My yacht runs on 110V 50 hz and has a USA NTSC TV set, DVD and VHS
Tape player both NTSC output.  In Australia I need a PAL tuner that
outputs NTSC to the TV set, and that copes with 110V 50/60 hz power.
I was told that some PAL VHS tape recorder/tuners can with internal
conversion output in either PAL or NTSC but I can't tell from
published specs.

The easiest solution might be a multi-standard VHS player. I'm using the AIWA 
MX-100 myself and could recommend it. This VHS player allows you to select
input, recording and output formats and even has multiple tuners built in (the
Channel selection is different between multiple areas in the world). You could 
use it as the tuner for your TV (watching thru the Video player) in areas where
your normal US TV tuner doesn't work, use it "just as a converter for
a DVD player or VideoCam from another system, or play any VHS tape
(from any system -
there are multiple PAL systems, and different SECAM systems in addition to the
NTSC) and have the output always converted to NTSC (or PAL or whatever you
like). Selection of recording and output system are independent of each other.

Disadvantage: It's not cheap. I think this player runs around $500 in the US.
There might be other VHS players doing the same - but I don't know them (so I
can't vote for them)

Another alternative (completely different) might be using a PC with Video
Input/Output. If you have a PC anyways, you may consider using it - even 
replaceing the TV and the DVD player. It would play DVD discs and TV programs 
without much issue (however most tuner cards only have one tuner for the region
where they were bought - not multiple tuners for the whole world).