You need either a "Tide Table" or tide program, so you can figure out
the clearance based on the stage of the tide at the time you will pass
under.
The tables or a program will give you the mean tide ranges (or in the
case of a tide program it will frequently tell you MHW) from which you
can calculate YOUR height above or below MHW for the particular time or
stage of the tide, in question.
If the bridge clearance is 55', and you find that MHW for a particular
bridge ( or nearest datum point)is 5' and you will be passing under the
bridge at a time when the tide is at +2', you (technically EG) should
have 58' of clearance ..... or if you are in some extreme tides and the
tide at the particular time will be +8', you will only have 52' of
clearance.
Always approach these numbers with caution and check any local "tide
gauge" on a bridge pier when available.

Steve wrote:

Ok, now that I have had time to compare NOAA charts with the reproductions
in the Maptech portfolio, I see that vertical clearance is from MHW (mean
high water), as stated in the title block of each NOAA chart.

It seems Maptech 'washes out' the title block as they copy and crop the
charts for their book pages.

But, that still leaves me with the question: How do you calculate or adjust
the stated Vertical Clearance when all you have is a chart and a tide
table.

In my example; The charts show a vertical clearance of Hood Canal Bridge,
east span, as 55 ft and the tide table shows a height of 8 ft at the time I
want to pass under it. My mast height is 54' 7", including the VHF
antenna...

Do I need the NOAA tide tables or can this be calculated from the usual,
locally reproduced, convience tables??

I realize, after the fact, that there would have been sufficient clearance
yesterday, but there may well come a day when I arrive at the bridge at
something slightly higher than MHW.

Steve
s/v Good Intentions