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Old 09-26-2013, 11:23 AM   #1
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Question 2 Engines ONE PROP SHAFT

Hello. I need some expert advice. We are looking at 2 boats that have twin engines but only one shaft One prop. What is the advantage of that, and does that mean if one engine should quit that you can not use the other one to make it to shore?
Is the good engine now pulling the dead engine around??
I know there is many Mechanical experts on here so I need some of you to tell me the real reason for having this set up and how it works in a break down.
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Old 09-26-2013, 11:41 AM   #2
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I saw a boat like that for sale several years ago. The two engines were stacked vertically in the engine room.

It appeared to be a neat concept, but I never go my head wrapped around its functionality.
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Old 09-26-2013, 12:23 PM   #3
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The ones I have seen were military units (LST) or similar and had (2 ) 6-71 hooked up to an Allison unit.

Either or both engines could power the shaft. Either engine could be disconnected from the shaft by manually pulling a lever.

This was a dog gear so the boat has to be stopped , with engine off to reconnect .

The concept was with a single engine taking a round , the vessel could still operate.

Today for a cruiser it would be a superb setup.
Both engines when making huge waves and wish to defuel at speed.
A single engine for long range cruise.

If the engines were identical one could be robbed for operating parts.

Even with similar engines 3-71 and 6-71 there are loads of parts commonality.

The 3 would be good for 60-90HP cruise , the 6 for 120-180-HP cruise (with modest sized injectors), and the pair for big wake making.

With a large diameter shaft abd prop , protected in the centerline the boat should be able to push mud and not loose the wheel.

A CPP would be icing on the cake , but there fairly expensive , worth it for a circumnavigation , but probably not for most other recreational use.

Great find!
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Old 09-26-2013, 12:32 PM   #4
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That arrangement was used by WW2 tanks, and as FF noted, military boats. There were a number of RCMP boats built in BC in the 50s/60s using a pair of 6-71s on a combining transmission. My brother owned one for a while. One engine at a time would give a decent cruising speed, together made it fast enough for RCMP pursuit purposes, but again as FF noted, "defuel at speed".
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Old 09-26-2013, 02:59 PM   #5
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Old 09-26-2013, 03:13 PM   #6
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If I recollect, Phil/Fill's Roughwater 58 has a cone/spline PTO coupling with a chain drive set up on the same shaft as his single DD671 single engine for get home. I've had this set up on at lease one John Bean fire pump that ran the same way off the main driveshaft of a 1947 Ford fire engine pumper.....very reliable and with the cone spinning on the driveshaft when disengaged, seemed to be very smooth.
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Old 09-26-2013, 04:57 PM   #7
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Look in the archives.

There was much discussion on a drive in an article in PMM about 5 years ago. Called "Geared up ...." ??

We did have an interesting discussion about it and several advantages surfaced.

Two 20hp engines in Willy would be great but I think this system was one engine two prop shafts.
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Old 09-27-2013, 02:03 AM   #8
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I want to thank everybody for giving me good advice and it is always great education to read your Forum.
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Old 09-27-2013, 02:40 AM   #9
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As an owner of twins, having the back up engine is good, having it drive the same shaft is less good, it could have its own drive system which gives more back up, eg. in case of prop damage or shaft failure. But then you maintain 2 stuffing box systems. Maybe a sail drive/outdrive would work. Few things are black and white.
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Old 09-27-2013, 05:29 AM   #10
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Having 2 engines on one shaft will require the prop and shaft to be sized to the max output of the combined engines.

This usually creates more immunity to a damaged shaft , tho when the shaft wears out , $$$$

Also props are pretty cheap up to 24 inches or so, once you price a 40 inch wheel it can be heart stopping.

Reliability and efficiency are not cheap.
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