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Measuring Wind Direction: What is Upwash?

Here’s a term that is bandied about in measuring Wind Direction, but is commonly misunderstood: upwash. Many people think they understand what it is, but are wrong by 90 degrees. What do I mean by that?

Most people hear the term upwash and take it for face value. It is the motion (or wash) of a fluid over something; in effect, the opposite of downwash. In the case of sails, the fluid in question is air. However, the misconception lies in what people assume to be “up” in relation to the wash direction. People assume that upwash refers to the motion of air up and over the top of the mast as the boat moves through the air (or the air moves around the boat). While this does occur, it is not what the term “upwash” is referencing. If we are to be pedantic, this motion of air up and over is termed axial flow (amongst many terms used).

So what actually is upwash? Let’s explore where the term comes from, and that should provide an insight into what it actually is, and how it relates to measuring Wind Direction.

As most people may know, sails on a modern boat form an airfoil shape, like those found on heavier-than-air aircraft. This realization didn’t come until the science of aerodynamics had been well-established to service the aircraft industry. Airfoil shape strongly determines an aircraft’s performance, so the flow of air around the airfoil has been very extensively studied. Air primarily moves in two directions around an airfoil: under and over. The difference in air speed between the two paths results in the Bernoulli effect, which is the conventional source of lift.

The portion of the air that travels over the top of the airfoil is called upwash. Airfoils are typically oriented so that one surface faces down and one faces up, so it’s natural to reference the flow of air according to this. When we apply the science of aerodynamics to sails, we are presented with a small problem: the frame of reference is rotated 90 degrees. What was once “up” is now to the side – in the case of sails, to leeward. So instead of flowing up and over the sail, the air is flowing to the leeward side and around.

Measure_Wind_Direction_What_is_Upwash

 

A diagram from p. 212 of “Aero-hydrodynamics of Sailing” by C.A. Marchaj

Czesław Marchaj wrote a seminal book in the mid-1960’s titled “Sailing Theory and Practice.” In it, he applied many of the principles he had learned as an aeronautical engineer to the design of sailing yachts and sails. He established the convention of calling the leeward flow of air around the sail “upwash”, as a convenient way of marrying the worlds of aeronautical and nautical design. Since many boat and sail designers studied his work, upwash kept Marchaj’s frame of reference.

On the Ockam system, the upwash calibration allows the user to remove the effect of the sail plan and rig on the apparent wind angle. Since the air is forced around the sail, the apparent angle bends as it travels around the sail. Since the wind angle sensor is embedded in this distorted flow, it is necessary to remove this distortion to properly figure the true wind solution. The additional calibration of upwash slope allows the value of the upwash calibration to change with wind speed. The effect of upwash is lessened at higher wind speeds as the air travels closer to the sail before turning. This requires that the base upwash calibration is de-rated which is what upwash slope does.

The ability to correct for the effects of upwash haven’t been commonly found on sailing instrument systems measuring wind direction, so it is not a term that is commonly understood by the sailing community. It is a term that is worthwhile to understand however, since it so strongly affects the way a boat sails.

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