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Old 04-25-2015, 11:50 PM   #1
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One Vs Two

I am reopening a old can of worms. the issue of the fuel burn of a single motor Vs twins. Theoretically on the same boat the single should be more efficient and the motor size closest to the HP needed should be the most efficient. What I have noticed over and over again is that on a practical real world basis this is not always the case in any significant measure particularly if similar but not identical boats are compared. I see many circumstances where twins even large twins do as well as single engine boats. As an example that is not in any way unique we can compare the fuel burn published for the Helmsman 43 at approx. 35,000 lb disp. against my boat at 48 ft and approx. 35,000lb disp. The helmsman has a single 370 HP engine and my boat has twin 330 Hp engines. The Helmsman at 8.3K burns 4.9 G/Hr and at 9.5K burns 8.4 G/Hr at 10.5K the burn was 16.3 G/Hr. Compared to my boat we find 8.3K at 4.4 G/Hr and 9.6K at 7 G/Hr and 10.3K at 11 G/Hr. There are realistic reasons against twins but for me the fuel efficiency argument does not fly. I am of the school that believes the boat and prop or props will demand x amount of power for y amount of speed and the x from one or two engines is pretty much in the same ball park in real world terms and whatever drag and lose from two engines is not significant to speak against twins.
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Old 04-26-2015, 12:00 AM   #2
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The twin engines cost much more than the single engine and crowd the engine room as well, and you've twice the engine maintenance. Further, if one is moving at less than hull speed (like a good trawler should), fuel consumption is the least of one's concern. I worry, however, of running one's diesel engines too easy.


Schulman, why does the boat pictured in your avatar look much shorter than 48 feet?
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Old 04-26-2015, 01:08 AM   #3
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Originally Posted by eyschulman View Post
I am reopening a old can of worms. the issue of the fuel burn of a single motor Vs twins. Theoretically on the same boat the single should be more efficient and the motor size closest to the HP needed should be the most efficient. What I have noticed over and over again is that on a practical real world basis this is not always the case in any significant measure particularly if similar but not identical boats are compared. I see many circumstances where twins even large twins do as well as single engine boats. As an example that is not in any way unique we can compare the fuel burn published for the Helmsman 43 at approx. 35,000 lb disp. against my boat at 48 ft and approx. 35,000lb disp. The helmsman has a single 370 HP engine and my boat has twin 330 Hp engines. The Helmsman at 8.3K burns 4.9 G/Hr and at 9.5K burns 8.4 G/Hr at 10.5K the burn was 16.3 G/Hr. Compared to my boat we find 8.3K at 4.4 G/Hr and 9.6K at 7 G/Hr and 10.3K at 11 G/Hr. There are realistic reasons against twins but for me the fuel efficiency argument does not fly. I am of the school that believes the boat and prop or props will demand x amount of power for y amount of speed and the x from one or two engines is pretty much in the same ball park in real world terms and whatever drag and lose from two engines is not significant to speak against twins.
You're comparing two very different boats. Different shape hulls, different lengths, different widths. I assume different waterlines and yours is longer and therefore has a faster displacement speed. Then don't even know the type engines or vintage and that could be another factor. Definitely an apples and oranges comparison.

I will agree that in some situations twins get nearly the same fuel consumption but in others they consume more.
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Old 04-26-2015, 01:11 AM   #4
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The twin engines cost much more than the single engine and crowd the engine room as well, and you've twice the engine maintenance. Further, if one is moving at less than hull speed (like a good trawler should), fuel consumption is the least of one's concern. I worry, however, of running one's diesel engines too easy.


Schulman, why does the boat pictured in your avatar look much shorter than 48 feet?
Does not look so short when I pay for 52 feet of dock space from anchor to end of swim platform. Maybe the boat would look bigger if it were a triple decker. As a mater of fact we often marvel at the same issue when we view our boat from the dinghy in a mooring field. As to the extra costs of two motors that is not the issue I address. I merely point out the fuel burn fallacy. The cost of the fuel burn after paying for a custom one off built by a high priced artist like Sam Devlin is not much of an issue. The difference between what we paid for the boat and what we could get for it a day after delivery could pay for fuel for two life times. Just pointing out that in many boat set ups twins really don't always use more fuel than a single.
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Old 04-26-2015, 01:25 AM   #5
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You're comparing two very different boats. Different shape hulls, different lengths, different widths. I assume different waterlines and yours is longer and therefore has a faster displacement speed. Then don't even know the type engines or vintage and that could be another factor. Definitely an apples and oranges comparison.

