Twins are more efficient, got it?

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.

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.

Nomad Willy said:

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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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.

markpierce said:

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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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

NBs

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.

Eric
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 won’t find these guys on TF.

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.

Nomad Willy said:

IDon’t remember but I could bet money on the Nordhvn Sunchaser alluded too had more total power w the twin installation.

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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.

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.

FF said:

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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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".

diver dave said:

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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Yea, each engine has a clutch, then the gearbox has FNR gearset.

These came about when they developed the 6-71 detroit in the late 30's. That engine was about 185hp then and some military apps it was not enough hp. So they ganged them up. Two engines per shaft, and also four engines per shaft in a quad "H" configuration.

It was not done for any efficiency reasons, but for wartime logistics. They had gotten an efficient production line on the very reliable and compact 6-71, but in some apps they needed more hp. There was not a larger hp engine at the time in high production numbers until you got up to the locomotive engines. And in wartime those were needed for locos, subs and the larger landing craft.

After the war, they developed the vee-block 12V71, 16V71, 149 series, etc that took the place of the twins and quads. But by then the war was over. And those twin and quad packs were everywhere in the war surplus market, and could be had cheap. Last one I saw running was only a few years ago. Still running fine!

The transmissions for twin engines have a dog clutch for each engine to pull a bad running engine.

To engage both engines are stopped so the dog can be engaged.

The tranny then is normal F,N R , usually a mechanical if old..

Yes, there not small BUT many operate with no need for water cooling.

My Twin Disc from that era needs 5-Q of non detergent oil and nothing else.

Sorry one last question on the conjoined twins.
With the two locked in rpm , how is throttle managed? Is there a tendency for any instability with hunting? Managed remotely with the throttles is ok?