Why forestay sag is a critical factor in upwind speed

Getting the optimum speed and performance from your yacht, especially upwind, comes down to a combination of many factors. 

One of the most important variables is forestay ‘sag’, but why is this so critical?

Eddie Warden-Owen, RORC CEO, former sailmaker and America’s Cup coach explains:

“Forestay sag is a key part of controlling the aero-shape of the headsail and enables you to create different sail shapes for different wind conditions. Modern material is less stretchy and allows a sail to be used through a much wider wind range. 

Forestay sag is measured by forestay load, and controlled by rig and backstays. More sag equals deeper headsail and more power. Less sag equals flatter headsail for stronger wind.

Accurate knowledge of forestay sag enables you to repeatably adjust this to give known optimum sail shape for best performance throughout the race. Finding the right shape is achieved by trial and error or working with your sailmaker who can provide feedback on the photos you take.”

So, how to measure forestay load accurately and repeatably? 

For many years, most of us have relied on ‘low-tech’ solutions such as pieces of tape, rulers and pen marks on the deck, which seems more than a little out of place in 2019! 

This is why we’ve created smarttune, a new sensor technology that wirelessly measures forestay load and displays the numbers live on the boat’s instruments. This allows crews to adjust the load (and hence the sail shape) in response to wind conditions as they race. 

Simple to retro-fit, the smarttune package consists of a wireless forestay load sensor, which simply screws into the forestay as a direct replacement for the existing turnscrew, and sends data wirelessly to the instrumentation gateway mounted below decks. This plugs data directly into NMEA2000 systems and can also send live data to smartphones over WiFi. 

Ultimately, it’s all about helping to get the boat moving fast, then keeping it fast as well – giving you maximum enjoyment and pleasure.

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