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It's a stadard tide clock. The extra hand moves approximately with a tide cycle, which is a bit more than 12 hours. Of course they will only tell the tide time for the location to which they have been set (e.g. Gorleston Bar), not for your actual location if you are on the move.
It's a generic tide clock. You still need tide tables to set it.
You set the clock as a normal clock. The background bezel can be turned so that the red high tide pointer can be set to the appropriate hour. You can then read what the tide is doing and going to do over the next period.
However, it is a generic clock working on a 6 hour flood then 6 hour ebb cycle, as experienced on most coastal waters. As you will be aware move about one hour each 24 hour cycle so you only see what is predicted for up to 12 hours ahead.
Remember tidal rivers including the broads, the Solent and other coastal locations do not follow the 6 hours flood / 6 hours ebb rule so the clock is no good for these area.
In summary very pretty, impresses the visitors, but is actually of little use as you have to be able to work things out yourself from proper tide tables.