Staring at Sails

I spend my life staring at sails from the coach boat, trying to work out what is fast, what is slow, how the shapes match the modes, and the modes match the conditions. At the root of this, is the challenge of understanding the 3 dimensional shape of the sail from far away, and as we start to collect more and more sail shots of our own setups, as well as the competition, I think it’s worth breaking down the types of photos that we’ll be looking at, the strengths and weaknesses of each one, and how we can assess each perspective qualitatively (which factors into the quantitative evaluation with the computer).

From the coach boat, I generally take four types of sail shots, which we’ll focus on here (six possible angles with a drone, and eight possible if you count photos from inside of the boat. Even more if you start counting kite photos!). There are a number of other shots that I use from time to time to see things like mast bend, but these are the photos you should get used to looking at on a daily basis:

Aft Shot Twist Shot Low Depth Top Depth

Aft Shot Twist Shot Low Depth Top Depth

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Aft Shot: This angle gives you a good perspective on the lateral mast bend - how much is your mast tip falling to leeward, and how much is the bottom section staying in column, sagging to leeward, or poking to windward? This is obviously also a good perspective to look at the boom position relative to centerline, as well as heel of the boat. When headstay sag is significant it can often be seen from this perspective, but it’s definitely not great for that, so don’t put too much stock into those observations unless you have the  head on (drone!) shot to back it up.

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Twist Shot: This photo is taken directly down the boom. The primary function is to look at the twist and curvature (rate of change of the twist) of the main, but it can also be an interesting perspective of the slot between main and jib.

Low Depth Shot: This perspective lines up the leech of the main with the luff of the main at the height of the lower spreaders. The thickness of the sail at this point can be used as a metric for main depth in the bottom half of the sail. This same trick can be applied anywhere along the mast, but by doing it at spreader height, we can make it consistent. Again, this is a good shot for looking at jib position, and will often actually line up a “twist shot” on the jib, where the clew lines up with the forestay (like sighting down the boom.

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Top Depth: The same as the low depth shot, but this time looking at the top of the main. This is an important one, especially in lighter conditions. 

There are many other angles that get used from time to time - side on, head on, directly above, looking at the max mast overlap from the leeward luff of the main, etc. - to figure out what a sail looks like in three dimensions, in a given condition, but with these four, we can learn a lot, and decide what shots to chase on our competition.