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Old 04-09-2015, 03:06   #1
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Efficiency of single vs twin engine

ok don't kill me please

i know the thread has been very widley discussed but...

i was looking at the power vs liter per hour cosumption for a given boat with x shaft horsepower required to reach a y speed.

i calculate i would need about 180 shaft horse power (133kw) for my boat to reach the hull speed (displacement trawler) 46ft 35 ton (70.000lbs) 9 knots hull speed

take a look at this 3 engine


http://www.nannidiesel.com/downloads....115%20ITA.pdf

http://www.solediesel.com/portals/0/...as/sd21_en.pdf

http://www.solediesel.com/portals/0/...as/sd28_en.pdf

2 x nanni diesel not very big and not much efficient (245gr kw at rated speed)

or 1 serious heavy duty sole diesel in 205 hp version or the same engine 272 hp version



let's make 3 example; i take care to convert kw to shaft horse power to read the nanni's power graph


88kw needed (running 85% hull speed) cruise speed

2x nanni at 1200rpm, 2 lph each, tot 4liter hour

1x 205 sole diesel about 1300rpm 7,5 liter per hours

1 x 280hp sole diesel 1100rpm , about 8 lph




120kw almost full speed

2x nanni 1400rpm 3,25 lph each, tot 6,5lph

1 x205 sole 1700rpm 17lph

1x 280hp sole 1450 rpm 16 lph



140kw needed max speed + rought water

2x nanni 1800rpm 7 lph each total 14 lph

1 x205 sole 2100rpm 32lph

1 x 280 hp sole 1600rpm 21lph


conclusion:

two it's always cheaper to run...but more expensive to mantain, and so on...as all the ordinary discussion..

it's since i began to plan my boat that i'm squeezing my brain about one vs twin engine...



let's the fight begin
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Old 04-09-2015, 04:59   #2
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Re: efficiency of single vs twin engine

Point ➥ Single Or Twin Engines- Which Is Best? | | PassageMaker

Counterpoint ➥ LetterToPassageMaker
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Old 04-09-2015, 05:07   #3
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Re: efficiency of single vs twin engine

I hadn't read Gord's articles, but I can tell you it's always cheaper and more efficient to run one engine, but two gives you docking ease and redundancy,
I ran twin engine outboard Center Consoles for a few years, never had a single engine problem ever, but did have a hydraulic steering hose blow, rendering both engines almost useless, and did once have severe water in the fuel problem, killing both engines as I only had one fuel tank.
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Old 04-09-2015, 05:19   #4
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Re: efficiency of single vs twin engine

Many threads on the topic on trawlerforum.com


I think a common conclusion is that when using cash outlay as a litmus test, a single (assuming appropriate power) is always more "efficient" even when compared to twins that are also in the "appropriate power" range (i.e., not oversized).


But that doesn't take into account other considerations, like get-home, easier docking, etc. These can sometimes be solved/mitigated in other ways (PTO from genset? Thrusters?), but anyway, at that point, it might depend on what your definition of "efficient" is.


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Old 04-09-2015, 05:23   #5
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Re: efficiency of single vs twin engine

I only wish for a second engine when the one fails.
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Old 04-09-2015, 05:46   #6
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Re: efficiency of single vs twin engine

I read all the articles on passagemaker and so on at least 5 times lol


what do you think about my numbers? Why does according to me twice is more efficient than one?
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Old 04-09-2015, 06:31   #7
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Re: efficiency of single vs twin engine

Quote:
Originally Posted by stefano_ita View Post
what do you think about my numbers? Why does according to me twice is more efficient than one?
Can't tell, but it seems to me you've used engine HP instead of prop HP for your calculations?

Any changes to conclusions would be proportional, so maybe not much difference in outcome.. but you might be better off for planning using propshaft HP anyway.

Unless I've mis-guessed...

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Old 04-09-2015, 07:18   #8
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Re: efficiency of single vs twin engine

You are comparing fuel consumption when running at very high speed. Why?

