Apparent and True Wind

The short answer; What Happened and Why?

As some folks noted the wind seemed to double in strength as we headed up for home.  Actually, the wind didn’t just choose that moment to increase in strength it was always there but the captain and crew were not paying attention to it.  They were “had” by the “Apparent Wind”.  No one should feel embarrassed by this event, all who sail will sooner or later get caught with their proverbial pants down!  Scary, but a good pub story and a lesson will come of it.

The Long Answer,

To understand what caused our knockdown we need to understand the difference between “true and apparent wind”

Simply stated “True Wind” is the wind we feel when we are standing still.  It is the True Wind that creates the ripples and waves on the sea.  “Apparent Wind” is the wind we feel as soon as we start moving, whether it be walking, riding a bicycle, in a car, or sailing a boat.

There are 4 basic rules about Apparent Wind on a boat, they are;

1. The apparent wind is always forward of the true wind unless you are running or motoring dead into the true wind.
   1. When the Apparent Wind moves behind the beam so we are broad reaching or running the strength of the True Wind will be decreased by our boat speed. In this instance the Apparent Wind will be less than the True Wind speed. (see 2 below)
   2. When the Apparent wind is on or forward of the beam the Apparent wind is a positive combination of our Speed and the True Wind, in other words the Apparent wind will be stronger than the true wind.

   It is been discovered in studies that people who have got tired of spending too much money on why not check here levitra no prescription can simply switch to this smart and effective way of ED treatment. This means that eggs discount priced viagra retrieved may result in a failed IVF cycle. The recommended time to take the pill is cialis line order from 30 min to 12 hours before sex. It is said that you must avoid excessive use of these medications might be explored to determine if they’re able to last during sex much more time. viagra effects women

2. As the true wind moves aft, apparent wind decreases in strength.
3. When the true wind is well aft a small change in true wind direction makes a large change in apparent wind direction.
4. Close hauled to a beam reach apparent wind is always faster than the true wind.

We also need to understand one other concept about wind, and that is, “When wind speed doubles, wind force quadruples!”  This means a 10k wind has 4 times the power of a 5-knot wind and a 20-knot wind has 8 times the power of a 5-knot wind.

So back to This Week’s Question:  What happened and why?

When you were enjoying your 6-knot downwind sail and feeling just 12 knots of wind over the deck you were likely flying full sail as would be right given the point of sail and the apparent wind conditions, but there was in reality an 18-knot true wind.  The 12-knot wind you felt was from your speed being subtracted from the true wind as you sailed in the same direction the true wind was blowing.

When you turned the boat to head up suddenly you had full sail and 24 knots of apparent wind.  This apparent wind comes from adding your forward speed of 6 knots to the true wind speed of 18 knots. At 24 knots your boat should have been double reefed with a small jib.  Instead as you rounded up with full sail from your run you were suddenly overpowered.  The wind that caused your knockdown was always there, it just felt reduced in strength by your forward motion.  Again, look at wind force, as the apparent wind doubled in speed from 12 to 24 knots its force increased 4 times.

The real lesson here is you were not paying attention to the events happening around you.  You were enjoying the company of your guests and enjoying what was probably a very pleasant downwind run.  As skipper you can still enjoy the company and the day but you need to always be out in front of your boat, recognize what is happening around you and make decisions based on your observations.

The proper procedure here would have been while still running, to either roll the jib up completely or to the point you would for 24 knots of wind close hauled and as you made the turn to head put in two reefs before hauling the mainsheet in for a close hauled course.   If later after retrimming the main sheet you find you have too little sail area then shake out a reef or let out a little more jib.  It is always better to err on the side of caution and over reef than to discover you still have too much sail area.