Understanding the Rule of Twelfths for Tide Prediction - Sail Magazine

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

04-GOPR0511

Book Review: Sailing Into Oblivion

Sailing Into Oblivion by Jerome Rand $15.99, available through Amazon As refreshing and inspiring as Jerome Rand’s 2017-18 solo-circumnavigation may have been, his account of the voyage in the book Sailing Into Oblivion: The Solo Non-Stop Voyage of the Mighty Sparrow may be even ...read more

01-1970-Dec

50 Years of SAIL

Back in early 1970, Bernie Goldhirsh and the recently founded “Institute for the Advancement of Sailing,” publisher of an annual sailboat and gear guide, launched something called SAIL. A half-century later, a look back at the magazine’s first few years provides a glimpse into a ...read more

Photo-by-Adobe-Stockpics721-2048x

Webinar: Navigating Post-Dorian Abaco

On Thursday, October 22 at 6 pm ET, Navigare Yachting presents a webinar on what to expect from Abaco post-Dorian. The event will feature the authors of The Cruising Guide to Abaco, Steve Dodge and his sons Jon and Jeff.Hurricane Dorian hit Abaco in early September of 2019 and ...read more

LunaRossaBoat2

Luna Rossa Prada Pirelli Christens “Boat 2”

Hot on the heels of the UK’s Britannia and the United States’ Patriot, Italy’s new AC75 Luna Rossa, formerly known as Boat 2, was christened in Auckland, New Zealand, this morning. As the moniker suggests, it was Team Luna Rossa Prada Pirelli’s second design. In his christening ...read more

m7803_DSCF6698-1

Challengers Christen Britannia and Patriot

October 16 proved an exciting day for America’s Cup fans with the christening of both the UK’s Britannia and America’s Patriot. Britannia will be helmed by four-time Olympic gold medalist and America’s Cup winner Sir Ben Ainslie. Olympic Gold medalist Giles Scott will serve as ...read more

HookPromo

Defender Product Spotlight: Lowrance Hook Reveal

Defender product expert Alex Lyons explains the benefits of HOOK Reveal’s new FishReveal technology: “DownScan sonar uses high frequencies to provide a picture-like image of the sea floor. The traditional sonar’s lower frequencies are best suited for locating fish in the water ...read more

701820_promo

Defender Product Spotlight: Lumitec Poco Lighting Control

Several key features of the Lumitec Poco Lighting Control system stand out right away. First, it is a 2-wire system, so the average wiring-savvy boater can install or retrofit the controls without an engineering degree. Second, it works with any Power Line Instruction (PLI) ...read more

Isotherm

Defender Product Spotlight: Isotherm Refrigeration Component System

Retrofitting a refrigeration cooling system has never been easier thanks to Isotherm’s quick mount bracket. And installation is more than half the fun of boat projects, right? These refrigeration component systems also have one of the smallest footprints on the market, and the ...read more