Many thanks for updating this thread and providing a positive ID.
With such a clear explanation, the yacht looks obvious. This Y-in-Yacht mark definitely dates from post-WWII, as it has nothing to do with the company's name or history. I imagine it was designed to match up the Y of Yamatogumi with something common in English that begins with the letter "Y." To update my earlier conjectures, while Yamatogumi might seem connected to the "yama" meaning "mountain" in Japanese, the characters for Yamatogumi are unrelated to the "yama" of mountain. Yamato is an ancient name for Japan.
Yamatogumi was still in operation in 1999, but its export production probably fell off dramatically after the 1970s.
What I've learned of the company's history can be found under Yamatogumi, in the CES and Far East Trade topic of Contributors' Notes:
viewtopic.php?f=38&t=24259&p=112898#p112898Regards
DR