yanmar 1gm

Hi, I have a Yanmar 1GM diesel that runs well at high rpm when stationary (and when in gear) at the dock and cooling water exits the exhaust. However, when I am underway at higher rpm the exhaust outlet sits under the water and the engine slowly dies out. And now, unfortunately it won't start...

I have heard that I should flush out my Yanmar 1gm10 inboard diesel engine every time I run it in saltwater. Is that true? Is there an easier way to flush it, because it currently takes me about 20 minutes after each time I motor which is a pain?

Hellowould be very he I am curious to know from other owners of a Yanmar 1GM what you get for max RPMs in neutral and in forward gear. If you have a boat around 27' that would be even more helpful. I have a Hunter 27 and get around 3200 max rpm in neutral and 2600-2700 max rpm in forward gear...