The maximum power or high power, which can be legally transmitted by a marine VHF radio is 25 watts. When reliable VHF communication is available, that is for ranges less than 10 NM, it is perfectly satisfactory to use less transmitter power. The advantage for the user is that there is less drain on the radio’s power source, that is battery supply. However, the main advantage of using less power is that by reducing the range over which one’s vessel transmissions can be heard, it allows others a few nautical miles away, to utilize the same channel. Another important advantage is the possibility to avoid capture effect explained below.
For these reasons, all marine VHF radios must be able to transmit on a reduced or low power setting of 1 watt. With one’s VHF antenna sited as high as possible, good communication ranges in excess of 5 to 10 NM on even the smallest boat are possible.
Place the mouse on the vessel and check who hears the broadcast through VHF radios on the chosen vessel
For this reason you should first try to transmit on low power and only if you are unsuccessful you may try on high power.