It is difficult to provide a number, because range will be affected by the following factors:
Transmit power: Higher transmit power gets better range. Transmit powers up to 25 watts are allowed.
Length of the antenna: Short antennas will get shorter range. Long antennas will get longer range.
Height of the antenna: The higher the antenna is mounted, the better the range.
Antenna tuning: Having a properly tuned antenna will maximize your range. Checking the SWR of the antenna will identify how well your antenna is tuned to the marine band.
Atmospheric conditions: Atmospheric conditions can affect radio propagation. If conditions are just right, there are times when you can transmit 100 miles or more. On other occasions you may be limited to only 10 miles. Although these extremes occur infrequently, it does serve to illustrate the effects of atmospheric conditions.
Your location: Your location will also influence the range. For example if you are located on an inland waterway where buildings and trees can block your signal, you will get less range than if you where in the middle of the ocean. With a handheld radio your range will vary depending on whether you are inside the cabin (worst range) or standing on the highest portion of the deck (best range).
Battery condition (handheld models): Low batteries will cause your transmit power to drop, which will reduce your range.
The combination and severity of the above factors will determine your range. Several of these factors can change on a day-to-day basis.
If we were to eliminate all the negative effects, a properly setup mobile radio transmitting 25 watts will communicate as follows:
-A powerboat communicating to another powerboat: approximately 20 miles.
-A powerboat communicating to a well-equipped shore station: approximately 50 miles.
-On a sailboat with the antenna mounted at the top of the mast, communicating to another sailboat: approximately 40 miles.
-On a sailboat with the antenna mounted at the top of the mast, communicating to a well-equipped shore station: approximately 80 miles.
Handheld radios will communicate as follows:
-A 5 watt handheld will communicate approximately 7 miles to another handheld, or approximately 20 miles to a well-equipped shore station
-A 2 watt handheld will communicate approximately 5 miles to another handheld, or approximately 15 miles to a well-equipped shore station.