Jim,
Find a mechanic who is familiar with Yanmar, and follow the advice. I have a 25h.p. Universal Diesel with a huge warning sticker not to sail in forward gear. (No explanation) Also, very bold detail in the owners manual saying the same thing. What I do is Shut down the engine in forward gear, and leave if for a few seconds, to feather my max-prop. Once feathered, I shift to neutral. Regardless, know that it is a very important topic as far as transmissions are concerned. Visit marinedieseldirect.com to learn more. (Universal site, but you can learn some anyway) Good luck
MattF