Tides and Currents

To Remember

  • Semidiurnal - 2 highs and 2 lows.
  • Diurnal - 1 high and 1 low.
  • For tides check Tide Tables.
  • For current check Tidal Current Tables.
  • The sun's influence on tides is 1/2 that of the moon.
  • The moon's location determines the time of tides.
  • The moon's phase determines the range (height) of tides.
  • For passages, a navigator wants to know when the current will be weakest or in their favor.
  • The maximum speed of a current - at a particular place - is proportional to the range of the tide at that place.
  • Semidiurnal tides are typically 6 hrs and 15 minutes between high and low tides.
  • Diurnal tides are typically 12 hrs and 30 minutes between high and low tides.
  • High and Low tides are 50 minutes later the next day.
  • The Tidal Day = 24 hours and 50 minutes.
  • High water in general happens 1-2 hours after the moon's meridian passage.
  • A tidal cycle max flow is on its way to the highs and lows.
  • The speed of the tide is given at its peak.
  • When Tides heights are needed check Tide Tables.
  • When Tide currents are needed check Tidal Current Tables.
  • The max speed of the tidal current at a given point is proportional to the range (height difference) of the tide at that same point.
  • Spring tides (full and new moon) are approximately 20% larger than average tides.
  • Neap tides (half moons) are approximately 20% smaller than average tides.
  • Tide station errors will be within 20% on tidal strengths and 20 minutes on time.

Important Dates

Strongest Spring Tides (Solstices)

  • June 21st - Summer in Northern Hemisphere
  • December 21st - Winter in Northern Hemisphere

Weakest Neap Tides (Exuinoxes)

  • March 21st - Spring in Northern Hemisphere
  • September 23rd - Fall in Northern Hemisphere
  • Approximately 20% weaker

Rules of Thumb

When it comes to tide you have 6 primary considerations.

  1. The tide height (Rule of twelfths)
  2. The resulting current
  3. The drift (speed) of that current (50/90 rule)
  4. The set (direction) of that current
  5. The distance that current travels (Rule of Thirds)
  6. The time this will occur

Cross Current Speed

Cross Current Speed in knots = (XTE/6) * (0.1 * Boat Speed)

To find the cross-track (XTE) angle take bearings off the stern and see how far in degrees it has veered from your starting point. This angle gets divided by 6 and multiplied by 10% of the boat speed. The result equals the net speed of the current in knots.

For the following you'll need to know:

  1. The tidal range is the difference in distance between MWH and MLW in feet or meters.
  2. The max speed of the current in knots.
  3. The direction of the current in bearing.

The below equals one-half the tide cycle. So if the tide is rising, this equals from low to high. From high to low would be the next 6 hours.

Tidal HourStart12345End
50/90 = Speed (% of max current speed)0%50.00%90.00%100.00%90.00%50.00%0.00%
Rule of thirds = Drift/Distance (1/3 maximum rate)1/3
33%		2/3
66%		3/3
100%		3/3
100%		2/3
66%		1/3
33%
Rule of twelfths = Height (1/12 the tidal range)
The difference in MHW and MLW		1/12
8.33%		2/12
16.67%		3/12
25%		3/12
25%		2/12
16.67%		1/12
8.33%

The table above was inspired by this article.