When sailing we all know that it's important that we sail in enough water to prevent us going aground. This is not rocket science.
In tidal areas, this requires us to know how to calculate how much tide we have at a particular place and time. We can identify chart datum by observing soundings on a chart. We then calculate additional water at that location at a given time of day or night by referring to tidal heights calculated from the appropriate tidal curve & tidal times and heights in the almanac. Given that we know our vessel's draft, we can calculate where we have enough water to sail and where we do not.
With bridges and other obstructions above us, such as cable runs, the chart shows us a clearance height. The clearance height is measured from a point on the bridge, usually the centre line to the sea level at Highest Astronomic Tide (HAT). This tidal height is effectively the highest Spring Tide of the year (as opposed to the Mean High Water Spring (MHWS) which is sometimes used but which will be, by definition, a little lower.
We know the vessel's Air Draft from our boat's papers, The air draft is measured from the waterline to the highest point on the vessel. Usually the masthead.
When calculating a safe time to clear under a bridge, we should first calculate the maximum amount of tide we can have and still pass safely under the bridge. Then, we check that with this amount of tide we have enough water to enable our vessel, with its draft, to pass under the bridge without grounding.
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