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Old 10-24-2017, 12:12 PM   #21
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These discussions make me think of a wreck dive i've done numerous times off Boca Raton, the Hydro Atlantic. German ocean dredge setup with 4 diesels, driving 2 shafts and a two story engine room. Great place to do these 1 vs 2 vs 3 vs 4 experiments. before it sank, of course.
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Old 10-24-2017, 03:20 PM   #22
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Chris Craft offered their 62í Motor Yacht w three CC flat head 6cyl engines of 160hp. Very big engines for an old gasoline 6. Lots of cast iron.
I attended a Chris Craft convention at Maple Bay BC about 20 years ago. There was one boat with triple Chrysler 426 hemi's. I'm not sure if they were original, but it must have been one efficient boat.
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Old 10-24-2017, 04:10 PM   #23
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The Fleming is a solid glass hull with no coring accordin' to the article...that has to be a plus.
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Old 10-25-2017, 10:27 AM   #24
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I attended a Chris Cnraft convention at Maple Bay BC about 20 years ago. There was one boat with triple Chrysler 426 hemi's. I'm not sure if they were original, but it must have been one efficient boat.
AusCan,
So smooth and quiet it surely must have been a pleasure to cruise.
But I canít see efficient. What would be more inefficient for a big CC cruiser? The hemi engines were prone to detonation because there was so little turbulence it the symmetrical and compact combustion chamber. Compression ratios were very limited compared to other engines of the day. Efficiency? ... probably a tad above a Buick straight eight w a c/r of less than 7-1.
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Old 10-25-2017, 01:42 PM   #25
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Hello all, I'm new here and you may discover that I'm sort of a science geek.

The "efficiency" being described is a bit of a red herring. What is being discussed, engine efficiency or vessel efficiency? Engine efficiency is simply a ratio of fuel energy to work, i.e., how much work can you do with the same liter/gallon of fuel? In general, fewer engine cylinders equal higher efficiency due to increased frictional losses as the number of cylinders is increased. My single-cylinder 650 cc motorcycle engine will always operate more efficiently than a multi-cylinder engine of the same displacement, with decreasing efficiency as the number of cylinders is increased.

Duplicating the single engine in a twin-engine configuration doubles not only internal friction losses (cylinder walls, rod bearings, valve trains, etc) but also adds new losses in a second gearbox, shaft and prop.

Owners, operators and builders of commercial cargo ships are motivated by one thing and one thing only: optimizing profit. These ships use single engines, enormous great beasts, with a relatively small number of cylinders.

Performance? Sure, add another engine or two. But if you're really looking at engine efficiency...the work obtained from each tank of fuel...it will decrease as the fun factor increases.

As far as loping along at low revs versus operating at higher RPMs, this is complicated by different engineering factors in modern diesel engines. In general, higher revs result in higher friction losses...but not always. In general, higher revs means a more favorable effective compression ratio and therefore increased efficiency...but not always. Direct injection versus indirect, variable turbocharging, electronic injection and/or valve timing...it just depends upon the design of the engine, whether it's designed to be operated at higher or lower revs.

Now if we're talking about vessel efficiency, effectively miles per gallon, the comparison of number of engines is just silly because hull shape and length are left out of the equation.

As was astutely pointed out by a previous poster, it's really apples-to-oranges to compare a multi-engine, semi-displacement vessel with a single-engine, pure displacement vessel designed to travel much more slowly. They're designed for vastly different performance and for vastly different owners.

Oh, and the single versus twin thing? Which is more efficient? What does the math reveal?

Who cares? The one for me is the one that gives me the biggest grin. That's why I chose a single-engine vessel...not because of percentages and ratios, but because it's the one I LIKE.
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Old 10-25-2017, 03:01 PM   #26
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Hello all, I'm new here and you may discover that I'm sort of a science geek.

...
Sabre, a very thoughtful, articulated and scientific response.

Thanks.

