Steve wrote:
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...

You have to know MHW. If it isn't on your tide table, I'd ask the bridge
tender. If he doesn't know, I'd ask him to open up.

I did ask the bridge operator, but the answer, sounded more like a guess
(seat of the pants) since he is about a mile from the fixed span I was
interested in passing under.

There is a clearance scale on the fixed span but according to my cruising
guide, it has some error (on the conservative side), several feet less than
the vert. clearance on the charts.

Even if I could trust these scale markers, I wouldn't hate to get in close
enough to read them only to find there wasn't enough clearance, with several
knot of current running.. Bad situation for a sail boat, trying to come
about with a deep draft.

Many of these scales that I have seen in New England, were badly faded or
covered with marine growth/bird droppings, etc.

I'm going to search out a Gov. tide table and see what the charted datum is
for both of these bridges.

Thanks again for the good response on this question. A good exercise.

40+ years of boating but not to proud to ask the dumb questions.

Steve
s/v Good Intentions