SailNet Community - View Single Post - Correction for Leeway - Coastal Navigation Question
 View Single Post
Old 07-30-2011
fryewe
Senior Member

Join Date: Dec 2004
Posts: 2,046
Thanks: 33
Thanked 58 Times in 58 Posts
Rep Power: 13

Puzzling problem...never started a journey knowing what my leeway would be (in degrees!) before I started out. Don't have a leeway instrument. A good substitute is bearing to next waypoint feature of gps as billyruffin described above.

In the real world this problem's solution can be estimated prior to getting underway, and course to steer can be adjusted to approximately offset the effects of set and drift due to wind and current by observation, experience and history, but after you are underway, it is worked out by calculating set and drift, which can occur due to current or wind.

By getting a fix and comparing it to your dead reckoned position, using your observed speed through the water and course for the time since your last fix, you establish a vector between the DR and the new fix. The vector's length is the distance traveled in the time since last fix (distance divided by time is speed, or set) and the direction of the vector is the drift, the direction of motion caused by current and wind. If you draw a vector from your original fix to the new fix, that is the actual course and speed you have made good.

The difference in bearing of your DR position from your last fix, and the bearing of your new fix from your last fix, is the amount you have to adjust your course on the opposite side from your steered course to offset the drift.

If you adjust your course in the direction opposite you are simply forcing your ship to make the same amount of speed to the right (or left) of your desired course that the set is pushing you to the left (or right).

Or if the destination is 2.9nm away, or 5800 yards, and a physical feature, look at it through the binoculars and steer toward it. Just as in the case steering bearing to waypoint using GPS, you will be steering a curved track rather than a rhumb line course, but my guess is after a few minutes you can estimate how much you have to crab to offset the wind and current. Another method when you can physically see the destination is to check the true bearing to it every few minutes. If it draws left, you are steering too much to the right; if it draws right, you are steering too much to the left.
fryewe is offline

For the best viewing experience please update your browser to Google Chrome