Depending on where you learn to sail, the different types of tides may play a significant role in your training. The majority of my sail training took place on the west coast of Scotland. There, miscalculating the tidal current could see you powering along at six knots yet still moving backward against a six or seven-knot flow! However, get your tide calculations correct, and you can reduce your sailing times and increase efficiency between destinations.
The other reason for understanding tides is the rise and fall of water levels. Tidal heights in Scotland can vary by up to 24 feet. In the United States, the most significant tide fluctuations are in Anchorage, Alaska, where it’s possible to see a tidal range of up to 40 feet. Why is tidal range important, you might ask? Well, consider tieing up alongside a dock or pier. If it’s high tide when you arrive, a drop in water levels could leave your boat dangling from its mooring lines. You wouldn’t be the first to make this mistake, and you wouldn’t be the last.
This article introduces what causes tides and how you can plan around different types of tides to improve your experience.
Effects of Tidal Flows on Different Types of Tides
What is a tidal flow, and what causes them? Without giving a detailed description of the rotation of the planets, the sun, and the moon, the simple answer is gravitational pull. The sun and the moon exert gravitational forces on the earth, with the moon’s pull being the most significant.
The moon’s gravitational pull causes two tidal bulges. One where the moon is closest to earth and the other on the opposite side of the planet. This relationship causes the most common type of tide, semi-diurnal tides, which simply means there are two high tides and two low tides every day. Semi-diurnal tides swap from high tide to low tide just over every six hours.
What Are the Different Types of Tides that Boaters Should Know?
There are four essential types of tides that boaters need to know and understand, and we briefly cover them here.
A high tide occurs when the moon is directly above a location, and the gravitational force is at its strongest. High tide occurs when the water depth is at its maximum. Knowing when high tide occurs allows a boat to enter waters and harbors that may otherwise be inaccessible due to the boat’s draught.
Low tide occurs when the moon is at 90 degrees to a location, exerting the weakest gravitational force. This is the point during the day when the water depth is at its lowest. These tides may expose otherwise hidden obstructions, such as rocks, sandbanks, or even sunken ships. A skipper must know when low tide will occur, the depth of the water around him, and the draught of his boat to avoid problems.
Despite the name, spring tides have nothing to do with the seasons. They occur twice per lunar month when the sun and moon align throughout the year. The term “spring tide” is thought to have derived from the phrase “springing forth,” describing how the tide range is exceptionally vast at this time. During a spring tide, high tides are higher and low tides are lower than the average.
In contrast to a spring tide, neap tides are the period in the lunar month when the lowest tides occur. A neap tide happens seven days after a spring tide when the moon and sun are at right angles in relation to the earth. The difference between high and low tide reduces during a neap tide, with lower high tides and higher low tides than usual.
In conjunction with high and low tides, we also need to understand tidal currents. As the water flows in towards high tide, you experience a flood current. As the water flows out towards low tide, you experience an ebb current.
Between the flood and ebb tidal current, there is a period known as slack water, where water is neither flowing in or out. Slack tides can last for just a few seconds or up to a few minutes, depending on your location.
As with tides, the moon and the sun both affect the speed of the tidal current. During spring tides, we will experience spring currents with greater tidal rates. During neap tides, the opposite occurs, with neap tidal currents being lower than the average.
Understanding the tides and tidal currents is an essential skill for any skipper. Tidal currents can run at very high speeds, making boat handling more complex, so tackling tricky harbor entries or difficult passages is always better to be timed with slack water if possible.
You should also bear in mind that the speed of the tidal current is not constant. The closer to slack water you are, the slower the tidal current, with the highest flow rate being midway between high and low tide.
Understanding Tide Tables
By now, you should have realized that if you are boating in a tidal area, then knowing how to understand the types of tides and currents is an important aspect.
Luckily for us boaters, help is at hand in the form of tide tables. One point to mention here is that tide tables are only predictions based on the moon and sun phases. Atmospheric conditions and weather can change the tides quite markedly. Now is also an excellent time to remind new boaters of the importance of checking the weather forecast before you head out onto the water.
Things won’t look obvious when you first view a tide table, but you’ll quickly get used to them with a little bit of practice.
Tide tables use a primary port to calculate other local area tides. Using the National Oceanic and Atmospheric Administration (NOAA) website, main ports are known as the Harmonic, while secondary ports are called subordinates. A large number of harmonic ports are used as a base to calculate tides, with conversion figures given for subordinate ports.
