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Old 10-04-2015, 08:26 AM   #1
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Twins on a single shaft?

This article in Soundings mentions two boats running twin 6-71s on a single shaft:

Chesapeake T-boats, happy in retirement | Soundings Online

How are those likely connected? Some kind of combining gear system? Or...?

However accomplished, does it mean one can run on one engine when desired, two when needed? Or would it always be on both?

If complete flexibility is there, would maybe seem similar to a get-home installation, with a main and an auxiliary (or genset)? Wouldn't need both engines to be large...

??

-Chris
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Old 10-04-2015, 09:01 AM   #2
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Greetings,
Mr. 42. Twins on a single shaft have always been attractive to me and from what I understand do allow the luxury of running on one via a gearbox (the workings of which I am totally unfamiliar). As mentioned in the T-boat article, the T-boats originally came with a single engine.
There is another member on TF (hasn't posted in a long time) that has just such a twin to single set up. http://www.strathbelle.com/
Some Scandinavian rescue vessels are also set up with twins on a single shaft with a variable pitch propeller. Although full spec's are no longer listed for this vessel I think it was set up with the 2 to 1 configuration... Shipsforsale Sweden - The Scandinavian Shipbroker
I was all set to pull the trigger on her but the Admiral took the wind out my sails REAL QUICK!
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Old 10-04-2015, 09:29 AM   #3
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The two engines shared a gearbox. The only reason this was done was to get more power into the shaft from the 671 engines that were in rapid production during WW2. They even made a gearbox with 4 671's. I think you could clutch out the engines to run on few than all, but not sure if that feature was in all gearboxes. If more power than that was needed, an 8- or 12-567 was used.

Later, I think in the 50's or early 60's, they came out with the v-block 71 and the 12-71 and 16-71 took the place of the twin and quad 671.

It was sort of a short term fix to get something around 400-800hp using the successful 671 200hp that was at hand. There was not a good single engine in that hp range in quantities the military needed. The 567 and cleveland 278 were in the 1000-1500hp class.
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Old 10-04-2015, 09:30 AM   #4
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I saw a boat Yacht World years ago that had two engines vertically stacked and one shaft.

It was a shippy looking steel pleasure boat in the 50' range if memory serves correctly.
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Old 10-04-2015, 09:32 AM   #5
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Quote:
Originally Posted by ranger42c View Post
This article in Soundings mentions two boats running twin 6-71s on a single shaft:

Chesapeake T-boats, happy in retirement | Soundings Online

How are those likely connected? Some kind of combining gear system? Or...?

However accomplished, does it mean one can run on one engine when desired, two when needed? Or would it always be on both?

If complete flexibility is there, would maybe seem similar to a get-home installation, with a main and an auxiliary (or genset)? Wouldn't need both engines to be large...

??

-Chris
I think the Nordhavn system is probably simpler; standard main engine with single shaft, and a small donkey engine on a separate shaft/ folding prop to get you home.

These folding prop engines on shafts are standard issue from Yanmar and Volvo for sailboats, and come in 70hp units. At 5hp/ton that can push quite a big boat along at displ. Speed.
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Old 10-04-2015, 09:54 AM   #6
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These are sometimes used on drivelines with variable pitch propellers, take a look here:

Finnøy Gear & Propeller

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Old 10-04-2015, 12:22 PM   #7
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These are sometimes used on drivelines with variable pitch propellers, take a look here:

Finnøy Gear & Propeller

Interesting site.

If only they made 2 into 1 boat gearboxes, instead of ones for ships. I wonder how heavy the lightest version is?
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Old 10-04-2015, 12:40 PM   #8
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Interesting site.

If only they made 2 into 1 boat gearboxes, instead of ones for ships. I wonder how heavy the lightest version is?
the lsts had 4 engines on one shaft
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Old 10-04-2015, 01:07 PM   #9
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the lsts had 4 engines on one shaft
I have a friend who was an engineer on coasters, 150-200' small cargo ships . I was surprised to learn that they have multiple engines.

They cruise along on the small 500hp wing engine when the conditions are good and the load is light, turning on the main 3000 hp engine when loaded up. So commercial vessels are constantly selecting the right engine for the right load.

