JoeSpareBedroom wrote:
,

John H

Essay: Your boat is 30 feet high, measuring from the waterline. The water is
10 feet deep. Clearance under the bridge is 42 feet. Which information is
missing here?


OK, I'll bite. If my the "bridge clearance" (height) of my vessel is 30
feet and the vertical clearance of a fixed span bridge is 42 feet,
there is only one possible answer- the draft of the vessel. If it
exceeds 10 feet, it's not going to make it. :-)

The other possibility would ordinarily be the state of the tide, but
that can't be a variable in this case based on the wording of the
question. Vertical clearances are measured to mean high water, not MLW,
so if you're dealing with a charted clearance of 42 feet there should
always be at least 42 feet available. Your question becomes tricky when
you have a 45 foot bridge clearance dimension for the vessel and a
42-foot charted clearance for the bridge. Definitely time to break out
the tide table and calculator before trying to pass under that same
bridge.