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Optimising sail trim: Upwind vs downwind

Sail trim is perhaps the single most important factor in boat performance. Whether you’re a weekend dinghy sailor or you sail on a 100-ft maxi, adjusting sail shape to maximise the wind that you have at your disposal is the name of the game.

But how do you hone sail trim to an exact science and win that game more often than your competition?  

Sail trim is fundamentally about optimising the load running through your rigging and sails. But at any given wind angle, optimal loading on different elements of your rig is totally different.

So, let’s take a look at how to nail the dynamic balance of loading – unlocking optimal sail shape and maximum boat speed…


A lot of people have a good feel for the boat when they’re going downwind, but nailing their settings when sailing upwind can seem something of a dark art.

“Matching forestay tension to the wind and sails is the key to upwind performance.”
– Stu Bannatyne, Four-time winner of the Volvo Ocean Race 

When you boil it down to the controllables, it really is as simple as that. Performing optimally upwind is about optimal sail shape, that is, the right amount of twist and depth in the sails for the conditions. You can try and judge this by sight/feel, but alongside the calibration of the primary soft controls, the sail shape heavily correlates with  forestay load, so being able to hit known optimal sailing loads is a more precise and repeatable method.

For light conditions, lower load through the forestay will increase depth in the sails and maximise power. For heavier conditions, higher forestay load will provide flatter sails and less drag.

Either way, the key variable is the forestay load. This video featuring North Sails Expert Charlie Cumbley explains how to measure and optimise forestay load using smarttune wireless load sensors:


When it comes to sailing downwind the conversation is a little more nuanced. The key loads at play when you’re searching for optimal performance downwind vary significantly from boat to boat. And, unlike upwind sailing, where forestay tension is the primary factor, downwind sailing involves a wider variety of sails and different load management priorities.

“Downwind you’re using a variety of flying sails at a wide range of angles, so understanding the optimal load share ratio between the sail luff and cable at all times is crucial”
– Cyclops Expert & Pro Sailor Ben Hazeldine

The best way to monitor and repeat this varies. For instance, prolific maxi’s like Pyewacket 70 have delivered results while monitoring their code zeros, while on a catamaran, mainsheet load is normally the key performance indicator. With this in mind, optimising loading downwind requires a more versatile tool. 

smartlink is a popular solution, as it can be used across various sails and running rigging installations to monitor key performance numbers and ensure loads do not exceed safe levels.

“We’re conscious when using the Jib Top we could exceed the safe working load in the Bowsprit & Halyard Lock. The smartlink was below the furler & we knew our Max Working Load so could push hard with confidence.”
– Tom Cheney, Navigator, JPK 11.80 ‘Sunrise’

The evolution of sail technology has increased the focus on optimal load sharing for downwind sailing even further, as structured luff technology is adapted to absorb more load and create a more positive sail shape. Load sensing has offered sailors a way to unlock its full potential and realise more powerful sail shapes more often.

As well as smartlink, which is often used to monitor sail loads without being installed directly into sails, this has also led to the introduction of smartluff as an upgrade to the tack fitting exclusively in North Helix Sails.

By revealing precise sail loading, smartluff or smartlink can reveal the load share ratio by process of elimination. But when combined with a sensor that also reveals the cable load, sailors can be even more scientific – with the exact sail / cable load share ratio on screen.

For example, TP52 ‘Jolt’ has a smarttoggle furler sensor installed in its code zero set up – allowing them to trim the sail based on what the cable load tells them about the overall cable/sail ratio.

Example smarttoggle / smartluff setup

Similarly, in this video, North Sails Annapolis’ Austin Powers explains how smartluff can be used in conjunction with smarttune to unlock optimal load sharing onboard Aerodyne 38 ‘Zuul’:

Putting it into practice

If you know what your critical performance loads are and you’re equipped to monitor them accurately, the only thing left to do is to put it together on the water – hitting your fast numbers as much as possible.

If you have Cyclops sensors, they’ll connect wirelessly to your boat displays and to the Cyclops app – providing live and logged load data. There are multiple ways you can put this to use:

  1. Practice using the sensors – take note when the boat feels fast and hit that ball-park figure again when you’re racing. 
  2. Get a little more scientific and use polar charts / dynamic tuning guides to stay on top of changing conditions – matching your numbers to the chart. Cyclops has produced a range of guides as a starting point. 
  3. Depending on your boat displays, you can configure your live load data to give you active targets. By combining the true wind angle with your load data, your instruments can provide exact numbers for you to hit, providing a real shortcut to optimum performance. 
  4. If you’re sailing in a class (like J/70) with rules prohibiting live load sensing for racing, practice hitting your numbers in training, either marking up your sheets or other equipment so you can easily hit them come race day – learn more about Training By The Numbers.
  5. Use the Cyclops app or sailing analytics software to analyse your performance based on your loads. Hone in on optimal performance to continually improve your sail trim.

Find the sensor for your boat or get in touch with a Cyclops expert for a consultation.

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