I’ve mentioned before that the best antenna for boats of the size most of us sail is a 1m whip antenna because it has a nice fat radiation pattern that suits the rock and rolly platform on which it needs to operate. With this plump rather than thinly focused pattern there is always some portion of the signal being directed at the horizon, where it needs to be, rather than at sea or sky.
The best place to locate it is at the masthead to gain the benefit of a longer horizon distance and, therefore, range.
On RIBs, motorboats and for deck mounted applications on yachts you benefit from mounting the antenna on a pole – 1.5m to 2m is typically enough to increase range a little and get the antenna above the level of cabin tops, people and other clutter which can block the signal.
You can buy proprietary poles which have a threaded top end and a threaded base so they can be screwed onto a deck mounting plate or a rail mount, and an antenna can be fitted to the top of the pole.
Another approach is to make your own extension pole using an appropriate length of stainless steel, aluminium or plastic tubing and a pole mount conversion bracket, seen in the picture. The pole can be attached to the boat using any of a wide variety of clamps and sockets available at most chandlers or hardware stores.