Trouble with tide times
Q
I wanted to calculate the difference between Dover HW times and Osea Pier (River Blackwater) for our mooring holders’ website (stonemoorings. com). I looked up Reeds for Dover times and Walton, which I adjusted for Osea, then calculated the difference. I was not sure that I had it right. So I booted up the chartplotter and looked up Osea and Dover to start working out an average. But then I thought again and decided, in true TV tradition, to phone a friend. He emailed me some tide times from an app on his phone. So, I have four different sources: a set of tables I found on the web, my mate’s phone app, Reeds Almanac and my chartplotter, but their tide times are all different!
Take HW Dover: the times vary by as much as 25 minutes at certain times of the year, but mainly by about 10 minutes. Can someone tell me why? Is Reeds the one to trust, or have I wasted £40? Sam Longley, Southminster, Essex
RUPERT HOLMES REPLIES:
We often assume tidal predictions are near-on perfect and that they have been calculated successfully for hundreds of years.
However, the first tidal predictions were only in 1924, more than half a century after the first regular weather forecasts. Initially these were calculated by hand, and then by mechanical computers at the Liverpool-based National Tidal and Sea Level Facility (ntslf.org/tides), which is now a branch of the National Oceanographic Centre.
The NTSLF’s tidal predictions tend to differ a little from those of the Admiralty’s UK Hydrographic Office (easytide.co.uk).
As you report these are generally within 10 minutes but at times may be close to half an hour.
For critical situations it’s clearly worth having a feel of the level of uncertainty inherent in the predictions on that day. I therefore recommend looking at data from both sources, before making further allowances for local weather conditions.
It’s also important to be aware that the apparent simplicity of apps can mask any assumptions made in their height of tide calculations.