RG, and anyone else having trouble with this. Yanmar didnt build the boat. The boat builder selected the powerplant, and installed it at an angle that is beyond Yanmars suggestions. Read that again, the engine is tilted beyond the manufactures recommendations. Had the engne failed early in life due to a lubrication failure, and a factory tech noted the engine was angled beyond outside its suggested range, they would likely have denied warrantee.
The engine is made to sit basically upright, and gravity holds the oil in the sump. The oil pan is made in such way as to hold a specified amount of oil, and keep that oil way out of the way of the crankshaft at the maximum range of tilt. But it wont work sideways, and it wont work upside down.
Race cars that run on a track, often have to make drastic alterations to the oil pan if they start doing hard cornering. As cars advance in capability, creating high G forces in long sweeping turns, no amount of creativity can prevent oil starvation or oil whip. Thats where we begin to see dry sump systems. Scavenge pumps in the pan continuously draw oil away, and pump it to a tank. A second, pressure pump, then feeds the oil back into the engines oil galleys, where it goes where it would normally go, and falls off into the sump... Aerobatic aircraft have similar systems, allowing them to fly upside down. Some advanced designs have multiple scavenge pumps to keep the sump and crankshaft clear of oil.
What we believe is happening here is his engine is tilted so far, that if the oil is filled to the line on the stick, the oil is almost touching the crank. In calm waters he hasnt had a problem. But in rough seas, the engine is getting tilted even further. Each time it does, the crank whips the oil. It can take a while, but soon the oil becomes an air entrained soup. It gains volume, and now even when the boat is level, the whipping continues. Only now its constant. The crankcase can no longer breath properly. Crankcase pressure builds beyond normal. It starts trying to push oil out anywhere it can. And because the oil is full of air, oil pressure drops. Now we have a oil warning going off. We have oil all over the engine. But after its shut down, after the oil settles and the mess is cleaned up, we cant really find anything wrong. He starts it up, it idles fine, it runs just fine at full power, and it seems a big mystery. Until hes back in rough water with a full sump of oil and running hard.