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:
rule%20of%20twelfths%20anchor

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.

rule-of-twelfths-chart

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).

Shoals ahead! Is there still enough water for you to pass safely over them?

Shoals ahead! Is there still enough water for you to pass safely over them?

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

GMR19_F53_0539

Boat Review: Beneteau First Yacht 53

Luxury performance-cruising isn’t entirely new. You can go fast and still be comfortable. You can even race if you make the right tradeoffs—or so says Groupe Beneteau, which recently launched its First Yacht 53, in the process also reinventing the company’s “First” brand. ...read more

BirdonBoard

Cruising: Birds of a Feather

One of the neatest things about sailing offshore is the other lifeforms we encounter. We smile when we see flying fish skimming over the surface of the sea. We cheer when dolphins leap and dance in our bow waves. We are duly reverent when mighty whales sound and spout, and ...read more

pirate-marlin-logo

Pirate’s Cove Billfish Tournament

This Spring has felt a bit like a slow day of fishing but it’s almost time to time to Bait…and Switch. The Pirate’s Cove Billfish Tournaments offer something for every member of your crew. Exceptional fishing, fun times with family, old friends and new. Our run is short, our ...read more

Tusk.00_00_16_21.Still001

Tusk by Spyderco

Spyderco's Tusk knife combines a blade and marlinspike to create a multitool perfect for marine use. Made with corrosion resistant materials like titanium and LC200N steel, a nitrogen alloyed steel, and Spyderco-exclusive locking mechanisms, this knife is one of the toughest ...read more

IY11.98_fTaccola©DJI_0200

Italia 11.98 Performance-cruiser

This past winter, SAIL principal editor, Adam Cort, check out the new Italia Yachts’ new 11.98 performance-cruiser at the boot Dusseldorf show in German, and says he can attest to the fact it’s a boat more than worthy of the attention of North American sailors. Available in ...read more

01-LEAD-2017_IDALEWIS_0219

Racing: Doublehanded

I was born in 1955, and although I was a tad young to actually follow the first Observer Single-Handed Transatlantic Race, I grew up in the age of the pioneers of solo offshore sailing—Francis Chichester, Robin Knox-Johnson, Alec Rose, all Knighted for their singlehanded ...read more

newport-400c-updated

Spectra Watermakers

The world's quietest and most energy-efficient watermakers. Our product line ranges from hand operated desalinators that can make 6 gallons a day to our largest system that produces 20,000 gallons of fresh, potable water per day. Spectra Watermakers has the complete package: ...read more

CAPE-COD---Under-sails

Boat Review: Cape Cod

The concept of “daysailer” has grown ever broader over the years. These days the label can be pasted on a boat as small and simple as a Sunfish, as fancy as a 40-footer with a nice big cockpit and plenty of brightwork, or any number of concoctions in between. This Cape Cod ...read more