Baz, I will post when I know more. I had read your previous post about how to check the impeller, and will not be doing that. When I mentioned how I wish it were designed so we could check it, I meant something MUCH easier to access/observe.
I do not believe it is a coolant issue.
I ordinarily do not mess with the seacock because I leave it open, but I had found bits of grass-like pieces in the strainer a couple of days earlier before taking the boat to the pump out, so I closed it, cleaned out the grass then opened it. I also experienced grass coming into the toilet when pumping, (I have the manual system) so clearly grass had gotten into the water line.
Before I left the dock the seacock was open and the strainer was clear. I did notice that the splashing I check from the swim step was not as lively as usual, but there was splashing, so I did not contact anyone. Jeff, thanks for the reminders and the info about the importance of that.
I presumed that since the oil was at the right level, the strainer was clear and the transmission fluid looked fine things were ok. I did not appreciate the significance of having major splashing when the engine is started up. If it needs to be significant splashing, that is probably the issue then.
I motored slowly through the marina and out into the San Diego Bay, getting up to 5 knots. After a bit i decided to increase the speed, and did not achieve full speed. I thought I heard a beeping noise, so I slowed and stopped the engine. No noise then. I started up again, and had very little acceleration. I figured I had indeed heard a noise and it was a warning, so I turned the engine off. I did not have engine temp up on my Garmin screen, which is something I greatly regret. That was a major error on my part, along with not realizing how significant the "vigor" of the splashing is.
As I said to the Yanmar rep at the Bremerton Rendevous, I am very frustrated that I cannot read the Yanmar screen that I should also be monitoring, and in my opinion it is a major issue that there is no backlighting so it is readable. The Yanmar rep said that issue had been brought up before...
I hope the next model will be READABLE (without using a flashlight!)
The Yanmar mechanic is coming mid-morning Tuesday. Then the next step will be to find out what the insurance company says. To be continued...