why smoke stacks so fat for diesels of this size?

hard to imagine "just for appearances sake" on USCG Cutter but then again that boat was commissioned to "fight" rum runners, and we aren't sure if that was a battle the Govt really wanted to win. lol.

maybe has to do with way exhaust travels once out of stack. Maybe without a big fat stack it wants to swirl around and choke out whoever is nearby on deck? That boat did 16kts max so on any given day it would be iffy-50 if the wind matched the boat speed and was in zero relative wind speed (but still swirly due to wind over water).

Maybe it had air inlets around the base and would heat a larger mass of air to both dilute and carry upward.

"smokeless campfire" involves setting the fire next to large vertical rock to cause the smoke to travel up the face instead of seeking out the other nearest vertical (a person).

 

There may be multiple exhaust stacks, combined into one near the top (such as generators and main engine(s) that all run into a common collector). You also want a lot of space around the exhaust for heated air to flow up and out (the vents mentioned by other posts). This air gap creates a natural heated draft, and keeps the outside of the stack and exhaust trunk through the cabin spaces from getting too hot. On smaller boats, like commercial fishing vessels, there is also a tube outside of the exhaust pipe to help divert rain and spray when the engine is not running.

There may also be mufflers in the stack, though I doubt on any old ships. Mufflers are kind of a medium and high speed engine thing. Fishing boats like to put the mufflers inside the stack if there isn't space in the engine room. The mufflers can be staggered up the stack so it isn't too large. Again air space is needed to keep the hot air moving up and out the vents.

Stack mounted exhaust fans, so they can push instead of pulling, EGE's, access walkways for servicing equipment inside the stack, and for painting and general inspection, all add to the size. Some engines or propulsion packages don't tolerate exhaust back pressure, and some I suspect were just poorly designed exhaust systems. Nothing like a fat exhaust, to slow the gases down, so it can cool on the way up, and deposit particulates, acids, and other wonderful things on the inside of the pipe.