[ I would like to simply add another standard VHF
radio at the helm and share the antenna (located at the top of a 55'
mast).

NO NO NO NO NO NO NO NO NO NO NO NO NO NO!!!!!

NEVER NEVER NEVER plug the transmitter of one radio into the receiver of
another, which is exactly what will happen if you try to do this,
destroying both of them.

We do share antennas between transmitters. But it takes expensive plumbing
and phasing networks called diplexers and the transmitters are not
frequency agile. Your local TV station actually has two transmitters, one
for the picture and one for the sound, on a single antenna with this
diplexer.

More practical is your main transmitter on the mast and your secondary
transmitter on a stern rail-mounted fiberglass whip. This gives you the
backup antenna you need when dismasted or lightning struck. I'd even like
you to run the backup transceiver off the starting batteries separated from
the dead house batteries.[/quote]

Additional research lead me to Defender.com who offers the Shakespeare A2 vhf splitter. The device can be found at: http://www.defender.com/product.jsp?path=-1|344|295760|299311&id=162352