I will agree that in some situations twins get nearly the same fuel consumption but in others they consume more.
Well aware of the apples to oranges issue which shows again that the single twin thing is not the big determinant of fuel burn. As to engines both are modern equivalent technology and that maters little since Diesels for the last several decades old or new get similar approx. 18-20 Hp per one gallon of fuel. What I am pointing out is that it is a fallacy to assume a singe engine boat of similar size and displacement (but not identical)to another boat with twins will get better fuel burn. The engines merely put out the HP and then other factors come into play. I meet so many people with displacement trawlers and one motor who just assume their boats are more fuel efficient than my twin and for some that is just not true.
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Old 04-26-2015, 03:22 AM   #6
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Single engine boats are great!!!

Right up Untill it unexpectedly quits working.
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Old 04-26-2015, 08:03 AM   #7
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From my following the debate..there are people here that have noted significant savings when shutting one of their two down.


Of course there's the uproar about this and that...as it should be.


Not only are there numerous factors in just design...there is also the range of speeds were all the design stuff goes out the window.


2 perfectly equal boats, designed to run at a given speed should be more efficient just for the efficiencies of the propulsion components. But the premise for efficiency must first be there and designed completely that way.


Just randomly selecting boats, powerplants and operational characteristics then comparing them is mindboggling to think a comparison can even be made.
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Old 04-26-2015, 08:22 AM   #8
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IMO, a great deal of what is missed in this discussion is the engine sizing, transmission ratio, shaft and prop, and finally hull / running gear setup.

Let me start with a non boating comparison. Dodge made a 3/4 ton 4 wheel drive pickup with a 220 HP Cummins diesel. It was offered with 2 rear end ratios. The short gears had the motor turing around 1800 rpm at 60 mph and yielded 23 +/- mpg. Same exact truck with tall gears turned 2300 rpm at 60 mph and yielded 18 +/- mpg. The truck required the same amount of HP to go down the road. The drive train and turning the engine faster required more fuel to accomplish the same amount of HP.

The part of the one versus two discussion that so often is missed has to do with the propulsion setup. If you have a large wide keel in front of a single prop, how much does that effect the efficency of the propeller versus the relatively cleaner water that a propeller sees without a keel in front of it? If the propeller in a single screw version of a boat is the same diameter as the twin version, which is more efficient for transferring HP to thrust? With the engine choices, is the single running more efficiently or are the twins? IMO, I really don't think most boat manufacturers optimize for fuel efficiency with either singles or twins. They do the best that they can with whatever engine manufacturer makes the most sense to do business with.

Final thought: I have a 35' Downeaster charter boat with a Cummins 6CT 300 HP engine, turning a 1.5 to 1 Twindisc transmission, and turning a 4 bladed 21" prop. I charter to scuba divers and run the boat fully loaded. 15 knots is the optimal cruising speed. Slower is less efficient as the weight doesn't have the boat fully on plane. Faster burns a lot more fuel pushing the big keel through the water. Originally I was slightly over propped. 2050 rpm made 15 knots at 1.2 mpg. Flattened the prop an inch and turned 2100 rpm at 15 knots at 1.3 mpg. Flattened the wheel another inch and now turn 2200 rpm at 15 knots at 1.4 mpg. Did the hull require less HP? No, the efficency of the drive train changed. My guess is the air to fuel ratio changed as I turned the engine faster making the fuel burn more efficiently (much less transom soot). These numbers are over the course of 3 years and about 240 trips. While the difference in fuel burn may not seem important to most of you, fuel is / was my number one business expense, larger than the next 3 expenses combined. Every dollar I don't spend in fuel goes in my pocket. The net fuel savings was 16% for just changing prop pitch! A side note: The fuel burn running without customers and dive gear went from 1.4 mpg to 1.8 mpg at 15 knots. That's a 28% savings!

