The 50-90 Rule

The 50-90 Rule

posted in: Navigation | 1

While navigating through tides it is important to know whether the tide is working for you or against you. If you are on a kayak and are trying to get to shore during low tide for instance, you will find it far more strenuous and difficult to reach it than attempting to do so during high tide. The way to explain this and how to make tides work in your favour is the 50-90 Rule.

First, it is important to know a bit about tides. Go through this article for a refresher on the reasons for tides and this article to know about the Rule of Twelfths.

A combination of the Earth's. Moon's and Sun's gravitational forces create tides. During high tide the water level rises and moves towards shore while during low tide the water level falls and moves away from shore. In this process of rising and falling of the water surface and the resultant horizontal movement of water, creates tidal streams. It is this tidal stream that can either work for you or against you when you are rowing or paddling through it.

The 50-90 Rule states that the speed of the tidal stream is zero at the beginning of slack water, at the beginning of the first hour, when the tide reverses. At the end of the first hour the speed of the current rises to 50% of the maximum, 90% at the end of the second hour and reaches its maximum 100% speed at the end of the third hour before starting to reduce again to 90%, 50% and zero at the end of the fourth, fifth and sixth hours respectively.

Think of it like a swing. As you are swinging, the maximum speed is attained at the bottom of the swing, while at the top of the upward swing the speed falls to zero before starting its downward journey. You can understand the 50-90 Rule using the same analogy. As the tide rises, the water level reaches a peak and it is at this point that the tidal stream is at its fastest and strongest.

Of course, to know the speed you will need to necessarily know the maximum speed for that particular location. Otherwise, you will just be left with theoretical percentages. For instance, if you know the maximum tidal stream for a location to be 12 knots, you can derive the speed of the current based on the time. For instance, at the end of the first hour the tidal stream will be moving at 6 knots, at the end of the second hour at 10.8 knots before reaching its maximum speed of 12 knots at the end of the third hour before reducing again to become zero during slack water.

If you are paddling at 5 knots in the first hour, your real speed over the water will be 11 knots, provided you are paddling the direction of the tide. If, however, the tide is going out and you are trying to reach shore, and still paddling at 5 knots in the third hour, your real speed over water will be -7 knots since the tidal stream will be pushing you back despite all your efforts. Unless you can manage a paddling speed in excess of 12 knots, you will not be able to move forward.

Therefore, it is extremely important to know the tide timings, the Rule of Twelfths and the 50-90 Rule if you want a smooth and comfortable journey on the water.

One Response

  1. Alex Foster

    Fantastic, I thought it simply worked as the Rule of Twelfth, but this 50/90 Rule is a great piece of knowledge to use.
    Thanks.

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