Other PicoMicroYacht

Saturday, 2 July 2016

Knowing the tides aged 11 years

When I was eleven my best friend thought high tide was always at tea time.

He was right.

Whenever we went sailing on the coast, usually Chichester Harbour, our parents chose to go in the afternoon and selected sailing days for when there was a high tide. He didn't have to worry about the tides and just turned up for the sailing. The high tide transformed the estuary into a huge lake where you could go everywhere.

High Tide at Bosham, Chichester Harbour, where King Canute, King of Denmark, England and Norway, is reputed to have decreed that the tide should stop coming in, but was not successful on that occasion.

I was reminded of this when studying a Yachtmaster training book and reading about semi-diurnal tides, in which there are two high tides a day, separated by about 12.5 hours. I thought this was universal, apart from some local variations caused by geographical features such as Islands.

Image result for semi-diurnal tide

I was right .... well that is to say, around the British Isles, North West Europe and most of the East Coast of North America, not to mention other places in the world.

But I was to find out that I was thinking like my friend - as my son said today, I was right given the limitation on the data I had to hand .... but then I was to discover the tides are also di-urnal, with  one high tide a day, or even mixed, a sort of hybrid, as in the Pacific Coast of North America.

The semi-diurnal tides set the rhythm for the PicoMicroYacht voyages in the UK  because PMY always aims to go with the tidal direction when going round the coast, and this follows approximately the same periodicity.

High tide to low tide and vice versa is about six hours, and this is a typical limit for a PicoMicroYacht voyage. Assuming a 3.5 knot average speed, this provides a range of  about 21 nautical miles.   The recent 42 mile voyage round the southern Isle of Wight required two voyages in one day and, of course, eventually turning in the opposite direction up the Needles Channel changed the game.

Below is a heat map of tidal rates for tide in full flow up English Channel and past the Isle of Wight - it would be impossible to voyage against the red flow areas.
round the island, cowes

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