Your bridge, your air draft, the tide for the day. One answer.
Two numbers and this is done
Open the setup below, put in the clearance printed on your chart and your air draft. It remembers them, so you
only do this once.
The next seven days
Your boat and the bridge
How to measure it properly, once
Pick a fixed mark on the hull you can see and reach, the top of the boot stripe works well.
From the dock, measure straight down from that mark to the water. Write it down.
On board, run a tape from that same mark up to the highest thing fitted. Not the deck, the highest
point, aerials and all.
Add the two together. That is your air draft.
Do it with the boat loaded the way you normally run it. Full fuel and water sits her lower and gives
you a little more room, so measuring light is the safer way round.
The clearance on the chart is the height at the middle of the span, and it is measured from mean high water,
which is the average high tide rather than the highest one. So on a big tide you get less room than the chart
says. This works that out for you off the real tide.
These are tide predictions, not measurements. A strong onshore blow or a low pressure system will push the real
water level up by a foot or more, and that comes straight off your clearance. If it is going to be within a foot,
go near low water and put someone on the bow watching the arch.
Tide predictions from NOAA.
First Mate Yacht Club · Confidence through competence.