Cruising on one engine

[Washopay]

From Pt Severn to Simcoe my 3807 drank a lot of fuel. While out playing around today I tried something. Cruising at 2200rpm I was moving along at 8 knots. I scaled back one engine to neutral and only list 1.3 knots in speed. On long voyages, I'm considering running on one engine only - alternating periodically between the two. Is there any harm in this?

Thanks

[RGrew176]

I know a few people who do that on longer cruises. They say it does not harm the engines. Give it a try and check your results.

[waybomb]

I think all you gain is the losses in the one engine, but then you are also working against a non-driven prop.
No matter what, to move xx pounds at xx speed takes xx horsepower.

If at hull speed, I'm not sure how much you'll save. What happens to the transmission if powered by the wrong end and in neutral? Will the oil pump still move oil?

Do you have a flo-scan installed? If you don't, you should. You would be surprised how a few degrees in trim affect fuel usage. You may save way more by monitoring fuel flow constantly.

[Midnightsun]

I did this all the time on my Avanti 32 express. With fuel computer equipped info I can tell you you will save big time in consumption running 1 engine if it is gasoline powered. Diesel is so economical at low rpm that the difference is not worth it. Savings were around 35% overall, I kid you not! I bumped the throttle to 1500 rpm on 1 engine compared to 1200 running 2 and basically kept the same "best" hull speed going. Sure miss the MPG readout but do have this on my to do list on the Carver.

Keep in mind this was on a stern drive boat with fuel injected 383's so there may be some differences in your application.

[Viper]

A couple of cautionary considerations when running on one engine:
- for those with dripless shaft logs that are plumbed in to one another, there is a possibility of the non-running engine ingesting water through this line as it is forced through by the running engine. For this reason, Merc. does not recommend plumbing the two logs together, or running on just one engine if they are.
- under the right conditions; exhaust system geometry, no exhaust shutters, transom exhaust flap, etc., the non running engine can ingest water in swells or due to a following swell resulting from slowing down. The exhaust pressure of a running engine helps minimize this possibility.

I'm not against running on one engine, but one needs to consider more than just fuel economy. If you're going to consider this type of boating, ensure that at least the above items don't pose a risk.

[waybomb]

Lots of unknowns. Driveline repairs cost bags of money. Not sure it's with it.

I still say fuel is the cheapest part of boating.

Even with the go fast.

[Viper]

I'd have to dig deeper but I've wondered before about the tranny's output shaft bearing under these conditions. The tranny is designed with the intent that it will be mostly pushed on under typical boating conditions not pulled on which is the case when running on one engine. While reverse applies the same forces, it's not for long durations. Does this affect internal hardware? Warrants some investigation.

For the most part, recreational propulsion systems are meant to be running when the vessel is underway unless there is a reason such as a failure. If you don't ensure the risks are addressed, fuel economy will be the least of your problems.

[Washopay]

Excellent feedback as always. It was the strain(or lacktherof) on the neutral tranny that had/has me most concerned. I don't think I'll risk it but man the fuel economy would be nice. Thanks all.