Understanding the Rule of Twelfths for Tide Prediction

The fog was rolling in quickly, and the sun would soon be setting. I was bound for Provincetown at the tip of Cape Cod, a long peninsula that extends east and loops north like the tip of an elf’s shoe.
Author:
Updated:
Original:

The fog was rolling in quickly, and the sun would soon be setting. I was bound for Provincetown at the tip of Cape Cod, a long peninsula that extends east and loops north like the tip of an elf’s shoe. The land curls so tightly around Provincetown Harbor that it makes for an ideal anchorage. However, the bottom has a very modest slope and a large tidal range, so it can be a bit challenging.

The water was +9ft at high and -1ft at low, a tidal range of 10ft. I was hoping to anchor as close in as possible without getting in water that was too shallow. The tide was falling. As I made my way deeper into the harbor, I calculated the ideal water depth where I should set the anchor.

You don’t need to know calculus to understand tides—you only need to know the Rule of Twelfths to determine the height of the tide at any time of day. Although it’s approximate and doesn’t account for every factor that might affect the tide, the Rule of Twelfths is ideal for the sailor on-the-go. It’s especially helpful when passing over a shoal or entering a harbor at a mid-point in the tidal cycle. It’s based on the understanding that the change in depth of water is not constant, but rather increases its pace until it reaches maximum ebb or flow, then decreases until slack water. It uses a simple six-hour cycle.

Think about the tempo of the tides like the motion of a swing. As you swing toward the highest point, you slow down before changing direction. Then you accelerate as you swing down, reaching your maximum speed at the bottom. Like the tides, your fastest point is halfway through the cycle.

Most tides are semi-diurnal, meaning there are two high and two low tides a day, approximately 6 hours and 12.5 minutes apart. Between high and low tide the water accelerates to reach maximum speed (max flood or max ebb) then slows until it changes direction (slack water).

During the first hour, the water level rises by 1/12th of the total tidal range. In the second hour, it rises by an additional 2/12ths of the total. During the third and fourth hour, it rises by 3/12ths. Then the increase begins to slow down. In the fifth hour, the water only rises by 2/12ths, and in the sixth hour it rises by 1/12th. The pattern is 1,2,3,3,2,1.

I was going to drop anchor in Provincetown Harbor at 2300, 4 hours after high tide. Given a tidal range of 10 feet, at 4 hours into the cycle, approximately 9/12ths of the water had ebbed, or 7.5ft.

I expected it to drop an additional 2.5ft. My boat draws 4 feet, so I wanted at least 6.5ft of water. Remembering that the Rule of Twelfths is just a rule of thumb and does not include all the details that affect tide height, I sought out 9 feet—better safe than hard aground.

Graphic by Ben Erikson

Related

BoatTalk-2048

VIDEO: Sailing Not just for Millionaires

Sailing and boating can come with a hefty price tag, but there are plenty of ways to get on the water without breaking the bank. In this episode of Boat Talk, SAIL's managing editor Lydia Mullan and Power & Motor Yacht's executive editor Charlie Levine share tips on getting out ...read more

Cornell-2048x

Elcano Challenge Resurrected

In late 2020, sailing legend Jimmy Cornell set off on his Elcano Challenge, a green-powered circumnavigation aboard the custom Outremer Aventura Zero. Unfortunately, shortly after setting out, the boat encountered major power-generation issues. "I took the decision to turn ...read more

F8V-BOOK-for-SAIL---1

Book Review: The Figure 8 Voyage

“What is the color of the ocean that rolls beneath Pacific trades? How does a wave curl and crash at 47 degrees south? Can an albatross remain awing in the worst of weathers?” Randall Reeves has always found images to be the most compelling part of the stories we tell about the ...read more

AC210117cb_23806

VIDEO: Capsize in the Prada Cup

American Magic's Patriot capsized during day three of the Prada Cup. If you haven't yet watched the catastrophe unfold with your own eyes, check out the above video or any number of others that are circulating on social media. It's truly a tip that has to be seen to be believed. ...read more

210115-AC36

Prada Cup: Brits Take First Two Races

Who saw that coming? After getting skunked in December, INEOS Team UK has swept the first two races in the Prada Cup elimination series of the 36th America’s Cup  Racing took place on racecourse “C,” sheltered between Auckland’s North Head and Bastion Point to take advantage of ...read more

ac-2048x

Hutchinson: 36th America’s Cup will be a Close On

On the eve of the Prada Cup challenger series, the official start of the 36th America’s Cup, New York Yacht Club American Magic skipper Terry Hutchinson says it’s anyone’s game. "As we've seen in the last week, everyone's gotten faster," said Hutchinson said at the event’s ...read more

Episode1_Thumbnail4_00000_00000_00000_00000

Sailing Docuseries Released Online

Endless Media's Reaching Reality is the story of three friends, a 24-foot sailboat and 1,200 miles. With candor and humor, this series proves that you don't need to be an expert or a millionaire to cast off on the journey of a lifetime. Produced by Emmy-award winner Barry ...read more

01-LEAD-nder-sail-3

Prepping for a Transatlantic

Growing up on the coast of northern England, I dreamed about crossing oceans on my own boat. Like most of us, though, education, a family and a career took precedence, and before I knew it, we had mortgages, young children and endless work obligations. We also became landlocked, ...read more