Important Components of a Tide Table:
Port and Time Zone
The port’s name is written at the top of the table, along with its longitudinal and latitudinal coordinates. It will also indicate the time zone, and this is very important to consider.
Date and Time
For each day the chart covers, you will see the time of each high or low tide and the height above the chart datum. You must use the data for the correct date to get accurate information.
The chart datum is also called the Mean Lower Low Water (MLLW). MLLW is the average low water height for a specific location over a set period.
The high tide height is shown as a positive number, sometimes with + before it, indicating the height of high tide above the chart datum. So to find the water level at high tide, add the high tide level to the chart datum.
The low tide figure can be positive or negative. A negative figure indicates that the tide falls below the chart datum level. As with high tide, add or subtract the low tide figure from the chart datum to get the low water height.
Using Tide Tables
It is vital to only use the table values for the day you need. The differences between days can be considerable. By knowing the chart datum, and the times of low and high water, you can work out when it is safe for you to sail through a particular point.
Two points to note that are beyond the scope of this article:
- If you are sailing through a subordinate port, you will need to adjust the figures using the conversion rates.
- Some ports have complicated tidal flows, and to get an exact tidal height, you must use a tide chart also.
There should also be tables for specific ports. We won’t go into detail here, as it is beyond the scope of this article. However, the charts allow you to calculate a more accurate water depth between the high and low water points.
The Rule of 12’s
Using tide tables to precisely calculate water height at any given point in time is not straightforward. It is easy to make a mistake, particularly as a beginner. Using the Rule of 12’s, a quick estimation can be made of the height of the tide and often provides enough information for safe coastal cruising without the need for detailed tide figure calculations. However, if the area has complicated tidal patterns, you should not rely on the Rule of 12’s method.
Rule of 12’s
- First hour: The tide rises/falls by one-twelfth of its range
- Second hour: The tide rises/falls by two-twelfths of its range
- Third hour: The tide rises/falls by three-twelfths of its range
- Fourth hour: The tide rises/falls by three-twelfths of its range
- Fifth hour: The tide rises/falls by two-twelfths of its range
- Sixth hour: The tide rises/falls by one-twelfth of its range
To use the Rule of 12’s, you need to know the low or high tide time and height and the tidal range.
Example of using the Rule of 12’s
1. From the tide table, find the low water time and height. We’ll use 3 pm, with a height of 1 meter above chart datum, with a high tide of 13 meters.
2. From this information, we can calculate the tidal range; 13 – 1 = 12 meters.
3. The tide will rise by one-twelfth of the range in the first hour, gaining one meter.
4. In the second hour, the tide will rise by two-twelfths of the range, gaining another two meters.
5. At 5 pm, two hours after low tide, the height of the water will be four meters above the chart datum – the low water level, plus 3/12 of the tidal range.
One use of the Rule of 12’s is as a quick double-check of your more detailed tidal calculation. If the figures are roughly the same, then you’ve most likely made the calculations correctly. If not, then a double check is worth doing.
We will be covering the Rule of 12’s in more detail in a future article.
How to Understand the Types of Tides Without Access to Tide Tables or Online Resources
You can make predictions on the tide height without tide tables, so long as you have basic information. Bear in mind that this will probably be a wildly inaccurate guestimate, and having the correct tide tables for the area is vital for safety.
The most important information will be the time and heights of high and low tide. You can get these from various sources, such as the harbor office, the Coastguard over the radio, or other boaters.
Once you know when the tides will be high or low, you can use the ] datum from the sailing chart to calculate actual high and low tides. Using the rule of twelfths, you can work out tide heights at various times.
Another method of predicting the height of the water is using the moon’s position and phase and the water levels from the chart you are using. Your chart will show the depth of water at the average low tide.
When the moon is above you, you are at high tide. Knowing that the moon tide cycle is 6 hours and 12.5 minutes, you can then work out when the next low tide will be and continue this process for as long as you need.
This kind of calculation is more in the realms of a Yachtmaster qualification, where you will learn to navigate using the stars. For most of us, it makes sense to ensure we have the correct information, including tide tables for any area we will visit before letting go from the mooring lines.
Need more information to better understand the different types of tides? Contact the folks on our community forum with a question or comment!
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