One reason to get a full displacement boat is for fuel economy and the boat should be run at a Speed Length Ratio somewhere between 1 to 1.2is but certainly not at 1.34. For the boats we are looking at owning, I think most owners are running at 1.0-1.2ish.

On the engines I have been looking at, the fuel burn per prop HP is about the same. The problem is at running at WOT because some engines are using more fuel for about the same HP. In the US, we also have EPA requirements on engines we might use, and the latest engine consumes more fuel for about the same maximum HP than the previous engine. This does not matter when running at reasonable speeds below hull speed...

Where it DOES matter is that you need to properly load the engine. Loading being the fuel burned per hour divided by the maximum fuel use. Some say 30%, 40%, 50%, 65% loading is needed on the engine OR that you need to run WOT for an hour every day to clean out the engine. An engine with a higher WOT fuel burn makes it more difficult to properly load the engine at the speeds one is going to use for a full displacement boat.

I would suggest comparing the fuel burn at the speed you will run most of the time. You will have to find the engine's power curves to do this and those power curves are not always easy/possible to find.

Gerr has several formulas to calculate HP requirements, Skeene has at least one, and there is Beebe's equation. Gerr's formulas produce a higher HP requirement for a given boat, followed by Beebe and then Skeene. Which is correct? I will be danged if I know.

And these numbers are for flat water, no current or wind so which ever equation is used, a fudge factor has to be added. But how much? And how much HP is needed to run the alternator(s) or any hydraulic pump(s)?

Later,
Dan
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Old 04-09-2015, 07:48   #9
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Re: efficiency of single vs twin engine

Let's take your first example:

nanni: To develop 88kw at the prop it needs to turn at around 2100rpm, which corresponds to around 10l/h or 20l/h combined.

Sole: to deveope iikw at the prop needs to turn at around 1900rpm, which corresponds to around 24l/h.

While the twin engines still appear to be a bit more efficent, it's not nearly the difference you were showing.

Of course, these graphs come with many assumptions that may or may not be correct for a specific installation. The obvious one I can think of is a large propeller tends to be more efficent at transfering power to the water. The single big engine is likely harnessed to a bigger more efficent propeller.

At 120kw, the sole is around 32l/h and the nanni is around 30l/h (15 each).

This is all pretty consistent with standard experience where if you can resist using the extra power, a boat with two large engines will come pretty close to the efficency of a single engine boat.
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Old 04-09-2015, 08:10   #10
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Re: efficiency of single vs twin engine

I think Ranger hit on it, if you size your engine so if you only have one, then your over engine running two, from an efficiency standpoint.
Example my first twin engine boat was an outboard, back in the late 70's, 23' Roballo, with twin 70 Evinrudes, one one engine, even with the second trimmed out of the water, it wouldn't plane, not even close. Now this is a planing boat. Size the engines so that it would plane on one engine, and running two, they are over sized.

But if you define efficiency as in fuel burn, cost to purchase and cost to operate, a large single beats two smaller ones every time.

But the purpose of twins to a large extent is what Weavis said, when one quits, it's awful nice to have a spare.
Plus, docking my full keel IP is child's play compared to docking a similar displacement single screw power boat, those tiny rudders don't essentially do anything at all at docking speeds, not compared to a sailboats rudder anyway, but give me twin screws and I can make the boat walk sideways and turn in it's own length.
Those big shrimp boat pilots are experts to not just crash into the dock all the time, way beyond the skill level I have.
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Old 04-09-2015, 10:02   #11
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Re: efficiency of single vs twin engine

yes i calculate (maybe wrong?) about the shaft power, not the propeller power. in this case you are right, the single is just a very little bit un-efficient than the single but far away from what i calculated


well i take care of this tables about hull speed- weight-shaft horse power required.


