Last time I talked about the meteorological effects on tides, and by extension, tidal currents. This time, I thought I would focus on a different aspect of this interaction, and talk about how wind and current affect the sea state. I will include not just tidal currents, but all ocean currents in the discussion.
As noted last time, many mariners time their activities, whether recreational or commercial, to the tidal cycle. In some cases, it is not necessarily the height of the tide that is an issue, but rather the strength of the tidal current. Indeed, in the age of sail, it was often not only just convenient, but necessary to depart with a fair tide, meaning that the tidal current would help carry the vessel out of its harbor, and not act as a hindrance. Most of today’s vessels have enough engine power to overcome a foul tide, but time and fuel are saved when departing with a fair tide.
Sometimes, however, what would seem to be a fair tide by looking at the tide and current tables may end up producing conditions that are anything but fair. This situation occurs when significant wind is blowing against the tidal current, which can lead to difficult sea states. When describing sea state, it is important not to focus only on the height of the waves. While this distance, measured vertically from the crests to the troughs of the waves is certainly important, information about the period of the waves is equally important in how they will affect a vessel. The period is defined as the time it takes for one complete wave to pass a given point. It is directly related to the horizontal length of the waves. When the period of the waves is shorter, full waves pass a point more frequently, and therefore they tend to be steeper. When the waves are steeper, usually it is more difficult to navigate, especially for smaller craft. The combination of high wave heights and short periods is especially dangerous, and in extreme cases will lead to breaking waves.
The wave pattern is never completely regular, but rather is a combination of locally generated wind waves, and perhaps one or more swell sets which were generated at some distance and propagated into the area, and within each of these wave sets, waves will have some variation. Swells tend to be longer waves while wind waves tend to be shorter. While swells will be affected by currents, it is the locally generated wind waves that I would like to focus on for the situation when the wind is against the current.
When the current is flowing in the opposite direction of the wind, then the waves that are generated by the wind will tend to become shorter, and, as a result, steeper. This means that the wave period will be reduced, and more waves will pass a given point during a given time interval. The wave heights can become a bit higher in this situation as well, particularly when the wind speeds become moderately strong, as the wind energy being converted into waves needs to be distributed through a smaller space. This can lead to a situation where the tidal current tables indicate a fair tide, and the predicted wind speed and direction, while adverse, is not that serious on its own, but the combination produces very rough seas.
Let’s look at a potential situation. Assume that a vessel has moved through the Chesapeake and Delaware Canal and is planning to head down Delaware Bay and into the Atlantic. The current is predicted to be ebbing (flowing seaward, or down the bay) as the vessel heads down the bay, but winds are forecast to be south-southeast in the 15-20 knot range. This would mean a moderate headwind for the vessel, but more importantly, would place the winds directly against the ebbing current, and in this situation seas could run as high as four to six feet and would likely be short and steep leading to a very pounding ride. If the ebbing current was not present, many vessels could power through the headwinds, and the seas, while uncomfortable would likely be manageable. In fact, even the situation with a flooding tide, which would place both the wind and current against the vessel, might be preferable because the seas would not be as short and steep as they would be when the wind was against the current.
Another situation that arises frequently has to do with winds blowing against the Gulf Stream. This, too, can lead to very difficult sea states, and with this particular current being very strong in many areas, when moderately strong winds oppose the current for a significant period of time, this can result in very high seas which are also short and steep, a dangerous situation for smaller vessels. For southbound vessels passing through the Hatteras region, a moderately strong northeast wind would seem to be an ideal situation with the wind behind the vessel, but with the Gulf Stream flowing toward the northeast in this area, very rough seas will develop, and this is one of the reasons that this area is known as the “Graveyard of the Atlantic.” Similarly, strong northerly winds east of Florida can make the short passage from Fort Lauderdale or Miami across to the Bahamas impossible with seas at times exceeding 12 feet with short periods.
Other locations where the interaction of wind and current can be very important include the Straits of Juan de Fuca, the Golden Gate, The Straits of Gibraltar, the Old Bahama Channel, the Kuroshio Current east of Japan, and both the eastern and western ends of Long Island Sound. All voyagers who have plans to move through these areas, or any other areas where either permanent currents or tidal currents prevail should make sure that they are aware of situations when wind opposes the current. In the case of tidal currents, it may be worth forgoing any possible current assist and moving through the area during slack current or even adverse current to avoid rough sea states. In the case of permanent currents, often a delay until the adverse wind situation changes may be required, perhaps as long as several days.