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Old 04-26-2015, 08:30 AM   #9
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IMO, a great deal of what is missed in this discussion is the engine sizing, transmission ratio, shaft and prop, and finally hull / running gear setup.

Let me start with a non boating comparison. Dodge made a 3/4 ton 4 wheel drive pickup with a 220 HP Cummins diesel. It was offered with 2 rear end ratios. The short gears had the motor turing around 1800 rpm at 60 mph and yielded 23 +/- mpg. Same exact truck with tall gears turned 2300 rpm at 60 mph and yielded 18 +/- mpg. The truck required the same amount of HP to go down the road. The drive train and turning the engine faster required more fuel to accomplish the same amount of HP.

The part of the one versus two discussion that so often is missed has to do with the propulsion setup. If you have a large wide keel in front of a single prop, how much does that effect the efficency of the propeller versus the relatively cleaner water that a propeller sees without a keel in front of it? If the propeller in a single screw version of a boat is the same diameter as the twin version, which is more efficient for transferring HP to thrust? With the engine choices, is the single running more efficiently or are the twins? IMO, I really don't think most boat manufacturers optimize for fuel efficiency with either singles or twins. They do the best that they can with whatever engine manufacturer makes the most sense to do business with.

Final thought: I have a 35' Downeaster charter boat with a Cummins 6CT 300 HP engine, turning a 1.5 to 1 Twindisc transmission, and turning a 4 bladed 21" prop. I charter to scuba divers and run the boat fully loaded. 15 knots is the optimal cruising speed. Slower is less efficient as the weight doesn't have the boat fully on plane. Faster burns a lot more fuel pushing the big keel through the water. Originally I was slightly offer propped. 2050 rpm made 15 knots at 1.2 mpg. Flattened the prop an inch and turned 2100 rpm at 15 knots at 1.3 mpg. Flattened the wheel another inch and now turn 2200 rpm at 15 knots at 1.4 mpg. Did the hull require less HP? No, the efficency of the drive train changed. My guess is the air to fuel ratio changed as I turned the engine faster making the fuel burn more efficiently (much less transom soot). These numbers are over the course of 3 years and about 240 trips. While the difference in fuel burn may not seem important to most of you, fuel is / was my number one business expense, larger than the next 3 expenses combined. Every dollar I don't spend in fuel goes in my pocket. The net fuel savings was 16% for just changing prop pitch! A side note: The fuel burn running without customers and dive gear went from 1.4 mpg to 1.8 mpg at 15 knots. That's a 28% savings!

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Old 04-26-2015, 08:47 AM   #10
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Well, as a new boat owner with more engine (twin Cummins 5.9 250hp each) than the boat will ever need, I'll just sit here on the sideline and observe.
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Old 04-26-2015, 09:52 AM   #11
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There are some that remove perfectly good twins and put in a single, it can be done. Sometimes it is not about the cost and rationale, it is about our druthers and free will.
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Old 04-26-2015, 09:54 AM   #12
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Originally Posted by psneeld View Post
From my following the debate..there are people here that have noted significant savings when shutting one of their two down.


Of course there's the uproar about this and that...as it should be.


Not only are there numerous factors in just design...there is also the range of speeds were all the design stuff goes out the window.


2 perfectly equal boats, designed to run at a given speed should be more efficient just for the efficiencies of the propulsion components. But the premise for efficiency must first be there and designed completely that way.


Just randomly selecting boats, powerplants and operational characteristics then comparing them is mindboggling to think a comparison can even be made.


And in fact, comparing boats that are not identical, except for engine(s) is nonsensical and tells you nothing.
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Old 04-26-2015, 10:18 AM   #13
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I am shopping now for my next boat. I prefer single screw. Less maint. and more room in the "already crowded" engine room. I guess it a personal choice.
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Old 04-26-2015, 11:12 AM   #14
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Interesting discussion.

We have a single FL120. I have toyed with the idea that if it ever dies on us to replace it with an FL80. We only cruise at 6 to 7 knots. Have no desire to go any faster. Our #1 concern is economy. That being said from what I have gathered over the last couple of these discussions is that it might not save any fuel at all.

