Safe Boating

Know your tides when out on the water

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Time and tide wait for no man. True, but what does tide have to do with boating in Narragansett Bay? It certainly affects your height above the bottom. There are areas you can safely pass over at high tide, but you’d run aground at low tide. Conversely, there are bridges you can get under at low tide, but not at high tide. How high is your mast if you are a sailboat owner? How about your tower and antennas if you are a powerboat owner?

You should know how much water your boat needs, and how high it is from the waterline to the top of your highest antenna. We once watched a foreign tall ship sail up the Mystic River without consulting their chart and run firmly aground. Unfortunately, it was on an outgoing tide, so she had to sit there embarrassed, for many hours. It was a training ship and I imagine all the trainees spent those hours learning all about tides and charts.

Narragansett Bay has an average depth of 26 feet, and its maximum depth is 184 feet in an area off Castle Hill. The depth difference between low and high tides is generally three or four feet. That’s one of the considerations in tying your boat up to a fixed dock or a piling. You don’t want to tie it up tightly at high tide and return at low tide to find it hanging on its lines! Of course we do not have to contend with the large range of tides found in some places. In Nova Scotia the shape of the land with long inlets makes the tide difference as much as 50 feet! When the tide changes, the resulting tidal bore in the Bay of Fundy, particularly at a full moon, is a wall of water surging up the narrow inlet. A similar tidal bore, although not as spectacular, flows up the Amazon River at tide change. The power of tidal water moving can be harnessed. In La Rance River in France they have had a dam with 24 turbines, which has produced power for Paris and Brittany since 1966. Closer to home, in Hingham, Massachusetts, for over 300 years they operated a tide-powered grain mill.

Tidal currents do affect us locally. If you are going through the Cape Cod Canal, at certain times you may encounter a current up to five knots. Certainly enough to cause a sailboat to lose ground!

What causes tides? The sun is 27 million times larger than the moon, but it is over 350 million times as far away. The moon varies from 221,463 to 252,710 miles from Earth, about a quarter of a million miles away. Of course when the sun and moon pull together, the tides are higher (and lower) than when they are at right angles. The alignment of the earth, sun and moon corresponds to new moon and full moon. These are referred to as spring tides – not named for the season, but for springing up higher (and lower) than normal. The lesser high and low tides found at first and last quarter moons, are called neap tides. Some Warwick roads are regularly flooded over during spring tides, particularly when the moon is closer to the Earth. This would be increased further if a wind from the south adds its push of the water into Narragansett Bay.

Local tide data can be found on the charts available at any marine store, or online.

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