Sail Twist

There are a lot of theories of why we need sail twist. They are mainly disputable, but the fact is that boats go better with twist. That’s all we need to know to understand the importance of this factor of sail trim. For us its simple to prove – when the tell tales are flying, and most importantly at the head the leech tell tales, the flow is good and the boat goes faster, and the leech is invariably open – from a few degrees on a Laser to maybe 30 plus degrees on a skiff or windsurfer. This is a huge part of being able to tell when a sail is well tuned, without being on the boat and “feeling” it, which is what we do from the coachboat. So let’s look at some outlines of how each class works.

We’ll begin by looking at the “plan” of the sail, and labelling some areas so that we can refer to them.

There are two important lines to consider to understand most sails. One line is from the top of the mast to the gooseneck (front of boom) – forward of that is the luff. By far the more important one is the straight line from the masthead to the end of the boom. The area between that line and the edge of the sail (the leech) is known as the leech roach.

What is critical to understanding the ability of a sail to adjust itself to the power required is how this leech roach works, and more to the point how we make it work for us. This is probably the biggest single advance in (conventional, not wing) sail design. In sailing, we all tend to copy what works for the leaders of the fleet. That’s usually a truly good point to start at, but actually making our own setup giving us the control that we need to sail at our best requires some understanding of function of parts.

For this reason, we are going to assume that you know how to bend the mast, and focus solely on leech roach for this blog.

If you look at the diagram, the lines representing the straight line from the top of the mast to each end of the boom give us what is a good approximation to a Laser sail. That doesn’t make a Laser sail bad, but it does limit it for tuning opportunities and gust control. The lack of leach roach and “tunability” means the sailor has to respond to much smaller pressure differences, and be prepared to muscle the boat through gusts and increased velocity, and respond rapidly with kicker and downhaul to decreased velocity. Different skills – the same could be said of the Finn, which has little leech roach. The 470 starts getting a workable area, and the 49er and RSX windsurfer have a large leech roach.

So let’s look back to the diagram and see the potential of the large leech roach. When increased pressure hits, if the leech roach is supported by very stiff battens, it will behave like a wing and not twist. As the battens are more tuned in to the weight of the sailors, and the righting moment they can make to the boat, the sail will twist off automatically at the head. By changing the stiffness of the battens we can see massive control possibilities, and need to make sure that the battens we use suit our bodyweight. Despite the claims of “one design”, battens come in massively different stiffnesses for 49er and FX, so it’s worth getting a fishing scale, measuring the battens, and trying some bespoke sail tuning. The willingness of the class to accept a certain optimum weight range can be bent in our favour, just by understanding the way the leech roach and mast tip work when a gust hits. This works for both heavier than optimum and much lighter than optimum crew weight.

All these small areas of tuning make sailing the boat a lot more fun and pleasure, and certainly a lot faster. At Toplevel Sailing, we always adopt the attitude of “try it, you should like it”, and encourage sailors to experiment in the areas where we know there’s a massive performance gain. Fast is fun!