By the way as an ex-sailor a constant 6 or 7 knots is flying, unless you came from high tech multihulls.
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Old 04-26-2015, 11:19 AM   #15
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Richard,

I have been wanting to ask you how you changed the oil on your FL during the crossing. I think I read where you did not shut off the motor during the whole crossing. I tried to go to your blog but Norton blocks the site won't let me access it.
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Old 04-26-2015, 11:53 AM   #16
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The following comparison has just as much meaning as the initial one in this thread....none.

Premise: Three engines are more fuel efficient than two.

Comparison: Contender 39 LS with triple Yamaha 350's vs. Rivarama 44' with twin 800 MAN's.

Contender cruise is 38.8 knots at 4000 rpm using 37.2 gph for just over 1.0 nmpg.

Riva cruise is 36 knots at 2000 rpm using 56 gph for .64 nmpg.

Proof: 3 Engines are more fuel efficient than two.

Next week we'll compare sports cars and SUV's.

Just teasing, but the point is that we can't and shouldn't reach any conclusions based on comparisons of different type items. All we can say is that Boat A with Engines A uses more or less fuel than Boat B with Engines B.

Even if we had an identical boat with twins vs single we couldn't use that to extend beyond that boat because much would depend on that boat and it's ideal pairing.

And I'd encourage people to just get and enjoy what they like. You don't have to justify or prove to anyone. Twins vs. singles ends up largely a matter of personal preference as one weighs the many different factors. Sometimes it's actually builder's preference as some builders only go one way. If you want a new Grand Banks 43' you will prefer twins. lol. If you want a new Nordhavn 43' you will prefer a single. lol.
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Old 04-26-2015, 12:12 PM   #17
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From my following the debate..there are people here that have noted significant savings when shutting one of their two down.
Who? I have never seen an actual test where there was a significant difference at the same speed. A few guys on the Hatteras owners Forum tried it. I once had the need to do this test myself on a chartered Mainship 430 with Floscans over the course of 120 miles when one engine wouldn't start. I don't have the stats anymore, but it was basically a push, at some speeds slightly better with two.

I think Mark summed up the main advantages of singles; I'd add to that easier to protect the running gear behind the keel and with a skeg and shoe. We bought a twin engine boat, because that's what the type of boat we wanted for our purposes had; if a single engine boat met our other criteria (the 48 Krogen Whaleback came the closest, but not close enough) we would have happily bought one. Look at all the single engine airplanes; I've flown happily as a passenger in those, knowing the owner/pilot was on top of maintenance.
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Old 04-26-2015, 12:16 PM   #18
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Maybe a more realistic evaluation could be done with the same length GB single and twin, or even a KK 42 which also had several twins mfg.
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Old 04-26-2015, 01:03 PM   #19
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I was not interested in opening the whole can of worms just the fuel burn issue. There are good reasons for a single and good reasons for twins and hopefully the builder and designer matches the propulsion to the boat and visa versa. Tony Fleming and Fleming yachts has done some long term thinking and testing relative to the twin verses single motor and fuel burn and range were included in their decision to stay with twin motors. I come across owners of the typical Trawler type like the Helmsman multiple Asian build's including GB etc. who just assume their single engine boat is more fuel efficient than my big motor twins and it just isn't so even when the boats are close in size and weight meaning with good design you can have twins and good fuel burn.
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Old 04-26-2015, 02:17 PM   #20
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The efficiency of the engine itself running in the boat and the way the engine is matched to the power requirement of power for the boat and that in turn matched to the engine speed that produces the best efficiency for that engine. The difference between a single and a twin isn't worth talking about if both are run at 60 to 75% load.

The main reason singles are considered more efficient that Twins is that Twins are almost always equipped w twice the power. So while cruising at any economical speed the twin engines will be very underloaded. A twin running at the same load as a single will be just as efficient as a single except for the consideration of the drag of the keel and exposed running gear of the twin. Probably fly stuff.

So load the twin the same as the single and there won't be enough difference to talk about.
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