Someone else asked this question a day or so ago, so I'll just copy my original answer here.
Air draught is the term used to describe the distance from the top of a vessels highest point to the waterline of that vessel.
Ship captains should know the height of their highest antennae on the ships' superstructure, for example and Yacht Skippers need to know the height from the tip of their VHF radio aerial on the mast to the yachts waterline.
This knowledge is essential and as basic a requirement for navigation as knowing the vessels draught (how deep the vessels keel is).
When navigating under a bridge, Skippers consult several sources of information in order to determine whether or not their vessel can safely pass under the structure at a given time of day.
Basically, one looks at the chart or an Almanac to find out how high the bridge is above a predetermined height known as HAT (highest astronomical tide).
One then determines the current (or intended time's) actual height of tide, adds that height to the known air draught of one's vessel, then works out whether there is enough clearance or not by comparing that figure to the HAT height and the height of the structure above HAT.
Until recently, this height of the structure (bridge) above the sea / river used to be taken from the MHWS (mean high water springs) of the relevant area and was a useful guide.
However, MHWS is only the average of highest tide states recorded over a period of time. Working from this average could actually cause a vessel to become stuck under a bridge even if the skipper worked everything out perfectly!
Prudent ship captains and Yacht skippers will take HAT (highest astronomical tide) as the level from which to begin their calculations for air draught. HAT is the level of the highest ever recorded tide for the relevant area.
A skipper would have to research this for himself until recently, but thankfully now modern chart publications are stating HAT instead of MHWS. Some almanacs this year for the UK and NW Europe are all using HAT and as of next year this will be a compulsory inclusion in nautical almanacs.
I note that my original answer to this question (that someone else asked) has received a thumbs down rating. I presume by an individual who also answered it on that occasion, since the asker is only a level 1 and not able to provide thumbs up or down ratings. There is no reason to have done that to my answer to this question, I am completely correct. Individuals who disagree with this are not up to date or are probably day skippers who have a little knowledge (but not enough) on the matter.
Hope this answers your question.
I answered this question for someone else yesterday - same person Geoff answered - and I am dismayed to find that I too received a thumbs down for my answer.
WHAT THE F***?
This IS the correct answer. I agreed with Geoff totally, still do, his response is very thorough and absolutely right.
I want to know who the PRAT is who thinks otherwise, because he / she could actually be putting people in danger!
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