Think of your antenna's broadcast pattern as like the light from a lightbulb. Light shines out horizontally and vertically. Antenna gain is the focusing of that energy, like the light from a lightbulb, into a narrower pattern. There's no actual increase in power, just less power sent upwards. So as you increase gain in a typical VHF marine
antenna, the signal is being flattened out to more of a doughnut shape. The problem on a sailboat is if the signal is too flat, the signal will not be sent along the horizon, but into the sea and sky when heeled over. It's best to stay with the 3dB antennas because it still has significant amount of power in the vertical plane. It will perform well when heeled.