Originally Posted by Seaduction
You can get a rough estimate by pulling a 100' tape measure up the mast with the main halyard.
(If you have no tape measure, just pull a rope up to the top of the mast and measure the rope.)
Better yet, duct tape a broom stick marked in 1' intervals to your halyard, attach a downhaul and haul the halyard up it up so that the broomstick extends above the top of the mast. Then you can eyeball how high the antennas/etc extend above the mast using the marks. Measuring with a 100' tape down from where you attached the broom stick to the halyard will give you good measurement from the highest point of your boat to the cabin top deck then you can get a measurement from there to the water to get a really accurate measurement of your air draft.