What Burgee does not know
Burgee is a planning aid. It is not a navigational instrument, it is not a substitute for a chart, and it must not be used as one.
The short version
Burgee does arithmetic on a tide table you imported, against numbers you typed in. It has no way of knowing whether any of those numbers are right, and no way of knowing what the water is actually doing on the day.
The skipper is responsible for the safety of the vessel and everybody aboard. Check every number Burgee gives you against a chart, a pilot book, local knowledge and Notices to Mariners before you rely on it.
Where the numbers come from, and how each one can be wrong
- The sill or bar height is one you typed in
- Published drying heights disagree. Marina literature, the chart and the pilot book routinely give different figures for the same sill, and sills silt up between surveys. Burgee uses whatever you entered and has no way to check it.
- The tide table is a prediction, not a measurement
- Predicted heights assume average barometric pressure and no wind. A deep low, or a hard blow onshore or offshore, can change the actual height by half a metre or more, and surge can change it a good deal further. On the East Coast a northerly blow over a spring tide is not a theoretical case.
- The curve between tides is an approximation
- Burgee models the height between low and high water as a cosine. Real tidal curves are not cosines – they are distorted by the shape of the estuary, by river flow, and by the local stand or double high water. In the Thames Estuary and its rivers this distortion is significant, and it is largest around low water, which is exactly when a sill matters.
- Secondary port differences are not applied
- Burgee uses the table you gave it, as it stands. If your berth is at a secondary port, the differences must already be in that table or built into the height you set. Burgee does not apply them and cannot tell whether they have been applied.
- Charted depths change
- Drying heights and charted depths are surveyed, not permanent. Check the current edition of the largest-scale chart, and check Notices to Mariners for anything issued since.
- Times are British local time, from the table you imported
- A table published in Universal Time and one already adjusted for British Summer Time look identical on the page. Burgee asks you which yours is, and believes you. If that answer is wrong, every time it gives you is an hour out.
Your safety margin is doing real work
Three separate uncertainties land on the one number you set as a safety margin: published sill heights disagree, the curve is an approximation, and prediction is not observation. That is the argument against running it thin. Burgee will do the arithmetic with whatever margin you give it, including one that is too small.
What Burgee says nothing about
- The weather, the forecast, the sea state or visibility
- Whether the passage is sensible for this boat, this crew, on this day
- The state of the boat, her gear, her engine or her ground tackle
- The competence or fitness of anybody aboard, including you
- Traffic, commercial shipping, firing ranges, or any local restriction
- Depth anywhere other than the one sill you told it about
- Whether a berth or an anchorage is available, safe or suitable
Always check against
- The current edition of the largest-scale chart for the area
- The pilot book
- Notices to Mariners, and any local notices from the harbour authority
- Local knowledge – the harbourmaster, the marina office, other skippers on the pontoon
- The observed depth on the day, with your own echo sounder
Never rely on a single source for the height of water over a sill or a bar, and never rely on Burgee as one of them without checking it first.