Do you ever need to know the tide heights if you're not crossing a shallow bar? Maybe.
It could be that problems arise above the water. A low bridge can block passage for fly-bridge cruisers and sailboats on high tide, yet let them pass with space to spare on low. Sometimes even runabouts can be blocked by low-hanging roadways.
You obviously need to know your boat's bridge clearance if you navigate anywhere that passage beneath one of these structures is common. The bridge clearance (BC) refers to the height from the waterline (where the surface meets the hull) to the highest point of any structure on the boat, including the mast, tuna tower, or outriggers. (If your radio antennas and riggers fold down, you don't have to measure them in the equation.)
It's always best to punch around in the backwaters on a rising tide, and to head back for deeper water shortly before the tide peaks and starts to run back out. That way, if you go aground inside, the rising water gives you a good chance to get off anc escape; while if you ground on falling water, you're stuck until the next incoming tide.
Bridge clearance (BC) is the distance from a boat's waterline to the highest point on the topsides. It indicates the height of a bridge under which a boat can pass.
Bridge clearance is listed on marine charts, with the height above mean high water given. So, if you want to pass under a bridge that has 20-foot listed clearance with your sport-fisherman boasting a BC of 19'6'', you aren't going to be able to get through at the peak of a spring tide that's a foot taller than normal high tide (more on spring tides later in this chapter). On the other hand, you can get through that bridge with a 19-foot listed clearance on the bottom of a -1.0 low.
Bet You Didn't Know
In most areas there's a normal progression through four tides, but some days have only three tides. How can this be?
As the 50-minute-per-day regression takes place, the P.M. tide gradually advances past midnight, thus leaving one 24-hour period without four tides, even though the tidal movements maintain their steady procession.
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