Of course they have take on water ballast to make sure the vessel doesn't turn turtle when lightly loaded; that would be difficult for us leisure boaters!
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Old 10-04-2015, 01:10 PM   #10
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Some of the CG Light ships had the 4 6-71's running through a Falk quad gear box.
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Old 10-04-2015, 02:18 PM   #11
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About ten years ago Ramsey Silent Chain built "stackable" chain drive units for a Navy competition for a twin screw fast assault boat (40 foot range). The boat had four 600 HP Yanmars, with two on each prop shaft. I spoke with Ramsey engineers about using the chain boxes for converting a typical twin engine mid-size cruiser to one engine driving two props. The Ramsey engineer said they actually did a private cruiser conversion with this set up. Simple, light weight design with pressure lube in each box. Power sizing is managed with chain width. Ratio changes are as simple as adjusting tooth count on the sprockets. One way clutches in the Navy installation to deal with engine failure or shutdown. Project was eventually cancelled by the government(because of funding prioirities).
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Old 10-04-2015, 02:35 PM   #12
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I have four DD671's - two per shaft. Single gearbox per shaft. Each shaft protected by Kort nozzles and flanking rudders.
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Old 10-04-2015, 02:48 PM   #13
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The boat mentioned, the Mi-T-Mo, has two 671 detroits mounted on a big Falk and it is set up for 4 engines- two facing forward and two facing aft. In Mo's case the engines sit side by side facing forward. Each engine is clutched so you can run one or the other or both. It is a nice set up but it would not work well in anything that was not fairly large. The box is gigantic probably about 5 feet across and long and quite deep too. Stout is a word that comes to mind. We had the great good fortune to have a tour of Mo and a short ride on her when we were in Baltimore a few years ago. Mike is a great guy and makes handling such a large boat look like a piece of cake. I see, looking on YW, that Mo has recently sold. Someone got a great boat for sure.
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Old 10-04-2015, 03:40 PM   #14
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I have four DD671's - two per shaft. Single gearbox per shaft. Each shaft protected by Kort nozzles and flanking rudders.
Please elaborate. What are Kort nozzles and flanking rudders? How are they used, etc. Thanks!
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Old 10-04-2015, 04:16 PM   #15
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A recent visitor to our dock was an early 1960's Romsdahl 65. The owner had recently purchased her and was in the early throws (I know it is "throes" but he is still throwing things out) of a major rebuild.
He purchased her with two diesels that ran a single prop; the port Lugger was slightly forward of the starboard Volvo and they were connected by belts to a pulley mounted on the propshaft. The prop is the variable speed type.

He has since removed the inoperable Volvo and is happy with just the Lugger; no current plans to replace the Volvo.
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Old 10-04-2015, 05:02 PM   #16
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A recent visitor to our dock was an early 1960's Romsdahl 65. The owner had recently purchased her and was in the early throws (I know it is "throes" but he is still throwing things out) of a major rebuild.
He purchased her with two diesels that ran a single prop; the port Lugger was slightly forward of the starboard Volvo and they were connected by belts to a pulley mounted on the propshaft. The prop is the variable speed type.

He has since removed the inoperable Volvo and is happy with just the Lugger; no current plans to replace the Volvo.
I think that was called a HYVO drive setup. Blount marine made a similar set up on a friends 60' research boat. A remote mounted gearbox with a belt to an output shaft on a high block 6-71 (SCREAMING Jimmy!). Robert Beebe talks about these in the original VUP.
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Old 10-04-2015, 05:05 PM   #17
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A recent visitor to our dock was an early 1960's Romsdahl 65. The owner had recently purchased her and was in the early throws (I know it is "throes" but he is still throwing things out) of a major rebuild.
He purchased her with two diesels that ran a single prop; the port Lugger was slightly forward of the starboard Volvo and they were connected by belts to a pulley mounted on the propshaft. The prop is the variable speed type.

He has since removed the inoperable Volvo and is happy with just the Lugger; no current plans to replace the Volvo.
I'm just wondering what happens in both engines are driving the shaft, but get out of sync?

Would the belt drive tear itself apart?
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Old 10-04-2015, 06:11 PM   #18
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I'm just wondering what happens in both engines are driving the shaft, but get out of sync?

Would the belt drive tear itself apart?
My thoughts as well, and I never thought to ask the owner while he was here. Although he never ran it on both engines due to the inoperable Volvo, so may not know.
It's especially interesting considering that he had two different makes of engines.
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Old 10-04-2015, 06:34 PM   #19
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Wonder if that setup was for combined HP or simply redundancy.

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Old 10-04-2015, 06:47 PM   #20
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Greetings,
Mr. OC. I think, in the case of the Swedish rescue vessels it was for extra HP for towing if necessary with the benefit of redundancy. The VP prop could make the best use of either the one or two engines. What I would have loved was a single driving two props but I'm WAY too old to embark on THAT exercise.
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