Hello Pirate Dan,
The air draft of the GH47 varies by boat. Some have fly bridges, some have different davits mounted high to store the dinghy above the cockpit and some have a collection of antennas and railings.
Since there are only nine GH47s built, it is puzzling. I own GH47 - hull 1 named Seven Tenths (that is for sale). We had a 30" railing on the roof that we cut down to bring our air draft to 16.6", to allow us to cruise Lake Champlain and through downtown Chicago.
Which boat are you looking at - I might can give you an answer for that boat.