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Old 15-12-2006, 14:26   #1
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NTOTD: Set

Set is another of those multi-purpose terms, including some very precise uses.

Noun: point of trim
  1. The position of the sails relative to the angle of the wind.
  2. (generically) A person's values, ethics, political views, or methods of doing things. "I like the set of your jib."
Verb: to deploy or put into use
  1. Setting sails, raising and trimming sails.
  2. Setting anchor, deploying an anchor, insuring it is securely attached to the bottom, rigging requisite additional gear (chafing gear, snubbers, kellets, tripping gear, buoy, etc.), and initiating anchor watch.
Verb: departing
  1. of a vessel leaving port or anchorage.
Verb: angle of force applied to the vessel resulting in a difference between course steered and course made good.
  1. Current direction (the speed of the current is its drift)
  2. leeway, keel/rudder lift, wave-induced drift, prop walk are other potential sources of set.
(Navigational digression) You can calculate the set your vessel is experiencing. Get a fix (an exact known position). Proceed on your course for a specified time, plotting the course and distance (call the dead reckoning (DR) position 'A'). Get a second fix (position 'B'). The direction from point A to point B is the total set, but in most cases you can assume the majority is due to current.

The drift can be calculated as the distance from A to B divided by hours. e.g. if the distance is .16 nm and the time on course was 10 minute, the drift is 1 nm per hour.

Now the line between where you started and where you actually ended up is both the vector of your course/speed through the water and the current set/drift. It's also your course over the ground (COG).

At this point you want to know what course to steer to get to you where you want to go, a course that will put your COG in the right place so you know what you'll be going over.

First, plot the course from where you are (point B) to your destination. Then extend the current set line as long as the drift. In the example, it would be 1 nm in length. This is point 'C'. Take your compass and measure the distance your boat can tavel in one hour on the scale of the chart, and with one point of the compass on C mark where the other point lands on plotted course to the destination, call it point 'D'.

Plot in the line from point C to D, and find out its direction. This is the course to steer to get to your destination from where you are now, traveling on the line between point B and your destination.

A nifty side effect is the distance from B to D is the speed your boat will be making on the course (speed over the ground, SOG). You can determine how many hours you are from the destination pretty easily.

All of this assumes the current you're dealing with is constant. Most likely it isn't. If you're dealing with tidal currents, you'll be doing a lot of fudging and recalculating to see what's really happening.

(Now I need to figure out how to draw some graphics to illustrate this...)
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