Be interested to hear how the 4 footer (3dB) works. I will be mounting mine in this location (a friends 205).
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VHF communications are line of sight. So the height of the antennas are the major factor in determining how far you will be able to receive a signal. Of course better radios will have better performance but the maximum distance is limited to the antenna height. Putting a 4 ft antenna on the deck will result if a shorter working distance than if the same 4 foot antenna were mounted on a hardtop.
For example, say one vessel has an antenna height of 16 feet ( 8 ft antenna on a hardtop that is 8 ft above the sea level) and you boat has an antenna height of 6 ft( 4 ft antenna mounted 2 feet above the water. You line of sight distance is 9 miles. If you use an 8 ft antenna with a effective height of 16 feet( again 8 ft above the water line on the hardtop) you operating distance increases to 12 miles.
Note that the antennas used by folks like the Coast Guard can be 100 feet or more in height; With that height and the example vessel with the 16 ft effective height, operating distance increases to 20 miles.
There are other factors in antenna design that affect performance but you really can't get around the distance limitations due to the line of sight distance