The answers to all your questions are somewhere in:

http://pollux.nss.nima.mil/NAV_PUBS/APN/Chapt-09.pdf

Yes, you do have to be careful. Astronomical predictions often show tides a foot or two
high than MHW in some locations. Also, a storm or even a persistent wind can do the same.
And both can occur at the same time.

And, although most bridges are actually a few inches higher than listed, some are actually
lower, or have gear hanging down.


"Steve" wrote in message ...
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