S.O.W. vs S.O.G.
- Thread starter butchrobbins
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If there is a current or tide for or against you, It will help in deciding where to steer to in order to maximize gains or minimize losses. SOG will tell you via GPS how long before the next waypoint, mark, marina, etc.
All of us that started sailing before the age of electronics probably learned some basic navigation. We start from a known position and set off on a course at a speed. We have some idea of where we are and we draw a line that shows that on the chart. At some point we get another fix. It is hardly ever where we thought we were. The error is mostly due to the difference between Speed Over Water and Speed Over Ground.
We used to calculate this every time we had a new known position. With GPS, we get a new "fix" several times a second and seldom find ourselves a mile from our estimates.
There are three winds that are a concern for us.
Apparent Wind relative to boat.
True Wind relative to boat.
True Wind relative to ground.
We trim sails to Apparent Wind. From AW and boat SOW we can calculate True Wind relative to boat. To get True Wind relative to ground, we must also know set and drift.
With no set and drift, the True Wind relative to boat is the same on all points of sail. When set and drift are present the True Wind relative to boat is not the same on all points of sail. In 6 knots true Ground Wind, you might have 9 knots True on one tack and 3 knots true on the other tack due to current.
Randy
We used to calculate this every time we had a new known position. With GPS, we get a new "fix" several times a second and seldom find ourselves a mile from our estimates.
There are three winds that are a concern for us.
Apparent Wind relative to boat.
True Wind relative to boat.
True Wind relative to ground.
We trim sails to Apparent Wind. From AW and boat SOW we can calculate True Wind relative to boat. To get True Wind relative to ground, we must also know set and drift.
With no set and drift, the True Wind relative to boat is the same on all points of sail. When set and drift are present the True Wind relative to boat is not the same on all points of sail. In 6 knots true Ground Wind, you might have 9 knots True on one tack and 3 knots true on the other tack due to current.
Randy
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