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Tide Clocks: The "Almost" Unique Gift
by William J. Cook
     Here we are again, another year older and with a lifetime's events in our recent past. But with confidence in the economy and the grit to go undauntedly about our business, we head out in droves-checkbooks in hand-to stage our own attacks on malls, department stores and, in many cases, nautical specialty shops. The prime question this year is as old as gift-giving itself, "What do you get the person who has everything?"

Christmas on the Water
     Boaters have always had an affinity for brass and chrome, and during the Christmas season, the gleam of brass and chrome clocks and barometers will draw shoppers like moths to a flame. But, does the person or family you're shopping for already have a nice brass clock like this or a barometer mounted like that? And, what did their kids say they were getting them for Christmas? Time is ticking away, and your decision is not getting any easier. How about a tide clock? Not being as plentiful as its more popular clock and barometer cousins, the easily overlooked tide clock might be just your ticket to a gift that is thoughtful, tasteful, enduring and within the budget of all but the most miserly.

      See, wasn't that easy? Yet while this little device may fulfill the criteria just mentioned, there is still one thing West Coast sailors-or those shopping for West Coast sailors-need to keep in mind: tide clocks do not work as accurately here as they do on the East Coast.

Understanding Tide Clocks
     While we rarely consider it, clocks do not delineate time arbitrarily. Instead, their graduations in hours and minutes represent the spinning of the earth on its axis or the relative position of the sun at different times during a 24-hour day. The sun is at its highest point of the day at noon and is on the opposite side of the earth at midnight; on our eastern horizon in the morning and the western horizon in the evening. Similarly, tide clocks have an additional mechanism designed to keep track of the position of the moon relative to the rotation of the earth. Since it is the gravitational pull of the moon (and to a more limited degree, the sun) that creates tides, tide clocks have an additional hand to indicate the moon's position, and thus the relative height of the tide.

     As the position of a clock's hands are measured against a set of numerals, the hand of the tide clock is measured against a background marked: "High Tide," "Half Tide," and "Low Tide" with additional markings for "Before High Tide," "After Low Tide" and the like.

Using A Tide Clock
     With the various tide calendars, charts, software and websites available to tell us all we could ever want to know about the tides at any location on either coast, why would one even want to bother with a tide clock in the first place? The answer: simplicity. There are no dog-eared charts or booklets with microscopic numerals to hunt, no computers to fire up, no login errors to deal with. Checking on the tide simply becomes a by-product of checking the time. Well, that is how it would work in a perfect world. Unfortunately, we do not live in a perfect world and unfortunately tide clocks do not perform as accurately on the West Coast as they do on our eastern shores.

Limitations of the Tide Clock
     One might wonder why a disparity between the coasts exists. After all, are the waters of San Francisco Bay not subject to the same forces of the sun and moon as the waters in Boston Harbor? The answer of course is, "yes." If the influences of the sun and moon were the only facets of the phenomenon to be considered, tide clocks would work equally well on both shores. There are, however, a number of other influences to be considered. Only the most important will be discussed here.

      If there were no landmasses, tides would form two relatively uniform bulges in the ocean's waters that would follow the moon as it travelled east to west. But there are landmasses in the way and westward travelling waves on the Atlantic run head-on into the East Coast of North America. This results in a slight lag between the time the moon passes overhead and when the tides arrive at the various ports.

      Tidal effects on the West Coast work differently because the continents of North and South America are barriers to tidal flow and this "wave" of tides cannot continue to follow the moon as smoothly as it could on the open sea. Waters of the high tide along the west coast of Canada and the US are actually drawn eastward from the Pacific Ocean. The farther north the location, the more the arrival time of the high tide lags behind the time the moon passes overhead, causing the second high tide of the day to be significantly lower than the first. This difference is part of an effect that exists here on the West Coast called "diurnal inequality" and changes throughout the month as the declination of the moon changes from north to south. This is the primary reason tide clocks do not work as accurately here as they do back east. This is especially true in the lower reaches of Puget Sound where incoming tides must not only traverse the full length of the Strait of Juan de Fuca and Puget Sound but which also have their effects altered by the many rivers and streams that are constantly releasing water into the Sound.

Gauging Accuracy
     Just what kind of accuracy can one expect from a tide clock purchased for use on the West Coast? In the Puget Sound region, tide clock readings can vary from actual tidal measurements by as much as one-and-a-half to two hours during certain times of the month with discrepancies normally not so pronounced in other West Coast ports. Skippers of large military or merchant marine vessels will find such irregularities almost inconsequential. However, for smaller craft, especially sailboats, they could be quite troublesome. For example, if you were planning to leave port on a tight schedule thinking you were sailing with the tide-or at slack water-only to find yourself sailing against the tide, your whole schedule would be shot.

The Tide Clock's Greatest Niche
     So, why bother telling you what a great gift a tide clock would make if only to turn around and tell you they are not very accurate? Because most first-time tide clock shoppers, relying on their faith in today's technology and, being unfamiliar with the forces just mentioned, tend to believe that tide clocks are as accurate as any other time-keeping instrument. This Christmas, they will spend thousands of dollars purchasing instruments that may never perform to expectations. In addition, this fact is not readily available to consumers. Typing "tide clocks" into Yahoo's web browser, for example, turned up only three sites associated with them. All of these were small nautical gift shops on the East Coast and none provided any information concerning the instrument's history or limitations. Thus, being forewarned is being forearmed as you may find yourself shopping in a store where the salesperson is either reluctant to tell you about the tide clock's less-than-noble performance or who is unfamiliar with the problem altogether. Even when shopping in the ancient and honourable nautical supply shops up and down the coast, you are as likely these days to be helped by a new-hire as you are by an old salt-especially during the Christmas season.

      What to do? If so inclined, buy a tide clock anyway; it still fills the bill of being a thoughtful, tasteful and enduring gift. Just keep in mind that 95 percent of all units sold are sold as gifts and that, whether the tide feature is ever to be used or not, they may still be used to accurately tell…time! And hey, didn't Tom and Peggy move to Nova Scotia last year?  

An authority on optical and navigational instruments, William J. Cook is a former Chief Opticalman for the Navy and is manager of the Precision Instruments and Optics Department of Captain's Nautical Supplies.