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    Categories: Cruising Life

Tidal Boating: Know the Flow

By Mike Milne

Those of us who do most of our boating on the Great Lakes don’t spend much time worrying about tides. Very occasionally there is a seiche, the freshwater tidal near equivalent. But unlike tides, that rare event — caused by high winds, exaggerated oscillation of water and the shape of a lake or bay or also a phenomenon — is rare and hard to predict.

The need to keep close track of tides was driven home to me during a trailer-boating vacation to the Atlantic coast. I had spent about a week day-boating in the Gulf of St. Lawrence around Prince Edward Island, where tidal ranges are about four feet. We tracked the tides there on a daily basis, but did most of our boating on the open water of the gulf, or in channels deep enough that tidal changes were no concern.

On the way home, during a stop at a waterfront inn in Riviere Du Loup, Que., the wide channel of the St. Lawrence River beckoned and we considered a visit to a nearby Island. Isles Aux Lievres is about eight kilometres from shore and otherwise accessible only by ferry. Arriving in late afternoon, we checked out the local marina, where there was a decent launch ramp and docks with a few sailboats, power cruisers and fishing boats.

High winds the next morning shelved our plan, but we re-visited the marina before leaving town and discovered another unanticipated problem. The basin was almost dry, with boats that had been afloat either sitting in mud or leaning against dock pilings. The launch ramp ended in a muddy slope and a puddle! Further research revealed that the town had a tidal range of about 16 feet and tides in the area have ranged as up to 19 feet.

For a country with so much freshwater boating, Canada is also home to very extreme tides.

Like most large river estuaries, the St. Lawrence experiences large tides far upstream. Quebec City, 200 kilometres inland to the southeast of Riviere Du Loup, has tides that range normally from 12 to 17 feet and have been as high as 21 feet.

That kind of range in water levels may be impressive for freshwater boaters, but pales against the tides of the Minas Basin — at the northeast end of the Bay of Fundy that separates the northwest shore of Nova Scotia and the south shore of New Brunswick — home to the world’s highest tides.

At the head of the Minas Basin, at Burntcoat Head, N.S., there’s a normal tidal range of 47.5 feet, with ranges as high as 53.5 feet. Since the tides virtually fill and empty the basin twice a day, boaters and commercial mariners must plan navigation carefully, especially since the vast movement of water causes a tidal wave or bore, very strong currents and even whirlpools.

Like any current, tidal currents mean boaters either have to plan to travel with them or have the horsepower to move against their flow.

It should come as no surprise that there is not a great deal of pleasure-boating on the Minas Basin. For adventure-seekers, though, there are rafting companies that specialize in rides on the tidal bore, with the incoming tide.

In areas of more moderate, or more manageable tides, boaters’ main practical concerns are tracking them. But before looking at tide tables and tide predictions needed for trip planning, some background information is in order.

As the U.S.-based National Oceanic and Atmospheric Administration (NOAA) explains in a web-based downloadable tutorial, “tides are very long-period waves that move through the oceans in response to the forces exerted by the moon and sun. Tides originate in the oceans and progress toward the coastlines where they appear as the regular rise and fall of the sea surface. When the highest part, or crest of the wave reaches a particular location, high tide occurs; low tide corresponds to the lowest part of the wave, or its trough. The difference in height between the high tide and the low tide is called the tidal range.”

What causes those long-period waves is a bit more complicated, involving the competing gravitational forces of the sun and moon on the earth. The moon mainly wins that contest and has the greatest influence, mainly causing opposing “bulges” that influence the earth’s surface. On the side closest to the moon, the gravitational pull of the moon causes a bulge, while there’s a bulge caused by inertia or centrifugal force on the other side. The influence of the sun either moderates or exaggerates those forces.

Generally speaking, the highest tides — called spring tides, but having no relationship to the season — occur during the full moon phase, when sun and moon are on opposite sides of the earth or during the new moon phase when sun and moon are on the same side of the earth. Tides with the lowest range — called neap tides — take place when the moon is in its first and third quarter phases, at right angles to the line between sun and earth.

