Finding set and drift

The term “set and drift” is used to describe several external forces that affect a boat and keep it from following an intended course. Just as the term “flotsam and jetsam” is used to describe the assortment of remains from a lost ship, set and drift is often used as an encompassing explanation for factors affecting a boat’s position. If a fix does not plot where expected, the effects of set and drift are often blamed for the discrepancy.

Traditionally, set and drift is defined as the effect a current has on a vessel’s movement, with set referring to direction and drift to speed. The Gulf Stream off Florida’s east coast, for example, has a set close to due north and a drift of four knots.

Often the term “leeway” accompanies discussions of set and drift since leeway is the perpendicular distance a vessel is moved away from its intended course due to set and drift. Additionally, steering error, dependent on the attention and skill of a helmsman or the accuracy of an autopilot, affects a boat’s position relative to its intended course and is lumped into the set and drift ditty bag when needed.

In this age of sophisticated loran, GPS, and differential GPS (DGPS) navigation, with their highly accurate ability to compute course and speed to waypoints, a navigator might wonder if a discussion of set and drift is relevant. When a waypoint is entered into a loran or GPS receiver, the computation of course to steer and speed made good towards that destination can be obtained with a glance.

Though this information is, naturally, of use to a navigator, it is deficient in one area. Electronic instruments cannot, as of yet, see into the futurethey make determinations based strictly on a vessel’s past positions. Before the advent of “waypoint” technology, a vessel wishing to cross a moving body of water had to make manual course and speed computations to avoid being set up on rocks or reefs or miss its destination. These skills are still needed and can be complemented by GPS or loran waypoint technology.

On a voyage from Newport to Bermuda, for example, a navigator would want to compute a course to steer by taking into account the Gulf Stream’s northerly and easterly set and drift. In manually doing these computations a navigator can “look” down a course line and see a current’s changing speed and direction and make appropriate course/speed alterations when or before required, not after.

Solely following a course calculated by loran or GPS will lead a boat into a loxodromic spiral, resulting in a circling of the desired waypoint or destination. Only by a navigator’s looking ahead at the expected current conditions, incorporating that information with a boat’s performance characteristics, and then referring to loran/GPS waypoint information can an accurate evaluation of set and drift be accomplished.

Proper plotting technique to compensate for set and drift is to use a DR track line drawn for the course steered and speed through the water. Successive fixes, as they are plotted, will show if the desired track line is being followed. If fixes do not fall on the desired track line (course made good), then changes to the course steered can be made after first recomputing set and drift by comparing DR positions and fixes of like times.

Reading the clues

There are various clues in both coastal and ocean sailing that aid a navigator in assessing set and drift. Water passing by buoys, day marks, or other fixed objects such as docks or pilings will reveal current direction and speed. In Maine waters a plethora of lobster-pot floats provide updates on current direction and speed almost as frequently as GPS’s average of once per second. It is good practice to pass buoys sufficiently close to observe current flow and then to compare observations with anticipated conditions.

Range markers at the entrance to harbors and canals show precisely, and without delay, if a boat is being set outside a channel or fairway. Since ranges are commonly marked with both day boards and lights, they can be used day and night, and when both ahead and astern of a boat.

In ocean sailing, where there are few stationary landmarks to reveal ocean currents, it is deviation of a boat’s wake from directly astern as well, as the plotting of frequent fixes with comparison to a reliable DR track line, that reveals set and drift.

Two examples of set and drift calculation will show how they can used effectively. First, on a voyage from Miami to Bimini courses are needed to take a vessel across the Gulf Stream while minimizing northerly set (see figure 1).

Step one in determining an appropriate course strategy is to construct a rhumb line connecting Miami’s sea buoy and Bimini’s anchorage. This course, measured on an appropriate chart, should be near 093° magnetic. The next step is to measure overall voyage distance along this track, which should be approximately 43 miles.

Now the average passage time is computed using the distance of 43 miles and boat speed appropriate to the vessel. In this example, a speed of eight knots is used. (If boat speed changes during a voyage due to wind, seas, or mechanical reasons a new speed should, obviously, be introduced, and the courses recomputed.)

