Twin motors

im thinking about buying a 340 with twin 454’s my question is can you run on one motor to save gas

You can run on one motor for emergencies only... certainly not just to save fuel.

Problems are:
no cooling water circulating on the unused shaft (that is still turning)
uneven wear on the running prop/shaft

I didn’t believe this before I bought out 290 with twins but I’ll just put it out there - fuel is one of the cheaper parts of ownership.

You won’t think about it once you own it.

Technically, yes you can. Mainly to limp home or to a port in an emergency. As a practice, no.

All manner of bad things when trying to do it intentionally though. You will not be able to get on plane - basically max speed will be trawler speed. Twin engine boats are designed to have both engines running. If you could get it at speed, handling would be compromised due to the off-center thrust. Often one engine runs certain systems; power steering, alternator, etc.

Honestly, if you are concerned about the cost of fuel from twin engines then you should really evaluate your ability to operate and maintain such a boat. Twin engines = double the maintenance costs. If a part breaks / wears out on one engine you should replace the same on the other engine.

I wanted a bigger boat but didn't want to pay for the maintenance (or fuel, frankly) on twins. That eliminated all Sea Rays larger than what I had and drove me in a very specific direction of possible boats.

I installed fuel flow meters on my last boat (32' Wellcraft with twin inboard 5.7's) and tested that theory. I found that the boat did use a bit less fuel on one engine running at 1200 RPM or less. Any faster and it becomes less efficient. That doesn't really tell the whole story. Running at 5-7mph on one engine becomes challenging just to navigate in any sort of straight line. You end up zig zagging all over the place especially when encountering wind, waves, or wakes - so when taking those inefficiencies into account, my take is that running on one engine is not going to save you anything.

The fuel flow meters did show me that the most efficient running speed was just over 4000RPM. The only problem with that was it was more than the recommended 75% of WOT which would not be good for engine longevity. Each hull/engine combo would be different and the way the boat is loaded also changes things.

By the way, nothing was more depressing to my "Pleasure" Boating than watching in real time, the amount of fuel draining out of my fuel tanks while under way with those meters on my dash! Never again. Let me live in denial until I need to throw another "boat buck" into the fuel tanks.

It's a ridiculous affliction we all have - but afflicted I am...

Jimmy Buoy said:

By the way, nothing was more depressing to my "Pleasure" Boating than watching in real time, the amount of fuel draining out of my fuel tanks while under way with those meters on my dash! Never again. Let me live in denial until I need to throw another "boat buck" into the fuel tanks.

Click to expand...

That's such a common sentiment, but I have to say I don't share it!

My only regret about installing meters was that I hadn't done it sooner. On the first day I had them I started to see so much of the "dock talk" and tales about fuel consumption fall to pieces. I was pleasantly surprised about how efficient my boat does at higher speeds, so for several seasons I ran faster than I ever did while using less fuel.

As the engines aged, I decided to go easier on them. Now most of my time I'm barely on-plane. I have the tabs dug-in and similar boats are passing me, but she runs quieter and it's even nicer to my wallet and to the engines themselves. This speed btw, without the benefit of meters felt inefficient.

I now run with one of my MFD's dedicated to fuel (unless I need it for RADAR)

My advice: run both your engines as the boat was designed to do, but do so while informed. Install meters!

You can run them on one engine.....as long as you never have to dock the boat. The engines are so far from the centerline on 340's that they do not handle well at low speeds.

Twin engine boats are designed to run on both motors.