Sailing: Tides

So, what about tide?

Tide is usually displayed on instrument systems as Set and Rate, however, to calculate tide we need the input of a geographical location/positioning device – such as a GPS.

Tide is calculated by looking at the difference between where a GPS says we are actually going (relative to the ground) and where the instruments believe we are going (relative to the water – without any tidal assumptions).

Why Rate not Drift?

A common mistake (even for the experts!) is to refer to the tide speed as drift when the correct term is rate. Drift is the distance something travels on the tide over a given time. However as both terms are commonly interchanged there is usually no confusion if you get the wrong one.

Tide Set is the direction that the tide is flowing to. It is important to note the difference between a northerly tide set and a northerly wind direction. A northerly wind means that the wind is blowing from the north, whereas a northerly tide set means the tide is flowing to the north.

Tides Understanding and being aware of the effect of tide is crucial to a safe passage.

• The effect of tide on the boat means that there will be a difference when comparing Heading and COG, Boat Speed and SOG. Tide set and rate can be calculated by the difference between these values.
• The effect of tide on the water can result in smooth or rough water depending on the True Wind Direction. For example, Wind against tide produces large waves, making entering some ports dangerous if not impossible!
• The effect of tide on the (True) Wind – True Wind is reduced if the tide is taking us downwind but increased if the tide is pushing us up into the wind.
• And this is our tip of the week, but don’t miss the next chapter where we will deepen into the “Weather” topic. See you soon!

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⇒ Sailing: Weather