Time and tide

Greg Jones
On a recent sailing trip that involved passing through Woods Hole, we were again reminded of the important role that tides play in our boating activities. Arrive at the entrance to Woods Hole, a fairly narrow passage between the Elizabeth Islands that separates Buzzards Bay from Vineyard Sound, at the wrong time, and you could be bucking a tidal current that can reach five knots. Conversely, arrive at the right time and your boat will fly along, boosted by a fair tide that is free for the timing.

Tides, whether your boating activities take place in areas of wide-ranging tides or of fairly small tides, are always an important factor. Drop your anchor at high tide with a few feet under your keel and soon you could be listening the rhythmic bump of your keel on the bottom. The noise might stop when you’re high and dry, or it might stop when the tide comes back in. Best to avoid either eventuality.

There are tides, and tidal currents, in nearly every body of salt water, and, for freshwater sailors, virtually none in lakes, regardless of size. Spring tides in the Great Lakes, if they can be noticed at all, are around two inches. NOAA does not publish data on Great Lakes tides because the changes are so small they are in effect masked by other changes in lake levels caused by wind and weather. There is the phenomenon known as a seiche, but those intermittent surges of water have nothing to do with tides.

Establishing the currents related to tides is predictable, within limits, but they are always best if verified by observation. Although the currents are directly related to tides, the variables induced by land masses, the volume of water and the effect of wind make even the best current tables something less than an exact science.

For all that, the information is vital and can make the difference between a comfortable day on the water and a long slog beating into a foul tide. When the current table says to expect five knots of current, it is a certainty there will be a current, in the forecast direction, but the prediction might be off by a knot or two.

The slower your boat, the more effect the currents will have on the duration of your voyage. A 25-knot powerboat beating into a five-knot tide will lose 20 percent of its speed; a five-knot sailboat won’t be moving at all.

On charts produced by the British Admiralty, there are “tidal diamonds,” small diamond-shaped symbols with a letter in them that corresponds to a legend printed on the same chart. These diamonds tell the direction and speed of the current in spring and neap tides. Tide tables and currents in select locations are available online from NOAA and many other private sources, but haven’t been published as “hard copy” by the Government Printing Office since 1996. It seems there was a budget cut.

There are printed tide tables and tidal-current predictions available for your boating area, and a copy should be on board. Additionally, most data packages on chartplotters includes tidal information and currents. NOAA’s Electronic Navigation Charts include tidal currents and tidal ranges. The NOAA ENC vector charts are available online for downloading the most up-to-date version.

The tidal range for select areas is available on most paper NOAA charts, but without knowing the tide schedule, the information will be of limited utility. Note that the charted depth is for Mean Lower Low Water (MLLW), which is the average of the lowest tides at the location. The only lower tide category is Lowest Astronomical Tide, which is the lowest possible theoretical tide, combining every contributing factor happening at the same time. MLLW, practically speaking, is about as low a tide as will be seen.

When we left our home harbor, with the Woods Hole passage to occur on our first day, we calculated our estimated passage time to the entrance to the pass and planned our departure to arrive just after the current turned in our favor. The time for the current change was written on a sticky note and placed on the chart we were using in the cockpit.

Tidal information is a vital part of proper anchoring procedure. Leave the endless discussions over the merits of a CQR versus a Bruce anchor to the sailors navigating the dockside bar. The most important part of anchoring is to be certain you have enough water under the boat.

Part of a successful voyage plan includes tidal information at your harbor of intent and harbors of necessity. Lay your anchor and figure out how much scope to allow taking into account the state of the tide at that time. If there are other boats nearby, remember that sailboats, with their large keels, will be more heavily influenced by tidal currents than powerboats. That can mean, for example, that a wind against tide could set a sailboat into the anchoring radius of a powerboat. Large powerboats, with relatively shallow draft but a lot of freeboard, will weathercock, while sailboats, with their larger keel, will tend to “tide-cock.”

For example, you’re in 10 feet of water, and the anchor roller is four feet above the water. A conservative scope would be seven times the total distance, meaning you let out seven by 14 feet, or 98 feet of rode. If the bottom is good, no winds are forecast, and currents aren’t a factor, you can reduce the scope somewhat, but you still need to take into account the tide. If you’re anchoring at low tide and expect five feet of tide, you will then be in 15 feet of water. Now, for a 7:1 scope, you need to let out 19 by seven feet, or 133 feet of rode. If you have an anchoring neighbor within that distance, and your neighbor might react differently to the effects of wind and current, you might want to reconsider your anchoring position.

Setting two anchors in line with expected current, the so-called Bahamian moor, with an anchor set from the bow and the stern (or both anchors of the bow), will significantly reduce your swinging radius. Setting two anchors from the bow, at an angle of 45 to 60 degrees, will also reduce your swinging radius, but note that this may cause other boats, not similarly restrained in their swinging radius, to swing into your boat. Anchoring etiquette means that later arrivals establish their anchoring location and set-up to accommodate earlier arrivals.

Currents and tides can’t be successfully fought but the prudent mariner can use them to speed up a passage, and knowledge of those tides and currents will do much to avoid drama and possible damage to your boat.

If you nonetheless find yourself aground, the first important piece of information you need is what the state of the tide is at that moment. If it’s on the ebb, every minute you’re hung up the situation is getting worse. Try to back out, reduce draft by heeling the boat with everyone gathered on one side, but the chances are that you might be there until the tide turns.

Inspect the hull for any damage or evidence of leaks. Powerboats can damage their props, and revving the engine while aground can stir up sediment that can clog up the engine’s impeller, leading to overheating. Pulling a boat off a grounding can be difficult and is generally a task best done by experts. Even the professionals can’t get a boat off that is high and dry.

If the tide is on the flood, get out an anchor to hold your position and wait for more water to arrive. While you’re waiting, check the hull for damage. A pan-pan call on your radio, stating your location, the state of the boat and the tide, will alert the Coast Guard and TowboatUS to your situation.

How fast does the tide go out? The Rule of Twelfths is a handy formula: In the first hour of the tidal change, the water level changes by one-twelfth of the total range. The second hour, two-twelfths; the third and fourth hours, three-twelfths per hour; the fifth hour, two-twelfths; and the final hour, one-twelfth.

Carry with you information on your local tides and currents and be as aware of these important factors as you are of the weather. The good thing about tides is that the forecasts are generally more accurate than any weather forecast, especially if you’re planning for a trip next week.

 Greg Jones and his wife Barbara returned from their 21-month cruise to the Florida Keys and the Bahamas in June 2015. They sail Chamba, their 1979 Gulfstar 37, in Buzzards Bay and nearby waters when they’re not exploring farther afield. They live in Dartmouth, Mass., and are members of the New Bedford Yacht Club.