Is this exhaust soot from running rich?

Not sure but I want to know what boat that is with quad bravo's?

Are they diesels?

Nortech? I saw it when I pulled into my storeage one day. It looked like the owner was fixing the trailer brakes . . . I waited around for a while, but I never saw anyone
Pretty sure that they're gas

They are gas motors

Wow, thats pretty cool. Love to see the engines.

Could be but when I keep my boat on a lift I had to get 89 because thats what was on the water and would get soot like that. Now that I trailer I use 87 and no more soot.

Wow! I wouldn't want to feed THAT beast! And yes, mine with only two will soot up the transom pretty good.

Octane may be too high. IE, higher than necessary if they're stock engines.

Yes.....When I first got my boat with HP500EFI's I figured I would use the best gas.....the highest octane. My transom looked like that at the end of the day. Use the right octane and it won't look like that at all. But if they have superchargers....rich is better than not!