make myself slow down. Less is more in this endeavor.
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Old 09-19-2019, 11:56 AM   #9
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I could argue strongly that if you compare two identical boats run at the same speed (hull speed) a twin and a single, the fuel consumption will be practically the same. It's been proven.



I could also argue strongly that there is no efficiency gained in running a twin on one. (There's may be some specially designed boats that do have the capability where it is useful running on one, most likely with feathering props.) Doubt it in the typical trawler and cruiser boats on this forum.


And we've been thru the twin vs single many times. So, it's a choice.
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Old 09-19-2019, 12:00 PM   #10
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Buy the boat that fits your needs and is in the best condition. Whether it has a single or has twins, doesnít really matter that much. You may save a bit by running on one engine rather than two, but it will not be much. Probably not worth it. The biggest savings will be to slow down.
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Old 09-19-2019, 12:04 PM   #11
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Quote:
Originally Posted by bayview View Post
There is no such thing as fuel efficiency in boatig.

running single at 6 knots fuel use in my 38,000 # boat was a bit better than with twins at the same speed. But it was silly because the numbers went from about 2.5 NMPG to maybe 3 NMPG. At 8 -9 Kts with twins is got about 2NMPG and was much happier.
Fuel is not the big expense anyway.

Bayview,



Fuel efficiency is all relative. Yes, one can get phenomenal fuel efficiency in some boats while others guzzle down.



However, if one uses their boat much, fuel can easily be the biggest expense. If it sits at the dock, fuel is cheap. I've had times when fuel cost clearly the highest annual cost.
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Old 09-19-2019, 12:37 PM   #12
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There is no reason for any of us to be confrontational about this! There are lots of reasons to own a single and just as many as there are to own twins. Bess and I have had both and wouldn't have traded one for the other. It was a situational thing for us. We can discuss the pluses and minuses, but one is never better than the other and we should not berate someone that chooses differently than yourself.
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Old 09-19-2019, 01:01 PM   #13
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There is no reason for any of us to be confrontational about this! There are lots of reasons to own a single and just as many as there are to own twins. Bess and I have had both and wouldn't have traded one for the other. It was a situational thing for us. We can discuss the pluses and minuses, but one is never better than the other and we should not berate someone that chooses differently than yourself.
Absolutely agree, choose the best boat for you. And the best condition and live with the engine choice.
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Old 09-19-2019, 01:01 PM   #14
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Bingo! " The biggest savings will be to slow down", I can't drive 55! My snowmobiling analogy applies for me. If the power is there I don't trust myself to not use it. I will force myself to slow down by not making it available in the first place. I also pick up a much more spacious engine room with a single. I probably will end up with twins anyway because far more boats come with twins than singles. Maybe in that lies the answer?




Quote:
Originally Posted by Comodave View Post
Buy the boat that fits your needs and is in the best condition. Whether it has a single or has twins, doesnít really matter that much. You may save a bit by running on one engine rather than two, but it will not be much. Probably not worth it. The biggest savings will be to slow down.
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Old 09-19-2019, 01:07 PM   #15
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We have twin SP225s in our current boat. When we started bringing it home we were cruising at 10+ knots. About halfway home we had slowed down to about 8.5 knots and out efficiency went up about 40%. Now we never go above 8.5 unless it is to test the engines. If we would slow down more we would save more but we like 8.5 (10mph) and can live with the fuel burn. Our boat topped out at 17 knots during the sea trial but we never use that type of speed.
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Old 09-19-2019, 01:10 PM   #16
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It is nice having twins, handling is easier, but last year we stopped at our marina to pump out the head. The starboard engine would not start so we ran to our home on one engine and I worked on it the next day. Turned out it was a blown fuse that I didnít know was there. So we didnít have to have a tow as we would have had to do with a single. But again, buy what fits your needs and then look at the single vs twin issue. If you get a single maintain it religiously...
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Old 09-19-2019, 01:40 PM   #17
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We have logged nearly 3,000 nm this year. Two engines, 30 ton boat and around 8 knots predominately. Fuel costs about 30% of all in annual cost. Fewer miles would have meant lower %.

As others have said, get a boat that satisfies your needs, run it slow and don't obsess about fuel cost. If one desires to save really big bucks, don't buy a boat, just find another rabbit hole to dump your play time money.
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Old 09-19-2019, 01:50 PM   #18
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Quote:
Originally Posted by Comodave View Post
We have twin SP225s in our current boat. When we started bringing it home we were cruising at 10+ knots. About halfway home we had slowed down to about 8.5 knots and out efficiency went up about 40%. Now we never go above 8.5 unless it is to test the engines. If we would slow down more we would save more but we like 8.5 (10mph) and can live with the fuel burn. Our boat topped out at 17 knots during the sea trial but we never use that type of speed.
This has been our experience too. Right down to the speeds! I do my trip planning in statute miles and cruising 10mph makes it easy to calculate. Like you probably do, I set an rpm for 10mph in still water and my ground speed varies a few tenths (or more!) on either side depending on wind and tide. We’ve seen 18kts, but we don’t do it often. Even though our fuel burn and hp is much different from yours, the principles still hold true: stay a bit below calculated hull speed for best economy.
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Old 09-19-2019, 03:35 PM   #19
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Originally Posted by moparharn View Post
I am purposely looking for a single. If you want to run at hull speed or close to it, why have two engines? More fuel, oil changes, general maintenance, shaft seals, anodes, etc. One good engine well maintained should work fine I hope. I also argued that you could run twins at hull speed just fine, but I learned from my snowmobiling that you cannot use what is not there. By this I mean if I had twins capable of planing I would likely being planing more than I thought and using much more fuel. I am trying to make myself slow down. Less is more in this endeavor.
Fuel efficiency has almost nothing to do w how many engines you’ve got.
It has to do w how efficient your engine is, how efficient your prop is and how efficient your hull is at the speed you run. And most importantly how big your engine is.

The problem w twin engined trawlers is that too many people bought them so after these many years the new owner has only a small fraction of the fuel money the original owner did. So the majority of trawler owners have two 380 cu. in. engines for 760 cu. in. A lot of cylinders and displacement to feed.

Today a skipper w a 70’s trawler would be far better off w two much smaller engines. It can be done. Economically w two sailboat takeouts. They are plentiful and inexpensive.

Otherwise going the speed the boat was designed for or slowing way down to get good/reasonable fuel consumption.

So if you want fuel economy and have a twin engined boat (think two FL’s) you’ll just need to underload them (and see how that goes), burn at least 4gph, or get another boat as you have the wrong boat.
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Old 09-19-2019, 04:02 PM   #20
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See? How easy this become a circular argument?


Pick range....ant try not to get into the bigger tank...bigger boat....bigger engine....bigger tank, bigger boat....bigger engine....etc....etc.... round and round.


same with efficiency....start someplace and change one thing...off we go again....


If you don't pick one set of parameters.....round and round we go.
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