How To Calculate Set And Drift

Attempting to canvas the 114 degree course to Gun Cay, the boat must caput upward into the electric current to avoid being set northward

An understanding of the effects of gear up and drift is vital for coastal sailors; it can make the difference between an enjoyable passage or a struggle to attain your destination against a foul current.

As an case, permit's program a Gulf Stream crossing from Miami to Gun Cay, Commonwealth of the bahamas. Yous know it is safest not to cross the stream in any current of air with a northerly component, then you are excited to hear that south-southeast winds will persist for two days at a moderate 10 to 15 knots. Your course to Gun Cay is 114 degrees magnetic, making you confident that you lot can canvass in the predicted 160-caste wind. With no seas, in a boat that points well, yous may be correct—were it not for the Gulf Stream.

The stream averages ane.five knots where it intersects the route from Miami to Gun Cay. Depending on the speed of your gunkhole, this may have a significant impact on your form, since the current runs almost perpendicular to your road. In a powerboat the result of the Gulf Stream is negligible, simply in a sailboat, especially 1 wishing to sheet rather than motor, this current can make what at first appears to be a pleasant sail into an impractical and unpleasant experience.

Correcting the grade to allow for set up, the boat must steer 132 degrees rather than the 114 degrees rhumb line

Set and drift are ii terms used to describe the outcome currents have on your boat. Set is the management in which the electric current is flowing—in this case, 18 degrees. Drift is the speed of the current. Y'all need to take both of these into account when taking advantage of a atmospheric condition window to cross any potent current.

Some sailors mistakenly believe that because they have a GPS to tell them what class to steer, they do not need to sympathise the effects of a current such as the Gulf Stream. This is simply not truthful. While your GPS will indeed brandish the straight line between you and your destination, information technology volition not predict the effects of set and drift, which can change a favorable air current direction into a headwind.

Never forget that a GPS is reactive, telling you how to correct your heading to stay on form. This means that at the start and cease of your passage, you will exist sailing the predicted heading. However, in the middle of the stream, where the drift is strongest, you will accept to betoken further and further into the electric current in order to remain on your rhumb line. Such a drastic change in your heading as well brings the wind further forrard so that you will be beating into it and, eventually, motorsailing. Not simply does this eliminate the sailing choice, it tin likewise make for a less pleasant crossing.

Past being able to predict the current'south effect, you tin choose a conditions window that allows for an easy Gulf Stream crossing under canvas. You can use set up and drift to plot the form you should steer to go far at your destination on a more consistent heading, spreading out the effects of the stream beyond the entire passage, rather than simply reacting to it when information technology is at its greatest.

It is now apparent that information technology will not exist possible to sail to Gun Cay with this wind direction. Wait for a southerly wind unless you want to motor all the way

1. Find the altitude to your destination in nautical miles.

ii. Dissever this distance by your average speed in knots. (Using your average speed is of import. Nosotros all like to call up our boats canvass faster than they really do, and then it is important that yous exist honest here.) This gives y'all the length of time of your passage.

3. Multiply this time by the boilerplate speed (drift) of the current. (The Gulf Stream averages 1.five knots across this Miami-to-Gun Cay route.) This gives you the number of nautical miles in the direction of the electric current that you would be swept if yous did not right at all.

4. From your destination, plot the ready of the electric current.

five. From your destination, measure out the altitude you establish in Pace 3 along the line you plotted in Pace 4, in the opposite direction of the current. This gives you a point to use to detect your class to steer.

vi. Measure the begetting from your departure point to the point you found in Pace 5. This is your grade to steer.

By following this method, the bow of the boat will exist pointing in the heading y'all establish in Step vi, but the electric current will sweep you along the line that takes you to your original destination.

Let's expect at the numbers for the Miami to Gun Cay instance.

i. Altitude: Miami to Gun Cay is 47 nm.

two. 47 divided by our average boat speed of four.5 grand equals 10.44 hours.

3. 10.44 hours times i.v knots of current equals 15.67 nm.

four. Plot the current at 18 degrees through Gun Cay.

v. From Gun Cay, measure 15.67 nm in the management opposite the electric current: 198 degrees.

vi. Decide the grade from Miami to this new point. The course to steer is 132 degrees, though the bearing to Gun Cay is actually 114 degrees.

With the wind coming from 160 degrees, information technology now becomes apparent that it is impossible to sail to Gun Cay. You lot will have to await for a more than southerly current of air in order to sheet across the Gulf Stream between these two points. The other choice is to depart Florida at a point further south. But exist sure to calculate the gear up and drift and determine your new course to steer, which will differ for every location.

Connie McBride and her husband, Dave, are meandering around the torrid zone on their Creekmore 34 Eurisko

December 2016

Source: https://www.sailmagazine.com/cruising/cruising-tips-understanding-set-drift-tactics

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