Predict your timing with these simple rules.

A working knowledge of tidal currents for sea kayaking is essential to ocean sea kayaking trips. Especially ones that take place in areas that are subject to significant tidal current activity. It requires consultation of that area’s tidal current atlas to establish the times of slack, maximum flood and maximum ebb. Regardless of whether your goal is to play in currents or avoid their effects.

What are tidal currents?

The vertical motion of tides raising and lowering near the shore creates currents. These move horizontally depending on whether the tide is coming in or out. As the tide comes in, the current moves toward the shore. It is the flood current. As the tide goes out, the current moves away from the shore which is the ebb current.

Slack (or the “turn”) is the time when a tidal current reverses its direction and turns to flow in the opposite direction. This time is only for an instant as the tide is always either flooding or ebbing. Near this time the currents will be minimal or virtually non existent and the water is at its calmest for safe travel.

Information in the tidal current atlas will also provide you with the time of maximum flood and maximum ebb, and their respective speeds, in knots. Also found on ocean maritime charts.

Finding slack, maximum flood and maximum ebb in this manner is relatively straightforward. To estimate what the current will be doing between these times, however, is not as easy as you might think. The cycle of tidal currents is such that a flood or ebb current lasts approximately 6.5 hours. A tidal current accelerates from slack (zero) to maximum speed over approximately three hours. The current then begins 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 Thirds or 50/90 Rule

Using the Rule of Thirds or 50/90 Rule

The Rule of Thirds breaks each half of the flood or ebb tide into three one-hour segments and states that the current reaches approximately 50 percent of its maximum speed in the first hour, 90 percent in the second hour, and 100 percent in the third hour. The current then decelerates to slack in the same order.

The Rule of Thirds

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 percent of that speed for much of the time.

Doing some research

You should use the Rule of Thirds as a rule of thumb only. Do your homework, study guidebooks and seek out local knowledge, because current speed is far from the sole indicator of a tidal current’s potential for danger. Bottom geography, 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 predictions. So let the final word rest with a visual appraisal on the scene. If the math works out but the current looks bad, trust your eyes and make decisions accordingly.

An Example Of The Rule Of Thirds or 50/90 Rule.

If you have a tidal passage that floods at a maximum of 10 knots at 3 pm, you can assume the following pattern. The same formula holds true for ebb tide currents.

Hour zero (12 pm): 0% slack, turning to flood = about 0 knots
Hour one (1 pm): 50% increasing speed = about 5 knots
Hour two (2 pm): 90% increasing speed = about 9 knots
Hour three (3 pm): 100% maximum flood = about 10 knots
Hour four (4 pm): 90% decreasing speed = about 9 knots
Hour five (5 pm): 50% decreasing speed = about 5 knots
Hour six (6 pm): 0% slack, turning to ebb = about 0 knots

Acknowledgement

This article was first published in Adventure Kayak’s Fall 2008 issue. Subscribe to Paddling Magazine’s print and digital editions here, or browse the archives here. Alex Matthews is the author of Sea Kayaking: Rough Waters, from which this article was adapted, published by the Heliconia Press.

We are grateful for the above article and I fully endorse the value of the knowledge shared.

The following text is added by Ian Ribbons of Meridian Kayak.

Using the Rule of Twelfths

This rule is similar to the Rule of Thirds except it is generally used to predict the height of the tide during the change. Very important for vessels having a deeper draft than kayaks so the skipper does not run aground on a low tide. Sea kayakers are interested in tide height as launching and landings can be very different at various water heights.

Such considerations as a sandy beach at hight tide or rocks/ coral at low tide. Sand or mud banks next to channels need to be considered as they can extend great distances from the shoreline.

How to calculate tide height.

Again each hour is used as the unit of measure. However, the full 6.5 hours of the cycle is used. After the first hour the change in height of the tide is 1/12th of the total of the tide range So if the tide range for that particular day and location is say 2.4 meters, then the first hour of change in height will be 0.2 M. For the second hour, 2/12 of the total movement ( another 0.4 M giving a total of 0.6 M). Then for the third hour 3/12th of the movement (another 0.6 m, giving a total of 1.2 M) . So, in three hours we have 6/12ths (1+2+3) of the movement, Continuing for the second half of the flow we have 3/12th, 2/12ths and 1/12th

Here is an example of the rule of 12ths.

If the tide tables for a particular date and location indicate a low tide of 0.6 M at say 12:00 pm and a high tide of 3.0 M then the tidal range is 2.4M.
The heights of the tide at each hour is thus.

Slack water at 12 pm Height is 0.6 M
First hour change at 1 pm is 1/12 of 2,4 M = 0.2 M. Height is now 0.6 + 0.2 = 0.8 M
Second hour change at 2 pm is 2/12 of 2.4 M = 0.4 M. Height is now 0.8 + 0.4 =1.2 M
Third hour change at 3 pm is 3/12 of 2.4 = 0.6 M . Height is now 1.2 M + 0.6 M = 1.8 M
Fourth hour change at 4 pm is 3/12 of 2.4M = 0.6 M Height is now 1.8 M + 0.6 M = 2.4M
Fifth hour change at 5 pm is 2/12 of 2.4 M =0.4 M, Height is now 2.4M + 0.4 M = 2.8 M
Sixth hour change at 6 pm is 1/12 of 2.4 M = 0.2 M. Height is now 2.58 M + 0.2 M = 3.0 M

As you can see the rate of change in height of the tide follows a similar pattern to the rate of flow of the tidal current. That is, the greatest rates of flow and change are in the middle of the cycle.

A working knowledge of tidal currents for sea kayaking goes a long way to enjoying safe, exciting and fulfilling experiences in the great outdoors. Many sea kayak operators offer suitable instruction programs.

Big storms can disrupt the flows of tidal currents and heights. They can reduce the accuracy of current table predictions. So pay particular attention to the weather as well.