Tides and how they work

When planning  a sea kayaking trip it’s useful, and sometimes essential, to know something about the tidal conditions.  The effect of tide will often make or break a trip, you can go with it or against it and it’s useful to have some basic understanding about tides before thinking about going on the water.  The tide is governed by tidal constants and there are basically two constants – tidal amplitude and tidal epoch. Epoch is the time between the moon’s meridian passage and the high tide and when combined these constants will completely specify a simple tide.

The great thing about tides is that they are easy to predict.  But why is knowing about tides important?  Firstly, tide height will determine how much beach is exposed and is important in planning where to put in or take out, for example on islets connected to mainland by spits which may be exposed or not exposed.  Secondly, when planning crossings over tideflats, knowing the tides can sometimes save you long distances of paddling since you may be able to paddle over a tideflat rather than going around it.  Also it’s also preferable to have deeper water over a tideflat since it’s easier to travel over deep rather than shallow water.  Lastly, when approaching a rocky shoreline, it’s important to know about rocks that cover or uncover with tide especially when there may be an unplanned landing.

What causes tides?  Tides are caused primarily by the attraction of the moon and to a lesser extent by the sun.  High tide is formed from the bulge in ocean on that side which is closest to the moon and centrifugal forces do the same on the opposite side of the earth as the moon passes to other side.  This results in a second high tide.  So in this part of the world there are two high tides and two low tides, i.e. tides are semidiurnal.   Interestingly in the mid ocean, the effect of the sun and moon results in a bulge of only about 18 inches and this builds as it approaches land resulting in the tides we see on shore.   The moon follows a meridian passage over the earth and this happens every 24 hours 50 minutes (a lunar, or tidal, day) and it  is highest either in the north or the south.   The sun also has an effect but it is about half that of the moon.  It is the location of moon which determines the tide time and the phase of the moon, that is the effect of the sun which determines the range of the tide.

It’s quite straightforward to predict the high and low tides using published time tables.  The tide will depend on the location, the month and the time of year.  First, locate your position on a nautical chart.  Then, using tables in publications such as Eldrich Tide and Pilot Book or the New England Edition of a current Tidelog, get the times and heights of low and high waters for the nearest primary station, making a daylight savings correction if needed.

Tide heights at intermediate times can be predicted beforehand.  The height of tides between high and low tides does not follow a straight line but instead follows a bell-shaped curve.  Tide height can be calculated using the Rule of Twelve as follows: first, divide the range (difference between low and high tide) by 12 to get a “range step”.  Then, in the first hour the height moves by one range step, in the second hour by two range steps and in the third hour by three range steps e.g. for a 12 feet range the tide will rise 1 foot in one hour, 3 (1+2) feet in 2 hours and 6 (1+2+3) feet in 3 hours.  Similarly, in hours 4, 5 and 6 hours, the tide will have risen by 9 (1+2+3+3), 11 (1+2+3+3+2) and 12 (1+2+3+3+2+1) feet, respectively.

Lastly, here is some of the tide terminology which may be useful when reading about tides or planning a trip.

1. Tide – vertical movement of water governed by the moon and the sun.

2. Current – horizontal movement of water; if caused by tide rise and fall is tidal current (tidal stream outside US).

3. Ebb and flood – fall and rise of tide.

4. Slack – between ebb and flood, no flow.

5. Tidal Height – vertical measurement between surface of water and tidal datum / reference plane which is noted on chart (e.g. mean lower low water (19 year average of the lower of the two daily low tides)).

6. Tidal range – vertical distance between high and low water.

7. Spring Tide – maximum tidal range, when sun and moon are in line (Saxon springan is a Saxon word meaning ‘swell’).

8. Neap tide – minimum tidal range, when sun and moon are at right angles (Neafte is a Saxon word meaning ‘scarcity’).

8. Set – direction of tidal current flow, if current flows from west to east it an easterly current. Note that this is opposite of wind, wind coming from the west is a westerly wind.

9. Drift – speed of tidal current in knots.

10.Tidal race – where velocity of tidal flow is significantly increased due to topographical features.

11. Perigree, moon closest to earth, greatest effect on tide height; apogee furthest from the earth, least effect on tide height.

Happy trip planning!!

Sources: “Fundimentals of Kayak Navigation by David Burch;  Sea Kayaker’s Handbook” by Shelley Johnson