Thanks for the feedback, everyone. I'm a little late in figuring this out, but I have a plan that combines several ideas.
My spare halyard is currently in a continuous loop through a standing block on deck next to the mast. I'm going to strap or tape a wood handle from my deck brush to it and hoist it up past the masthead. Then row out in the dinghy
and use my binocs to estimate its relationship to the vhf antenna. Then mark the halyard with tape at the block. And bring it down while marking every ten feet with tape until it is all measured.
For the rest of the distance to the waterline I'll use the handle or my boat hook lying level on the deck next to the block to overhang the gunwales. Then I'll measure down to the water and add it all up.
Then I'll scratch my head
, drink a beer
, and do it all again.
Frankly, my heart skipped a beat when I went under the overhead power lines going out to St. George Island just before the 65 foot bridge. I knew they weren't underground like the chart says. And I knew a Coast Guard boat took them out a few weeks ago and killed the power to the whole island for a day or two.
Probably had more than fifteen feet clearance but on approach it sure looked liked it was going to touch.