You can't see it in my picture, but there is a narrow shelf under the helm area that the radio is just above. I also normally just rest one knee on the captains chair while I pilot the boat. This way I am above the windshield and have an unobstructed view of the water ahead.
All fixed mount radios transmit 25watts on their highest setting. So in that regard they are all equal. The number one factor affecting VHF transmitting distance is antenna height. There are no guarantees as to how far the signal will travel.
VHF electromagnetic propagation is known as Line-of-Sight (LOS). Like a flashlight, it can only transmit in straight lines. This differs from lower-frequency radios in the HF range, such as HF-SSB or Citizen’s Band (CB) radio that can rely on "skip" to transmit long distances. For this reason, the transmission range for VHF marine radios is effectively limited by the curvature of the earth and antenna height. In this regard, the higher you can mount your antenna on your boat, the further you will be able to communicate. Of course, this is simply because the line-of-sight for the higher antenna is greater. There is a mathematical formula that can help in determining the line-of-sight distance:
To estimate the communications distance, you must perform this calculation for both the transmitting and receiving boat. Generally you would make the measurement from the top of the antenna to the waterline.
In the above scenario, we can determine the likely communication distances between the two boats, as well as the distant boat and the tower.
The boat with the 12ft antenna, the distance to horizon is approx. 4.2 miles.
The boat with the 18ft antenna, the distance to horizon is approx. 5 miles.
The antenna on the tower, the distance to horizon is approx. 12 miles.
The maximum theoretical distance that the two boats can communicate is 9.2 miles (4.2mi + 5mi)
The maximum theoretical distance the distant boat can communicate with the tower is 17 miles (12mi + 5mi)
It must be noted that this is a theoretical distance due to the line-of-sight limitation. The gain of the antennas will not increase this distance; however, they will make a marginal signal stronger - within the limitation of the line-of-sight distance. In reality, envionmental conditions, such as islands that obstruct the line-of-sight path, weather conditions such as rain or fog, or atmospheric ducting all affect the distance the signal travels.
OK, so you have been on the East side of Lake Michigan, and have heard the USCG from the other side of the lake - more than 80 miles, what gives? There is weather phenomena such as atmospheric ducting that act like a conduit for longer distance communications, but this is not reliable, since it relies on changing weather conditions. And, it is not likely that even though you may hear the transmission, you will be able to engage in a 2-way communication.
(Special thanks to AL C. from BoatingABC for educating the boating world)