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Catamaran, Learn to Sail

More Nautical Terms: Apparent and True Wind, Boat Speed, SOG and COG

This post, written by Captain Al, explains some of the sailing terminology you’ll need to understand while learning to sail. It’s all about how the wind and currents impact your sailing speed and direction.

Let’s start with COG and SOG because they are fun to say.

SOG is speed over ground.  How fast are you really moving?  Or as I like to say “When will we get there so that we can kick back, relax, enjoy our location with a drink and a snack?”  This is how fast you are sailing in your boat, or in our case, in a catamaran. It is comparable to miles per hour in a car. SOG is made up from the boat’s speed through the water plus other factors that may affect its speed such as current and wind. For example, if the knot meter on the boat is reporting a Boat Speed of 5 knots and there is a 2 knot current behind you pushing you, the resulting SOG is 7 knots. Conversely, if you’re fighting against a 2 knot current, your SOG would be 3 knots.

Let’s talk about how the wind and current effect the boats course, heading and COG (course over ground.) COG can also be referred to as Track. We all know about a compass on the boat. Let’s say we are sailing along at due north or 0 degrees. We are on a beam reach and the wind and current are from the east or 90 degrees, effectively pushing us sideways towards the west. Because of this sideways force on the boat, the boat will be going to the North West even though the compass is saying we are heading due north. Our actual course through the water is the COG. A sailor must compensate for wind and current forces by changing the actual heading of the boat. This can be called “Crabbing,” after the sideways locomotion of a crab. This term is also used in flying a plane. In a cross wind the pilot needs to head into the wind to compensate if he wants to land straight down the runway.

If you plot a course on your map from A to B it is called your compass course. Importantly, you want your COG to be the same to ultimately get to point B. You may have to head to the left or right of point B depending on how the current or wind is affecting your boat. Let’s say your compass course from A to B is due north or 0 degrees. You start off on the compass heading 0 degrees but you realize there is a 2 knot current coming from the east.  If you do not compensate your compass heading, your actual course/COG would be to the left or NW of destination B. So to maintain a 0 degree COG your actual compass course would be further to the right or to the NE.

Modern day GPS instruments will give you you COG or Track. If you are not using a GPS, you can plot on chart the corrected course you need to steer by doing some basic vector addition. Let’s say your boat is going 6 knots displayed on the knot meter. Draw an arrow pointing north on your chart. The length of the arrow is important.  Draw it at a length of 6 nautical miles on you chart. Then at the arrow end of your line draw the current arrow. It’s coming from the east or 90 degrees at 2 knots, so draw the line at the tip of the arrow extending at aright angle and the length of the line is 2 nautical miles. Attach the two arrows.  Now draw a line between the two tails of the arrow lines with another arrow pointing NE. The length of the new arrow joining the tails is your SOG speed over ground and the angle or magnetic course of the new arrow is your new heading in order to maintain your COG or track of 0 degrees.

COG Diagram

In simpler terms, using a GPS makes sure your track/COG equals that of your desired course from A to B. Our chartplotter on our boat makes it even easier. Put the cursor where you want to go. The cursor gives the distance and compass heading to our destination.  We then have a digital display at the helm that we generally have set to COG.  We then steer a COG equal to our destination magnetic course.

Apparent Wind Speed and Angle

Lets now address apparent wind speed and angle. Apparent wind speed takes into account the speed of our boat with the actual wind speed. If we are sailing upwind at 6 knots and the actual wind speed is 15 knots then the two speeds are added and the apparent wind speed is 21 knots.  If we are sailing down wind at 6 knots and the actual wind speed is 15 knots then the two would be subtracted, resulting in an apparent wind speed of 9 knots. Apparent wind speeds both upwind and downwind are important, as this wind speed is the actual force of the wind on our sails. For example,sailing down wind with a spinnaker in 15 knots of wind going 6 knots, the actual force on the sail is only 9 knots. Not a lot of wind. However, say you then have to round a mark or a buoy and have to change your course to sailing up wind at 6 knots, you will then be dealing with a fair amount of breeze of 21 knots apparent wind speed. In this situation you may want to consider shorting or reefing your sail area.

Apparent wind angle is relative to the angle off the bow of your boat. Zero degrees is on the bow; 180 degrees is on our stern. Ninety degrees would be apparent wind coming from abeam. Apparent wind angle takes into effect the speed and direction of our boat and the true wind direction of the wind on the water. If we were sailing directly downwind, in the direction the wind is blowing, the apparent wind angle would not change and would be 180 degrees. If we where motoring directly into the wind, the apparent wind would also not change and it would be 0 degrees.  However , if we where sailing either upwind, close reach, beam reach or broad reach, the apparent wind angle would change depending on the speed of our boat.  Like currents and wind, apparent wind speeds and direction are another case of vector addition.

Let’s do a simple example. You’re sailing on a beam reach, the wind is coming from 90 degrees off your boat. You are motor sailing at 6 knots.  The true wind speed is 6 knots.. This is an easy one. Vector forward length is 6 the side vector length of the wind speed is 6 and coming from 90 degrees. When you draw the two arrows tip to tip. and then draw what’s called the resultant vector from tail to tail, it will be at a 45 degree angle.  The apparent wind angle moves forward and is 45 degrees. If you suddenly slow your boat to almost stopped, your forward speed vector will be close to zero. The apparent wind will go back to very close to the 90 degree wind direction from abeam.. your apparent wind angle is 89-90 degrees.  Let’s say the wind totally died and your still motoring along at 6 knots, the wind would be right on your bow or apparent wind of 0 degrees. The important thing to remember is apparent wind moves more forward as your boat speed increases and moves further aft as your boat slows. The best and easiest way to determine apparent wind angle is to have a wind vane at the top of the mast visible from your helm. This wind vane. called an apparent wind indicator, will always take into account the speed and direction of your boat relative to the actual wind direction and will point 0-180 degrees relative to your bow.

Hopefully this explanation of SOG, COG, apparent wind angle and apparent wind speed shows how your boat is often affected by forces of currents, wind direction and velocity.  While it sounds complex in writing, with some practical experience it all becomes more intuitive and easy to understand .

We would love it if you would come sail with us, so Captain Al can explain all of this to you in person.

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