While talking with the folks over on the trawler forum about fuel efficiency, someone mentioned something that I hadn't considered before. 

Since we are running dual engines in our Commanders, does running on one engine only, damage the other?

For instance, my port engine seems to burn less gas than the starboard. Would it be ok to use only my port engine to idle down the ICW, which is usually a very straight course with very flat water? Or would the freewheeling of the starboard engine damage the transmission or the engine itself?

(I still haven't sold her either, and it looks like I may be taking her off the market pretty soon and continuing with the outside cosmetic work.)

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In short - yes, freewheeling can damage the transmissios. The way to do it properly is to secure the non-engaged transmission somehow to avoid “freewheeling.” This can be done with vice grips, ratchet straps, etc.

I often run with one shaft locked out and agree that wrenching off a shaft for short periods of slow running operation can be done safely but if you plan on doing quite a bit of single engine operation then a more substantial solution needs to be addressed. For instance installing a blocking post with enough beef to handle the wrench pressing against it, a determination of where the wrench could go and what it could damage should whatever is holding it fails. In my case the wrench would end up hitting my fuel tanks. You also need to set up a watch schedule to ensure the wrench is still in place as a three hour jaunt down the inter coastal without knowing the wrench has fallen off or started to slip means you may have damaged your tranny or shaft either of which could be  2-3K to repair and that would buy quite a bit of fuel. You also need to determine if your boat can steer well enough on the current rudders with only one engine running to avoid a potential obstruction or wayward boat without having to go to the bilge and disconnect the wrench, otherwise you may need to think about larger rudders. Not being able to steer around a drifting boat or navigate safely through a bridge opening can also ruin a good day on the water. It can be done, just make sure to do it safely.

And the “why” to this discussion is because with the engine off the clutches are turning and creating friction because the front pump is not providing fluid pressure to keep the clutches cool and wet.

Could you leave the transmission in gear? If you're moving slowly I can't see the prop turning the engine over?

The clutches will release when the engine stops due to no hydraulic pressure to hold them. 

Steve said:

Could you leave the transmission in gear? If you're moving slowly I can't see the prop turning the engine over?

The 327 isn't going to be much different than the 307q in fuel burn and the saving of running on one engine probably is non-detectable without flow meters. 

Chart

https://api.ning.com/files/W8zFTI3S9Em2JrZqLG*HR9vKoDZEyePIAzI-O0vx...

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When we race sailboats, we turn off the engine and put it in reverse, which locks the shaft and keeps it from spinning. What is the difference between the two scenarios?

Transmission design 

Kevin Fitzpatrick said:

When we race sailboats, we turn off the engine and put it in reverse, which locks the shaft and keeps it from spinning. What is the difference between the two scenarios?

Kevin sailboats use gear to gear with a clutch dog and synchronizer for direct shifting very similar to a stern drive. The oil is only there for cooling and lube. Powerboats use a hydraulic trans of the same design in your vehicle. The shifter transfers hydraulic pressure (oil) to lock in clutch discs to engage or lock in the gear. When the engine isn’t running the pump supplies no pressure. Hope this explains it for ya!

Thanks Lee! Makes sense now.

Lee H. Dahlen said:

Kevin sailboats use gear to gear with a clutch dog and synchronizer for direct shifting very similar to a stern drive. The oil is only there for cooling and lube. Powerboats use a hydraulic trans of the same design in your vehicle. The shifter transfers hydraulic pressure (oil) to lock in clutch discs to engage or lock in the gear. When the engine isn’t running the pump supplies no pressure. Hope this explains it for ya!
Lee,

What about having the 2nd engine idle in neutral? Would that provide enough cooling, etc. to keep that engine's trans safe and cool?

That would also give you a backup in case of emergency, etc. No need to undo a locking wrench.

I'm wondering where that would put you on fuel burn overall. Our 8.1s burn 1.1 gallons per hour at idle. Might be a good compromise for a long run slow ride.

-Darin

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