Lost your tide chart? No problem!

Low tide at the Lubec breakwater. Tides here average about 19 feet. Photo by Homer Shannon

December 2021

By Homer Shannon
For Points East

Boating in New England’s ocean waters requires that you pay attention to the tide. With local average tidal heights from three to twenty feet, ten feet being the norm for most of the ocean-facing coast, inattention to the tide can easily result in a grounding or serious accident.

You may have learned about the “Rule of Twelfths” If you have not, here is a quick introduction. The tide does not come in evenly. Its rate of rising or falling accelerates during the first three hours and decelerates during the last three hours. Because of this the tidal rise or fall per hour is not even. The rule of twelfths states that in the first and last hour of a tide, the vertical change is one twelfth of the total tide height. In the second and fifth hours, the tidal change is two twelfths. And, in the third and fourth hours of the tide the change is three twelfths per hour.

The piece that always seems to get left out of the explanation of the rule is, “How much is one-twelfth of the tide fall?” Not being a math genius, the answer to this question was not obvious to me, so I set out building a table that I could keep aboard with the tidal change for a range of tide heights. The equation for the answers is: Tide change in one twelfth of the total tide change equals the total tide height (in feet) times twelve (inches) divided by twelve. Well, if you do the math, you quickly see that the two twelves in the equation cancel out. The answer to the question is: One twelfth of the tide height equals the tide height in feet stated as inches. So, a local tide, say of nine feet, rises nine inches in each twelfth. All you need to do, to calculate the local tide change in any particular hour, is multiply the total tidal height times one, two or three. That number, in inches, is the tide change for that hour.

This does get a bit more complicated if you are way down east where the Bay of Fundy exerts its control and the tide starts climbing towards twenty feet. But before you go and get a calculator, try this trick. Taking a 17-foot tide (no one remembers their 17 multiplication tables!) in the third or fourth period where the tide rises three twelfths as an example, calculate the answer as three times twenty minus three times three. The answer is sixty minus nine. That’s math anyone can do in their head.

Knowing these facts, as long as you know the local tidal range, you can easily calculate the tidal rise and fall for any hour of the tide.

Another interesting fact about the tides is that, here in New England, the high tide on the day of the full moon occurs nearly exactly at noon. This rule has some assumptions. It assumes that the local time is eastern daylight savings time, which of course is when most recreational boating is done. Also, the rule assumes you are on the ocean facing part of the coast, Provincetown to Eastport. Once you get deep in bays, rivers or the southern Cape Cod area, local tide times get very weird. But for Boston and most of the coast, these rules hold.

The high tides on the day of the full moon and the new moon occur at noon and midnight. The high tides on the day of the first quarter of the moon and the third quarter (aka last quarter) of the moon occur at roughly six o’clock in the morning and six o’clock in the evening. To remember this fact, I created a simple saying:

Full moon, new moon – tide at noon. First and last – supper and breakfast.

The low tides are the opposite of the high tides – low is at roughly six o’clock a.m. and p.m. on the day of the full and new moon and roughly at noon and midnight on the moon’s quarters. Each day the time of the tide advances by about an hour and twenty minutes.

So, if you know the phases of the moon for the month, you can easily work out the general tides for the month and will only need a tide table if you need to know the exact time of the tide.

Homer and Dee Shannon live in Windham, N.H., and sail out of the American Yacht Club in Newburyport, Mass. They have been sailing Cinderella, a Bristol 29.9, for 22 years. The boat draws 4 feet 4 inches.