kayaker’s
journal TECHNIQUE BY ALEX MATTHEWS Perfect Timing
PREDICT TIDAL CURRENTS USING THE RULE OF THIRDS
Whether your goal is to play in currents or avoid their effects, when planning a trip to an area that is subject to significant tidal cur- rent activity you’ll need to consult a tidal current atlas to establish the times of slack, maximum flood and maximum ebb. Slack (or the “turn”) is the time
when a tidal current reverses its direction and turns to flow in the opposite direction. Tis is the time when currents will be minimal or nonexistent and the water is at its calmest for safe travel. Information in the tidal current atlas will also provide you with the time of maxi- mum flood and maximum ebb, and their respective speeds, in knots. Finding slack, maximum flood
and maximum ebb in this manner is relatively straightforward. To es- timate what the current will be do- ing between these times, however, is not as easy as you might think. Te cycle of tidal currents is such
that a flood or ebb current lasts ap- proximately 6.5 hours. A tidal cur- rent accelerates from slack (zero) to maximum speed over approximate- ly three hours. Te current then be- gins to slow again, heading toward the next slack, and this deceleration also takes about three hours. Common sense might lead you
to expect this acceleration and deceleration to progress in a nice logical, linear way from slack to maximum flow and back again. But this isn’t the case. To predict flows between maximum and slack, you need to use the Rule of Tirds.
Te Rule of Tirds breaks each
half of the flood or ebb tide into three one-hour segments and states that the current reaches approxi- mately 50 per cent of its maximum speed in the first hour, 90 per cent in the second hour, and 100 per cent in the third hour. Te current then de- celerates to slack in the same order. Te Rule of Tirds shows that
it’s important to travel close to the exact time of slack if you want to avoid paddling in current, because the speed accelerates quickly after the tide turns. It also shows that if you want to play in the current, it’s best to choose a maximum speed that you’re comfortable paddling in, because the current will be running at more than 90 per cent of that speed for much of the time. You should use the Rule of
Tirds as a rule of thumb only. Do your homework, study guidebooks and seek out local knowledge, be- cause current speed is far from the sole indicator of a tidal current’s potential for danger. Bottom ge- ography, wind, water depth, and shorelines all contribute mightily. Also, big storms can disrupt the
flows of tidal currents and reduce the accuracy of current table pre- dictions. So let the final word rest with a visual appraisal on the scene. If the math works out but the cur- rent looks bad, trust your eyes and make decisions accordingly.
ALEX MATTHEWS is the author of Sea Kayaking: Rough Waters, from which this article was adapted, published by the Heliconia Press (
helipress.com).
For example, if you have a tidal passage that floods at a maximum of 10 knots at 3 p.m., you can assume the following pattern. The same formula holds true for ebb tide currents.
24 ADVENTURE KAYAK | FALL 2008
Hour zero (12 p.m.): Hour one (1 p.m.): Hour two (2 p.m.): Hour three (3 p.m.): Hour four (4 p.m.): Hour five (5 p.m.): Hour six (6 p.m.):
Okisollo Tidal Rapids, British Columbia. PADDLER: ALEX MATTHEWS // PHOTO: JOCK BRADLEY
0% slack, turning to flood 50% increasing speed 90% increasing speed 100% maximum flood 90% decreasing speed 50% decreasing speed 0% slack, turning to ebb
= about 0 knots = about 5 knots = about 9 knots = about 10 knots = about 9 knots = about 5 knots = about 0 knots
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