For the most part, in most places, tides are what is called semidiurnal, with two high and two low tides daily. But at some places and in some times, there’s only one low and one high tide. Tides are also affected by the earth’s tilt on its axis (that affects the nearness and gravitational pull of sun, moon and other planets), sea floor topography (like the shape of the Bay of Fundy and Minas Basin that helps to create the world’s highest tides), plus effects of global and local winds.

Tides are also affected by the way landmasses are located on the earth. Generally, there are great tidal ranges in northern hemisphere areas including Europe, from Britain to Spain, in the eastern Canadian Arctic and along the coast of British Columbia and Alaska.

Tides in Tofino, B.C., a surfers’ mecca on the west coast of Vancouver Island, are usually seven to 10 feet, but can have a range up to 14 feet. That compares to normal tidal ranges in Vancouver of seven to 13 feet, with tides that have ranged up to 16 feet.

With often wide tidal variations, even within cruising grounds, boaters have to be able to track tides and plan accordingly. Changing depths affect boaters’ ability to cross shoals or other underwater obstructions, clearance under fixed objects such as bridges or wires and techniques for docking and anchoring. Boaters also have to be aware of tidal currents that come with flood (incoming tide) and ebb (outgoing tide), and understand differences in the strength of both, as well as tendencies for currents that run parallel to shores or strengthen when two bays meet.

Charts, whether paper or electronic, show depths in non-tidal areas that reflect datum, or historical low water levels. In tidal areas, datum on U.S. NOAA charts is mean lower low water (MLLW), the average of lowest low tides based on 19 years of tidal data. For overhead clearances, higher high water (HHW) is used as the gauge. Most Canadian Hydrographic Services coastal charts use lower low water, large tide (LLWLT) as datum, although it is called lowest normal tide (LNT) on some charts.

Definitions aside, for practical purposes, boaters can be assured that chart datums represents an average of water depth at low tide, as low as it usually gets. As for overhead clearances, averages of high water are used.

In home waters, most boaters develop a good sense of local tidal ranges and may use a variety of casual sources for determining ever-shifting high and low tide times. Local newspapers usually publish tide times along with the weather, while angler-oriented and boater-oriented stores often hand out promotional monthly pocket tide tables. Near Boca Grande in Florida, where I am often boating in winter, a local liquor store provides a handy complimentary guide showing tide times for a month, complete with phases of the moon, plus a list of legal fish size and catch limits.

For boaters needing more details on tidal range and currents, there are a variety of sources, including printed tide and current tables for different regions. In Canada, Fisheries and Oceans Canada publishes Canadian Tide and Current Tables in seven regional volumes; they are available through CHS chart dealers. For the U.S. and territorial waters, the NOAA also publishes tide and current charts. But for both Canada and the U.S. and the rest of the world’s oceans, web-based services are the best bet today.

For Canadian waters, go to tides.gc.ca and click on Oceanic forecast, then Index of Sites and Search Station List (to find the station nearest your location) and look under Data Available. There are tables as well as charts usually showing the real-time progress of the tide.

For NOAA information go to tidesandcurrents.noaa.gov. Under the PRODUCTS menu, choose NOAA Tide Predictions. Click on the region, then choose the station for which you want info.

Bookmarking the specific web locations of either of these two sites — on your computer, phone or other device — makes future searches easier.

Some commercial sites are also helpful. For tide charts anywhere in the world, check out tide-forecast.com. In southwest Florida, localtides.net can be helpful. A site called tides4fishing.com may be helpful for anglers.

There are also free apps that may make it easier to access info. Check out Tides Near Me and My Tide Times. Some boaters may find these or other apps easier to use (and upgrade for a fee). But for CHS and NOAA info, the official sources are free, easy to bookmark and access and provide reliable, easy-to-use information, with no commercial distractions.

To paraphrase an old anonymous proverb, “Time and tide waits for no one.” But boaters are always well advised to take the time to find out the time and range of the tides any time they plan to head out onto salt water.

This article is featured in the Summer 2017 issue of Boats&Places.

Mike Milne:

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