Understanding Tides

“We sail on the tide.” With that phrase countless generations of sailors have put to sea, although today power boaters and sailors alike can readily motor from port through a flood tide. Marina del Rey’s floating docks can lull us into complacency when we anchor or moor elsewhere. Tides are still a factor in boating, so it’s good to understand them.

Tides are the periodic rise and fall in the levels of the Earth’s surface water. Both oceans and lakes have tides, but while the seas’ height varies in feet, major freshwater bodies of water like the Great Lakes only ebb and flow by inches.

Tides are caused by the pull of gravity from the sun and the moon on the earth. The sun is much larger than the moon, in fact 27 million times bigger. However the sun is 390 times further away. Even though the moon is a lot smaller, the moon exerts twice as much influence on the Earth's tides as the sun. Every 27.3 days, the earth and the moon revolve around a shared point. Like clockwork a similar tidal pattern repeats in that time frame.

The pull of the moon's gravity is stronger on the side that faces the earth, and weaker on the opposite side. The moon pulls the Earth’s water into a bulge on the side that is closer to the moon creating a high tide. The highest tide does not occur when the moon is directly overhead, because the tidal lift cannot keep up with the earth's rotation.

As you see from watching a liquid slosh in a bucket, water has a lot of inertia, so the highest tide is delayed by about a quarter of the daily cycle. For any point on the globe, the day’s highest tide occurs about an hour after the moon has set there. On the side of the earth opposite the moon, inertia causes water to pile up, creating a high tide there as well, but that one isn’t as high as the tide closer to the moon.

At right angles to the moon are the low tides. Due to the earth's rotation there are two low tides and two high tides in a lunar day which is longer that the Earth’s day. A lunar day is 24 hours and 50 minutes because the moon revolves around the earth in the same direction that the Earth turns.

Since tides are due to the tug of both the sun and the moon, when the forces line up twice a month, we get the highest and lowest tides of the month. We use the term spring tides to describe those with the biggest difference between high and low. They can occur any season. The word “spring” here is used as a synonym to “jump.” Spring tides happen just after every full and new moon where the pull of the sun on the earth is in line with the moon's gravitation tug. Spring tides have their greatest variation between high and low at the equinoxes, the first days of spring and fall, usually March 21st and September 21st, when day and night are equal world wide.

When the sun and moon tug in different directions, the differences between high and low tides aren’t as great. These tides with the smallest change in level are called neap tides. Neap tides happen when the moon and sun pull at right angles to each other.

An ebb tide refers to the time that the sea level falls over the course of several hours. Slack tide or slack water is the point where the water turns. Flood tide refers to the period between slack and high tide when the water height starts to increase; in Marina del Rey, when it flows towards shore.

Tidal times vary due to local geography. In extreme case, such as in the south facing Panama City, Florida, there is only a single low tide and high tide each day. In most of the world, the time between high and low tide is consistent, approximately 12 hours and 25 minutes, half the lunar day. That is why high and low tides seem to advance by an hour each morning and evening.  Although most calendar days have two high and two low tides, there are days like December 31, 2010 in Marina del Rey that had two high, but only one low tide since the second low tide of the lunar day was in the wee hours of New Year’s Day 2011.

In some places the flood tide rises quickly after a long period of several hours of low water. One of the most dramatic places in the world to watch a high tide is in the Bay of Fundy between the Canadian provinces of New Brunswick and Nova Scotia. There the high tide rolls in with on tidal bore, a wave that quickly travels up river against the current.

The phenomenon is caused by the fact that the tide is funneled into a shallow, narrow river from a broad bay. I’ve watched it roll in myself, a true “tidal wave” in every sense of the word. This bay has some of the most dramatic differences in tidal height in the world.

Here are a couple of boating tips to keep in mind when it comes to tides:

Marina del Rey is constantly coping with shoaling, which the Army Corps of Engineers handles by dredging. When motoring in and out of the main channel, during low tide, stay closer to the buoys, especially if you have a sailboat with a keel. Be particularly wary during spring tides. It’s a good idea to learn the draft of your boat and study charts for where ever you plan to go.

You can pick up tide tables at most boating stores. I find a graphed tide calendar to be more effective. Tidelines offers a full sized, beautifully photographed calendar for over the desk, with a pocket sized edition for the boat.

As you leave your slip, pay attention to the angle the gangway makes to your dock. If it’s a severe slope, you’re at low tide and should be careful leaving the marina. If the angle is nearly level, take more care coming back because after a typical day sail, the water will be much lower.

Don’t assume that every port you pull into has floating docks. If you tie up your dinghy at a place like Anacapa Island without taking tides into account you can find it dangling from its dock line or submerged.  When I visited, I hauled the dinghy out of the water.

Santa Catalina Island presents tidal challenges. The first is the reef at Isthmus Harbor, the closest Catalina harbor to Marina del Rey. The reef there is 400 yards southwest of Bird Rock and extends in a northwest direction. It’s marked to the southeast by a lit red buoy and an unlit buoy to the west. At low tide the reef is only 2 feet below the water’s surface. If you inquire at the aptly named bar on shore, Harbor Reef, you can hear tales about boats that ignored this warning.

If you’re anchoring at low tide, be sure to let out enough scope to allow for high tide, or your anchor can break free. The opposite can happen on a mooring.  I was on a 38 foot sloop once in Catalina on a mooring for a 36 foot boat. The length difference wasn’t an issue at low tide but at high tide, it was impossible to free the lines and cast off to leave, without using a winch to literally crank the boat into the water to give us enough slack to depart.