I don't have the patience to explain as well as you, as all I was going to say was:

No, it's not a matter of opinion, but simply science and physics.
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Old 10-25-2017, 06:14 PM   #27
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Cruise ships typically have about five large diesel engines to generate electricity, no doubt not all are continuously used. Must be interesting to balance the electrical load as they spend much of their time anchored or docked.
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Old 10-25-2017, 07:13 PM   #28
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Sabre602 wrote;
“Duplicating the single engine in a twin-engine configuration doubles not only internal friction losses (cylinder walls, rod bearings, valve trains, etc) but also adds new losses in a second gearbox, shaft and prop.”

Why would there be added losses? One would assume trying to approach apples and apples an appropriately smaller gear, shaft and prop. Seawater pumps, cyl walls, rod bearings valve trains ect ect would all be 1/2 the drag of the bigger single engine. So both should have very close to the same % of loss per hp.

But I’ve never seen or heard of a comparison that involved 2 boats and engine combinations that would yield an actual apples to apples comparison.

Thanks for your very excellent post #25. Hope you become a long time poster and member.
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Old 10-25-2017, 07:50 PM   #29
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Old 10-25-2017, 10:35 PM   #30
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I commercial fished a 55' boat for salmon and tuna off most of the northern west coast. I had twins, uncommon, as most people had singles, but most engines were Detroits. I got similar mileage or better than folks with similar sized boats. After I sold the boat, the new owner converted to a single, from 2 Detroits to a Cummings and got slower speeds at higher fuel use. Not a scientific study, but the same hull.
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Old 10-25-2017, 11:02 PM   #31
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Lepke, that`s a basis for comparison, though knowing the hp of the engines would help. It might not be so helpful if the Cummins was small for the boat and had to be run hard all the time.
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Old 10-26-2017, 06:58 AM   #32
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I have seen same hull with twin vs single comparisons for Nordhavn 55/60, KK 52 and Nordic Tug 52/55. In all 3 cases the engines were same brand but obviously different size and HP.

The NT comparison data was written up in PMM. I have the written comparison sheets from KK and Nordhavn. In all cases the data shows an approximate 9 to 12% fuel nmpg advantage for the single. Not material IMHO unless one is planning a transoceanic trip where every bit of fuel becomes sacred.

For the outboard set, tons of data for same hull using twins vs triples or single vs twin. "Boating" magazine is your friend for this data with results all over the map.
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Old 10-26-2017, 08:55 AM   #33
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Sabre602 wrote;
ďDuplicating the single engine in a twin-engine configuration doubles not only internal friction losses (cylinder walls, rod bearings, valve trains, etc) but also adds new losses in a second gearbox, shaft and prop.Ē

Why would there be added losses? One would assume trying to approach apples and apples an appropriately smaller gear, shaft and prop. Seawater pumps, cyl walls, rod bearings valve trains ect ect would all be 1/2 the drag of the bigger single engine. So both should have very close to the same % of loss per hp.

But Iíve never seen or heard of a comparison that involved 2 boats and engine combinations that would yield an actual apples to apples comparison.

Thanks for your very excellent post #25. Hope you become a long time poster and member.
I suspect the efficiencies of a single 500hp is only slightly better than 2 x 250HP of similar design. Less windage loss. But, I think the far more major impact is the two shafts and props, vs one. Similar to a twin aircraft air drag with two on the wing rather than a single. Same GPH, but less speed for a given equal HP.
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Old 10-26-2017, 10:01 AM   #34
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Cruise ships typically have about five large diesel engines to generate electricity, no doubt not all are continuously used. Must be interesting to balance the electrical load as they spend much of their time anchored or docked.
Most of the cruise ship's electricity consumes other than the vessel's movement. Lights, air conditioning, cooling, etc. Energy in the hotel areas and other common areas. In addition, the ship has its own diesel engines for propulsion hydraulics and other items with its own machines. I think 3 engines run general electric and one engine run ship movemen normal speed and one engines is Depending on the need for extra power or maintenance work on other machines. When a ship is doging it does not take electricity from the beach but is self-sufficient

