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