View image: tableau Perkins


i know that my is just theory.

calculating a cruise speed of about 7,5-8knots (85% hull speed), and for price/avaibility i would use pratically the same propeller on both single and twin engine (except for the twin engine, the half of the pitch).

maybe the single Sole engine 275 hp version, overpowered but used at very low rpm, with a quite more reduction ratio such as 4 or 5,5:1 with a 26"x26" prop (the bigger available to buy in italy at a reasonable price)
in stead of a 2x nanni 2:51 gearbox and 24x17" prop


could be pratically the same if not more efficient alright?

with the $$ saved by do not buying the second engine i can get a very strong bow and stern thruster...

but still not convinced at all
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Old 04-09-2015, 18:56   #12
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Re: efficiency of single vs twin engine

I suppose when you calculated the power required you took into account going maybe 10 knots into a 25 knot headwind where you will have 35 knots of apparent headwind which is a considerable wind drag.
One engine is more efficient than 2 because there is less drag from underwater appendages and probably less total weight. But personally I would opt for the reliability of 2 engines as well as their maouversbility advantage.


Look at airliners. They used to commonly have 4 engines but now the modern engines are so reliable and powerful that they have cut back to 2 because that is more efficient with less drag and weight and a resulting fuel saving. They can safely fly on one engine which is a requirement.


They could conceivably make a single engined jet airliner but it wouldn't be certified and nobody would want to fly in it. It would be more efficient though.


Lockheed Orions on patrol routinely shut down and feather one jet prop engine. Then they have to increase power in the other 3 which is operating them more efficiently. This saves fuel. I've several times traveled in them on patrol with an engine shut down.


If you opt for 2 engines (as I would) consider your power requirements if you did need to shut one down and run on one. After all reliability is one of the reasons for having 2 engines.
The remaining engine is having to overcome the drag of the non operating propellor and appendages. It would also be operating off center and require rudder correction with resulting drag.


With a single engine installation you don't need to consider this because if it fails unless you have a sail you go nowhere.


Consider all that with one engine shut down trying to maintain speed into 25 knots of wind and wave and you might find you need 2 engines each at least 3/4 the power rather than 1/2 the power of a single engine installation.


As I said I'd still opt for the reliability of 2 engines but for me I'd make each engine the same size as a theoretical single. I'd put up with less fuel efficiency and likely a slightly lower top speed than a single. That's just me. I'm sure others have their own ideas.
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Old 05-09-2015, 08:00   #13
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Re: efficiency of single vs twin engine

Just adding to what I wrote; that I'd rather have 2 engines. That's because where I go boating in New Zealand the next stop is South America and I wouldn't want to be blown there. I was once on a chartered commercial vessel off our NZ East Coast and one of the 2 engines overheated and had to be shut down. We returned from volcanic White Island on one engine going a little slower that on our outward trip. If we had one engine I'm sure we could have radioed for help and eventually a fishing boat might have towed us back at great expense.

Needing the reliability of 2 engines might depend on where you intend to go boating. Practically every time I sail out from Auckland on a weekend I see our volunteer Coast Guard towing power boats back. Usually outboard boats, but also larger power boats.
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Old 05-09-2015, 08:35   #14
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Re: efficiency of single vs twin engine

It also depends on the boat design of a particular model. Reading Tanglewood's blog about engine selection it seemed that the Nordhavn 60 twin was 15% less efficient than the single. Or was the single 15% more efficient... On a big boat traveling long distances that is a lot of fuel difference.
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Old 07-09-2015, 19:12   #15
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Re: efficiency of single vs twin engine

If you're planning on cruising offshore, the safety factor should be considered. I've twice come in on one engine, 30 years apart. In the 70's I also returned once with a dead rudder (broken cable). But the twin screw allowed me to steer, maneuver and dock.
My twin Detroit 671 engines use a little more fuel than a single, but the boat moves faster. If I had a single it would probably have to be a 12V-71, so same number of injectors and cylinders to maintain and overhaul.
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