This is actually a question about an auxilliary yacht engine but since it is purely engine-related this seems like the right forum:
I'm restoring a classic yacht on a tight budget and it has a 37 year old Albin AD21 diesel engine. When I bought the boat we sailed her for about 7 hours with the engine running the whole time without apparent problem (once we actually got the thing running). Since then I've done some basic servicing including replacing badly corroded high pressure fuel lines, cleaning fuel system, new oil and air filters, and having the starter motor serviced. The engine now starts easily and seemed to run well, though up till now that has only been for 15 mins or so ata a time in a marina.
When I replaced the oil filter I also did an oil change and I noticed that the oil was quite emulsified (I think!) - it was a sort of milky coffee colour and quite thick; there was also some water in the bottom of the sump. I'm guessing the water is getting in via the water pump and that the seals probably need replacing, and doing so is on my (very long) to-do list.
I think the oil in the engine at the time was just basic mineral oil for a car engine as there was a part-used container of it on board. I have read that this is not really suitable for such an old engine and used a fairly expensive oil specifically for classic engines. It's SAE 30 (not multigrade) and it says on the company's website that it has enhanced anti-foam properties.
Yesterday I had to move the boat to a new mooring several miles away and motored up there. I checked the oil before leaving and found it had already turned milky coffee after just the few runs in the marina, but there wasn't much I could do about it and it hadn't caused any problem before so I set off. After maybe 30 mins a LOT of smoke started coming out of the exhaust. The engine seemed to be running fine and not particularly hot though some smoke was coming out round about the crankshaft at the front of the engine (it protrudes through the engine block to allow hand crank starting). We had no choice but to carry on and we got there ok leaving an embarrassing trail of air pollution behind us, By the time we arrived oil was actually foaming out of the filler cap on the rocker cover so I presume it was getting into the cylinders via the valves and that's why there was so much smoke.
So, sorry for the long intro but I thought that might help with answers to my questions:
Why is the oil turning to cappuccino so quickly? Is that simply down to water getting past the pump drive-shaft into the engine?
And why the incredible amount of foaming and burning this time but not on the much longer first voyage: was I in fact better of with the cheap basic modern oil?
One last bit of background: the last owner showed me the dipstick and proudly told me that the oil stayed so clean that you could hardly see it but I later found that it was really because it was pretty much completely empty. I topped up a bit before the 7 hour trip but maybe there was no apparent problem because there just wasn't enough oil there to foam up high enough.