Black smoke from diesel exhaust

[quickstep192]

I'm trying to help a friend who has a 32 foot Jarvis Newman with a Cummins diesel. He has a problem that seems to have developed recently. At 1900 rpm everything seems fine; he has light grey exhaust and clear discharge water, but when he gets into it a little bit, he gets dark black smoke from the exhaust and the discharge water is black as well. Anybody got any ideas on what could have caused this to start happening? I know this isn't a lot to go on, but if anyone has any insight, it would really be appreciated.

[Mistress]

Black smoke on a diesel is too much fuel (not enough air).

If it has air filters, change them.

Could also be a boost leak or turbo going out.

Is he down on performance? If he has an EGT gauge, is it reading hotter than normal?

[Pequod]

Sounds like overloading to me. I don't know the wot (full and no load) of your 6BT. I would first look up those stats first to use as a base line to start from. Then travel to Boatdiesel to visit the resident Cummins guru Tony Athens from Seaboard Marine. He is the definitive Cummins man. Btw, also get the CPL and other #'s off the tag on the timing gear cover, you will need those to identify the specific Cummins 6BT model/variation you have.

[craig]

i would go with a lack of air. check air filters, fuel filters also. when was the last time the aftercooler and turbo snails were cleaned.

[quickstep192]

Over the weekend, we replaced the air cleaner and cleaned the turbo. It seems to have made a big difference. We see a breif period of black smoke when throttling up before the boat speed can catch up (seems normal?), but goes away after a few seconds. Only at WOT do we see any smoke and it is grey. It's a displacement hull, so it seems possible to me that there's a point where it's just pushing water and is unable to make the boat go faster. The next step is to put in a mask and see if there's any significant fouling on the bottom or prop.

[Mistress]

It's normal to get some black smoke when throttling up (particularly after extended idle period) as the cylinders aren't hot yet and won't burn all the fuel. Long idle periods can build up soot too, which will blow black for a short while.