Understanding Tides and Currents

There are countless facts you should know about your yacht and navigation rules to stay safe on the water. But when it comes down to it, also understanding the basics about tides and currents is essential to ensure you’re doing everything possible to keep yourself, your boat, and your loved ones safe every time you head out on the water.

So, what do you need to know about tides and currents? While there is much to know, we’ve compiled 13 of the most important facts below to ensure you have the knowledge necessary to stay safe at all times.

1. Not quite sure what a tide is? It’s the water’s vertical movement. Tidal current flows in and out whereas tides rise and fall.

2. Even if the tide starts to fall, the current can still be moving in. These are just some of the important facts you can find in tide books like Eldridge.

3. Tides rise and fall by the moon. This means that even though the moon is much smaller than the sun, it actually exerts 2.25 times the amount of gravitational pull on the oceans as the sun does due to its close proximity to Earth.

4. As tides fall, water will move from the poles to the equator. This is because the spinning Earth creates centrifugal force that results in shallower water at the poles and an excess of water at the equator.

5. Neap tides will be very similar whether the tide is high or low. Neap, meaning near even as possible, tides occur when the moon and sun are positioned at right angles to the Earth, causing quarter moon.

6. Spring tides will be either higher or lower than normal tides and occur when the sun and moon are aligned with the Earth, especially during full and new moon periods. Because the gravitational pulls of the moon and sun work opposite each other, their alignment magnifies gravitational pulls that result in these higher high tides and lower low tides.

7. Spring tides occur during solar eclipses as well. Although the moon will block the sun’s light during an eclipse, its gravitational pull is the same, causing these tides.

8. Although tidal movement is controlled by the sun and moon, the tidal day is longer than our days being 24 hours and 50 minutes long. The extra 50 minutes is caused by the friction between Earth and sea, causing water to move more slowly.

9. Regions with semi-diurnal tides will have two high tides and two low tides every day. These are actually the most common types of tides in most regions.

10. In contrast to semi-diurnal tides, diurnal tides mean that there is one high tide and one low tide each day. These are more rare and are common in specific locations such as the Gulf of Mexico, Java Sea, and Tonkin Gulf.

11. Mixed tides are semi-diurnal tides where the two daily high tides and two daily low tides are extremely different.

12. The Rule of Twelfths is a simple way to estimate tide heights and the current flow in areas with diurnal tides. The Rule uses the total volume per hour as a fraction. If you know the time of high or low tide, you can assume it will move in or out over the following six hours in this pattern: 1/12, 2/12, 3/12, 3/12, 2/12, 1/12.

13. The Rule of Twelfths is most helpful if you’re cruising. This is because you can estimate when the most water will flow through an inlet (typically during the middle of a tide), which characterizes the roughest conditions you’ll face. Additionally, the Rule can help you estimate how much water will be beneath your keel in shoal areas at any given time.