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Old 29-03-2009, 11:27   #1
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Running Single Engine w/Twins - Advice?

I've got a twin 46' power cat and I usually cruise at trawler speeds (7kts)
I noticed if I shutdown an engine - I still make 6.5kts at same power setting (1400rpm),, so on a long slow cruise could i just run on one engine?
Is there any problems with this? I wonder about the prop free spinning on the non-running engine, will it burn up without the water cooling from the engine?
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Old 29-03-2009, 19:42   #2
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I don't know a thing about whether or not it's OK to freewheel a prop with your boat's setup (we have feathering props), but assuming it's OK, sure... We do this on our catamaran - burn 1/2 the fuel but still do better than 1/2 the speed.
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Old 29-03-2009, 19:46   #3
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It's usually best to put the non running one in gear to stop rotation. I dont know on a boat with outdrives if you have them though....
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Old 29-03-2009, 19:49   #4
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All the Leopards are delivered South Africa to the Moorings in the Caribbean on one engine. It's called "trailing". Just check with your transmission manufacturer if it should be locked or free wheeling.
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Old 29-03-2009, 21:25   #5
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I've got a twin 46' power cat and I usually cruise at trawler speeds (7kts)
I noticed if I shutdown an engine - I still make 6.5kts at same power setting (1400rpm),, so on a long slow cruise could i just run on one engine?
Is there any problems with this? I wonder about the prop free spinning on the non-running engine, will it burn up without the water cooling from the engine?
I had planned on doing this when mine goes in.

I spoke to ZF who's boxes I have and they said it was ok to let them freewheel, especially seeing as I had planned on a 4 hours on 4 hours off alternation between engines so as to keep even wear on them, though I suppose this would do extra wear on the starter.

I dont think gearbox heat would be an issue as you are travelling slow and not all gearboxes have watercooling anyway, but I did have concerns of my top and possibly middle shaft bearing and my shaft seal being affected frm lack of or limited water lubrication.

What sort of boat and engines Nauti Cat?

Dave
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Old 31-03-2009, 15:37   #6
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We took a 43' Defever, twin 120 Lehmans, with Borg Warner Velvet Drive Transmissions over 15K nautical miles, frequently running on a single to save fuel, reduce engine hours, and remarkably reducing engine noise! The previous owner quoted a loss of about .5 knot, we generally lost less than 1 knot at 1650 rpm, saving about 2.2 gal/hour. The old owner's manual from Borg Warner said it is OK to run on a single, allowing the other prop to "free wheel", no problem at these speeds!
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Old 31-03-2009, 19:00   #7
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Thanks Everyone
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Old 03-04-2009, 10:24   #8
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I have ZF transmissions and the operator's manual says it is ok to let them freewheel. you need to check with your transmission vendor.
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Old 03-04-2009, 10:52   #9
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We only run both engines when we are docking. Otherwise we exclusively use 1. No problems so far.

Running on one engine does suggest that folding or feathering propellors might be even more advantages. Just a thought.
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Old 03-04-2009, 12:00   #10
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Everything I've heard in the past has indicated that freewheeling is less efficient than locking the prop. It's a little sounter intuitive, but I guess it's so....
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Old 03-04-2009, 12:25   #11
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When we motor on Exit Only we use only one engine and usually leave the other engine in gear to reduce the noise of a free wheeling prop, particularly if someone is sleeping in a cabin over the engine.

We avoid running an engine in a cabin where someone is sleeping just in case there is a carbon monoxide leak, and to keep the cabin cooler and quieter for the sleeping crew member.
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Old 03-04-2009, 18:45   #12
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When we motor on Exit Only we use only one engine and usually leave the other engine in gear to reduce the noise of a free wheeling prop, particularly if someone is sleeping in a cabin over the engine.
This would only work with mechanical boxes though surely?

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Old 03-04-2009, 20:22   #13
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This would only work with mechanical boxes though surely?
Dave
We had Yanmar 3GM30F engines with mechanical transmissions. We have never had a problem with our gearboxes. For the first half of our circumnavigation we allowed the prop shaft to free wheel, and for the second half we kept the transmission in gear. The only difference I could detect was that it was quieter with less wear on the cutlass bearing when left in gear.
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Old 04-04-2009, 06:29   #14
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Fair enough, my ZF25's attached to Cummins B3.3's are a hydraulic box, so no pressure unless engine on.

Plus these will cause a bit of drag while locked.




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Old 04-04-2009, 07:13   #15
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Discussions on freewheeling are bit like those on the correct anchor - everybody has opinions.

However, there is a difference. With freewheeling, some gearboxes need the shaft locked, and some need the shaft not to be locked.

YOU MUST READ THE GEARBOX MANUAL
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