What Is The 50/90 Rule Of Tides For Boating?

Getting into boating means learning its lingo and the differences between certain terms that exist in that world. But it's also important to learn about some of the rules of thumb used in boating, including the 50/90 rule. This rule is a way for boaters to estimate how the speed of a tidal current changes between slack water and the point of the strongest flow.

The rule works by breaking the period between slack water and peak current into three intervals that are roughly equal. During the first interval, the current reaches about half of its maximum speed. Then, during the second interval, it increases to around 90% before reaching full speed during the third. As the current eventually begins to slow down, the pattern works in reverse, with its speed falling to about 90% and then 50% before returning to slack water.

This process produces a general progression of 0, 50, 90, 100, 90, 50, and 0% over the course of the cycle. Though this timing isn't always the same everywhere every time, the rule stands as a good approximation for understanding changing current speed rather than a precise prediction of local tidal conditions. The 50/90 rule has also been included as part of coastal navigation training by organizations including the American Canoe Association and the British Canoeing Awarding Body.

The 50/90 rule of tides can be especially helpful for kayakers and other small-craft boaters needing to navigate areas with strong tidal currents. In those situations, the 50/90 rule can help with planning a specific route and determining when conditions may be most favorable. Even when tidal information only provides the predicted maximum current, the rule can be used to estimate the current's speed at other points in the cycle.

Unlike some of the other maritime rules and boating terminology, the 50/90 rule is a fairly recent concept. According to navigation instructor David Burch, the technique was developed by Starpath in 1987 and first published in the book "Fundamentals of Kayak Navigation." The rule was later adopted by other organizations, and it continues to be used in some navigation materials today.

But the 50/90 rule does have similarities to the Rule of Twelfths, another method used to estimate changes during a tidal cycle. However, while the 50/90 rule involves current speed, the Rule of Twelfths focuses on the height of the tide. It divides the change between high and low water into six equal periods, with the amount of change increasing from 1/12 to 2/12 and then 3/12 before decreasing back down to 1/12.