Do most merchant vessels (large non military ships) run on diesel, or some other fuel?

Most large commercial vessels run off of what is commonly known as Number 6 Fuel Oil. It is also known by the name of RFO (Residual Fuel Oil) or the more crude term "bunker oil".

Though diesel is technically a type of fuel oil, it is of much higher quality (and price) than the grades of petroleum that are sold specifically as grades of fuel oil ranging from 1-6. With 6 being the lowest/cheapest grade.

Most all merchant ships that i have been involved with burn both Bunker C (not bunker sea) or another type of heavy oil as well as diesel oil.

The heavy oil they will burn while in the ocean, and typically they will switch over to diesel when maneuvering/navigation in traffic separation schemes or inland rivers and bays. The diesel is cleaner burning and a bit more responsive which is needed when giving the engineers many commands.

diesel will also power the ships' generators.

Smaller vessels use diesel. Anything over a few hundred tons (which is a small ferry or larger), generally will start on diesel and then switch to a lower cost heavier oil (bunker oil). Bunker oil must be heated before use. Only the smallest power boats use gasoline. Coal of course is not in use, except for a very few antiques like the steamer at Mystic Sea Port in Connecticut.

Most of the ships are low grade diesel, the motor runs at low rpm's so it can run on some low grade diesel

They run on black oil or bunker oil. It is pretty close to crude, barely refined. The black oil is used to produce steam . There is probably an equal number that use steam as diesel.

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Ronan is correct....No. 6 F.O. or low grade diesel...aka...Bunker Oil.

That's what they fill their tanks with...they can run on just about anything.

For sure though...they have Diesel Engines....very simple and reliable ones.

I think they mostly run on bunker sea fuel - marine fuel. It is a type of diesel.

diesel, or fuel oil. most of these engines are very low rpm units, so they can run on some pretty low grade stuff.

Those ships run on used motor oil.