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Old 10-26-2017, 10:23 AM   #35
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SO MANY VARIABLES.
I don’t think an apples to apples comparison exists. Trawler manufacturers looked at twins a a 8cyl car w a 4 barrel v/s a 6cyl w a single barrel. The designers, builders and marketing men probably all grew up in the 50’s and 60’s when deluxe cars had more chrome and power. I know I’ve said it before but we’ve probably spent 1000 times as many hours piloting cars as boats. We knew if the guy down the street had a V8 or a 6. We live and have lived in an automotive society.

Sunchaser I remember the Nordhavn twin v/s single comparison but not well enough to remember the specifics. As I recall the single had turbos and the twins did not or some other similar difference. At first I was excited that finally we had a good comparison .. but it was not. As I’ve said in post #28 I don’t think a good example of twins/singles example exists. Lepke points out one very close. If only the twin and single were both w DD .. but no. The configuration that burned less fuel was the twin. But it had the DD engines and 999% of members here think they burn much more fuel. But maybe the Cat had a small (comparatively) prop. Or some other feature like very dissimilar power ect. Looked like a good example at first though.

And the twin w two exposed prop shafts is usually mentioned as more drag that would render the twin less efficient. But if the boat had the same power engines the twin would have shafts w considerably less dia and the keel (on the single) may be smaller (not as wide) not having to accommodate a prop shaft.

I think a boat would need to be powered w engines props ect that didn’t favor the twin/single factor and had the same power. Don’t remember but I could bet money on the Nordhvn Sunchaser alluded too had more total power w the twin installation.

I don’t think there is an answer to this question. An engine is going to burn what it burns and if there’s two or one probably dosn’t make much or perhaps any difference.
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Old 10-26-2017, 12:16 PM   #36
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The Nordhavn and KK models I noted have twin keels. Hard for us non designers to quantify the increased drag. Also on a FD hull the drag considerations are different than on a SD hull operating on plane.

As stated, so many variables but for those in the know (NAs) answers are quantifiable. But you wonít find these guys on TF.
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Old 10-26-2017, 12:41 PM   #37
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Sunchaser,
On the twin keel boats do they disperse w the ctr keel?
If not that would be extra drag. Perhaps it dosn’t amount to much because of the slow speeds of these boats.
Why, do you think twin’s have the props so far apart? I would think a twin w props close in by a traditional keel would be ample protection for the prop w the boat high and dry laying on a significant list. Probably most buyers consider the asymmetrical thrust for maneuvering to be more important than prop protection. But we have thrusters for that. I think I’d rather have CR props close in. Better in the kelp too.
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Old 10-26-2017, 01:08 PM   #38
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IDonít remember but I could bet money on the Nordhvn Sunchaser alluded too had more total power w the twin installation.
Don't bet on it.
Lots of variables yes. 2" shaft vs 3". 2 4 blade props vs one 5. Extra weight from the 10 percent greater fuel capacity and 2 one ton engines. But horsepower remains the same.
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Old 10-28-2017, 09:23 AM   #39
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The Navy uses transmissions that allow two engines to be coupled to one shaft.

They are found in LST and similar work boats.

There have been built since WWII so used or rebuilt are easily found.

A twin with a modest engine (3-5GPH) for long range and a defuler (20-50GPH) for coastal cruising would be easy to do.
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Old 10-28-2017, 09:39 AM   #40
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The Navy uses transmissions that allow two engines to be coupled to one shaft.

They are found in LST and similar work boats.

There have been built since WWII so used or rebuilt are easily found.

A twin with a modest engine (3-5GPH) for long range and a defuler (20-50GPH) for coastal cruising would be easy to do.
I've seen these, but never fully understood them. They must have a clutch for both input shafts, and then the usual F-N-R clutch after that? And, I never seen a "small one".
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