Article by Dave Adler Courtesy of Dagger

Whether you're planning on crossing the mouth of a wide bay or catching a perfect glass wave on a reversing falls, having a big, giant party on a beach in Maine or just trying to figure out how far up the beach you need to lug your boat during lunch -- tides matter.

The tides aren't totally straight forward, and to understand the details requires more math than should ever be inflicted on an innocent boater. If you spend any time on the ocean though, youıll be better off if you have at least some understanding of the tidal patterns and how they vary.

Outfitting shop now

Otherwise, you may be hauling your boat across the mudflat where your glass wave used to be (done that), or watching it drift away without you as you finish off your picnic lunch (done that, too.) In the worst case, a misunderstanding of tides can lead to poor decisions and trouble at sea.

You should be able to look at the shore, know what the tide is doing and predict what it will do during the time you're on the water. You should be able to predict when the tidal currents in any area will be strongest, and in which direction they will flow. You should be able to predict whether you'll be able to paddle between two islands when you get there, or whether you'll have to paddle around. You should absolutely know when to show up at a reversing falls in order to catch the greasy wave, and you should be able to predict when you'll be able to pick mussels, and when you can't. There are only a few things you need to understand in order to make sense of the tides. Bear with me.

The Physics
If the earth were a smooth ball covered in water, and the orbits of the moon around the earth and the earth around the sun were circular, symmetrical and consistent, then the tides would be a piece of cake. But that's not the case -- the earth has continents, it spins on a tilted axis, the moon orbits on a tilted ellipse and so does the earth.

The combination of forces acting on the earth which produce the tides are complicated. I have no idea how they figured it out -- but they did. Tides are caused by two main forces: gravitational and centrifugal.

Just as the earth has a gravitational pull towards its center, so does the moon and the sun. The amount of gravity we feel depends on the mass of the object, and how close we are to it. Although the sun is much larger than the moon, since it's so far away, the moon has actually about two and a half times the gravitational pull on the earth than the sun does.

Paddle Jackets shop now

We'll deal with the sun later on, but right now just consider the moon. The gravity of the moon pulls a big bulge of water towards it. This bulge is always on the side of the earth directly below the moon. If you're at that spot, it's high tide. But the moon doesn't just rotate around the earth, they both sort of whip around each other.

It's like if you tied two rocks of different size together, held on to the big one, whirled the smaller one over you're head, and then let go. The small rock wouldn't just rotate around the big one, they'd rotate around a point somewhere on the string (and then crash to the ground or break a window). The same thing is happening between the earth and moon, so that on the side of the earth opposite the moon, there is another bulge of water caused by this whipping around motion.

Both bulges mean high tide. As the earth makes its complete rotation over 24 hours, a point on its surface will pass under both the bulge caused by the gravitational force, and that caused by the centrifugal force. Bingo -- two high tides, twelve hours apart.

When that point (or boater, or beach or mussel) is half way between the bulges, it's low tide. That happens twice a day, too. This is called a semi-diurnal tide, and is what we have on the east coast.

What About The Sun?
The sun has the same effects (both gravitational and centrifugal), but because it's so far away the bulges arenıt as big. The moon is responsible for most of the tide we observe.

Adventure Books shop now

When the two sets of bulges line up, though, we feel the effect of the sun. This happens during the new and full moon. (Both positions are called syzygy. Yes, this is a real word...now you can really impress your friends in Scrabble.) Since both sets of bulges line up, they reinforce each other. During these times, the high tides are higher, and the low tides are lower (which means the range is greater).

During the quarter moons, these bulges counteract each other, so the tides are smaller. The large tides during syzygy are called spring tides (think babbling brook, not robins), and the smaller tides are called neap tides (I think that's Greek for "I swear this wave was much bigger the last time I was here.") So if you want to get to the big mussels, go during new or full moon.

Timing
Timing If the moon were stationary, high and low tide would happen at the same time every day. But it ainıt. The moon is orbiting around the earth in the same direction the earth is rotating on its axis. So by the time a point on the earth rotates through 360 degrees, the moon has moved. It takes about 50 minutes each day for the earth to Œcatch upı to the moonıs new position.

For this reason, the tidal cycle is 50 minutes later every day. That means that if high tide is at 12 noon on one day, it will be at 12:50 a day later, 1:40 a day after that, and so on. This is a key piece of knowledge when you're planning a trip.

Tide Velocity Or The Rule Of Twelves
If you've ever sat on a beach and watched the tide come in or out, you may have noticed that near high or low tide, not much happens -- the tide changes slowly. But when the tide is about halfway up the beach, it begins to move faster (as if on a B-line for your boats). This change in the rate the tide moves (acceleration) is also the reason that reversing falls are formed, and the reason that tidal currents are not consistent -- sometimes they are faster than others.

Lifejackets shop now

The reason for this is connected to the slope of the bulges of water, and I'm not going to get into it here. But it's easy to conceptualize, without any physics. Imagine a 12 foot tide. Between high and low tide (6 hours), twelve vertical feet of water have to move. At high tide, no water has moved. During the first hour, 1 foot has moved, during the second hour 2 feet move, during the third hour, 3 feet move, in the fourth hour, its 3 feet again, during the fourth, 2 feet move, and during the last hour, 1 foot moves. Add them all up and you get 12. This means that the period of greatest tidal change (tidal velocity), occurs during the 3rd and 4th hours after high or low tide.

This piece of knowledge is also key: if you're aiming to surf at a reversing falls, plan to be there during this period, when the current is fastest (and the wave is sweetest).

For the same reason, if you're planning to paddle across the mouth of a bay (a situation where bad timing could mean a one way trip to sea), plan on doing your crossing during the periods of least tidal flow -- an hour on either side of high or low tide.

This is by no means a comprehensive discussion of tides. There are many areas I haven't touched on, and you probably have questions I may have missed. If you understand these concepts, though, you'll be able to make sense of the tides. Happy paddlin' -- don't miss the glass wave and enjoy the mussels.