Steve wrote:
Were is vertical clearance measured from (what datum)?

The vertical clearance is measured from 'Mean High Water' in places with a
single diurnal tide. In the Pacific Northwest, it is measured from 'Mean Higher
High Water' (MHHW).

The tide tables provide 'Mean Tide' for each subordinate station, and 'Mean
Range'. You can figure out what MHHW is for a place (within a foot or so) by
taking 'Mean Tide level' and adding one-half the Mean Tidal Range. When I do
that, I come within a foot of the MHHW given in my 'Tides and Currents Pro'
program, and the error results in a number less (more safe) than the listed one.

The vertical clearance of the east span of Hood Canal Bridge is 55 ft.
Yesterday the highest tide was 8.8 feet (at 1500). The Mean Tide for Port
Gamble is listed as: 6.10 ft. The Mean Range is listed as: 6.70. My
computation results in a figure of 9.45 feet for the MHHW.

Therefore, at the high tide (daylight hours) there was 55.6 feet under the east
span high rise. Of course, prudence leaves 2' of wiggle room to make up for
unplanned things like wind effect and such, or some dot.com yahoo with more boat
than brains zipping through at 55. But if your boat height over water is 53' or
less, well, just don't go on rec.auto.vacationing and mention it. ;=)

This information comes from Chapman, but also from U.S. Power Squadron courses
in Piloting and Advanced Piloting. Not a bad investment in time and a few bucks.