IMHO, an 8' antenna is too large for a small trailer-sailer boat. And, it won't buy you much more in range over the standard 3db antenna.
VHF range is determined principally by height above the water, as you imply with your question. Many find that a stern-rail mounted VHF antenna is adequate for their purposes, i.e., talking with other boats they can see and with transmissions over distances up to about 5 miles or so. If the other station's antenna is high, distance potential is increased. Power helps somewhat, too. That's why you can easily copy USCG and WX broadcasts with a handheld radio over pretty long distances.
If you're really in need of communication over longer distances, then you've little choice but to rig an antenna at the top of the mast. Again, a 3db antenna is most appropriate, because healing will greatly reduce the distance potential of more "powerful" antennas, which have a narrower vertical transmission pattern.
Last edited by btrayfors; 07-26-2009 at 08:09 AM.