A passage time of 5.4 hours (43 miles/8 knots = 5.4 hours) is calculated, which leads into the next step, determining the Gulf Stream’s effect on this vessel during a 5.4-hour period. Referring to NOAA chart 11460 shows average Gulf Stream currents marked at six-mile intervals from Florida’s coast to Bahama Bank.

Using this current information, courses can be calculated for each six-mile leg of a crossing to negate being set north. On the first six-mile leg a 2.4-knot current will be pushing north while the boat in this example is moving to the east (093°) at 8 knots. It will take this boat 0.75 hours to cover six miles at eight knots (6 miles/8 knots = 0.75 hours), and so to neutralize the Gulf Stream over this period the boat’s heading needs be towards a point 1.8 miles (2.4 knots x 0.75 hr = 1.8 miles) south of the original rhumb line.

This course-modification procedure is duplicated for each of the seven legs making up the Gulf Stream crossing and will result in the boat’s crew sighting Bimini dead ahead close to five hours after departing Miami.

A key aspect to this manual method of dealing with set and drift is plotting fixes at regular intervals during a passage, comparing those fixes to the DR track line, and making adjustments for non-average, or changing, current.

Accurate manual plotting

Is manual plotting more accurate than relying on loran/GPS waypoint and crosstrack error readouts? Yes, since, as mentioned earlier, adjustments from loran/GPS can only be made after a vessel has been pushed off the desired rhumb line and set and drift (crosstrack error) information calculated.

In another example, a boat sailing west to east through Bass Straitthe body of water separating Australia’s south coast from Tasmaniain December would need to pay particular attention to set and drift considering all the of rocky islands, oil rigs, and fishing vessels that inhabit this 500-mile stretch of ocean (see figure 2).

Consultation with a Pilot Chart reveals an average current in this area of 1.7 knots flowing in a direction of 167° true. Seas average eight feet with winds from the southwest at Force 5. Since wind and current are often at angles to each other in Bass Strait, it is known for its rough conditions.

A desired course through Bass Strait is 074° true, and with a current running to the southeast a correction left of track is necessary. In this scenario assume a boat with good off-the-wind sailing capabilities that can make a boat speed of 11 knots under Force 5 conditions.

For each hour this boat sails along the course of 074° it will be set 1.7 miles in a direction of 167°. To find a compensating course for this set and drift a simple vector sketch is drawn which solves for the correcting course. In this case the new course is 066° true. So, if set and drift remain at 167° true at 1.7 knots, and boat speed hovers around 11 knots, a course steered of 066° will maintain a track line of 074° indefinitely. Knowing this corrected course, loran and GPS waypoint functions can assist in monitoring a vessel’s progress and ensure, through frequent fixes, that the desired rhumb line is being followed.

Rules of thumb

A few rules of thumb on set and drift are worth mentioning. The distance a boat is set from its intended track line is not so much a function of a current’s speed but the length of time a boat is exposed to that current, so it is best to maximize speed when passing through an adverse current. This often involves altering course to increase boat speed. Current on the beam will have the most effect in setting a boat from its desired track line, but a small change in course to put the current abaft the beam will produce a substantial reduction in leeway and increase boat speed.

For example, in the earlier discussion of a boat heading from Miami to Bimini, if that same vessel had been departing from Fort Lauderdale, 50 miles to the north, its most efficient course upon departure would have been to head due south along the Florida coast, staying close to the beach to avoid Gulf Stream current, and then heading to Bimini after reaching Miami.

To head for Bimini from Fort Lauderdale in all but a fast powerboat would invite bucking a four-knot current forward of the beam. This would substantially reduce boat speed and lengthen the transit time.

There are more than 50 major ocean currents. Several of these flow at speeds in excess of four knots: the Somali Current off Africa, the Kuroshio Current off Japan, and the Gulf Stream off the eastern U.S. There are many more rivers in the ocean with currents in the three-knot rangethe South Equatorial Current and the Agulhas Current are good examples.

Navigators should use these currents to advantage just as winds from high and low pressure systems are used for course planningby computing the effect of a current’s set and drift on the voyager’s proposed track. Lack of attention to these currents can result in wasted time.

An excellent reference to ocean currents and their use is the recently published 1995 edition of The American Practical Navigator, known also as Bowditch, published by the Defense Mapping Agency and available from the National Ocean Service (NOS) or from many NOS-authorized chart dealers.

By Ocean Navigator