Our 1986 Catalina 27 with Universal 18 diesel tends to run hot
(although not immediately, takes many hours), has low RPMs (always,
although it ranges between 1750 and 2000 at full throttle), and always
give off some black smoke. Additionally, it appears that there may be
a tiny bit of black oil leaking behind the transon of the boat, but
not a lot. Finally, we get a maximum speed of 4 1/2 or 5 knots in
still water, which seems to be too little...

I am quite new to diesels, so know very little. Can someone recommend
some easy things to check/fix. I've heard black smoke means injectors,
but could that also cause the low RPMs?

Thanks!