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How Does Backstay Tension Affect Mainsail Trim?

If you have a mast that bends when you apply backstay tension, you have a powerful tool for controlling mainsail shape. Quantum’s Dave Flynn answers a sailor's question about how to use this tool and how it interacts with other sail controls. 

As a follow-up to your article on mainsail twist/trim, what is the role of backstay tension on mainsail shape? 
- Richard G.

The way it works is simple. However, before we start, you need to get past the concept of the backstay simply pulling aft on the mast. In reality, much of the force is directed down towards the deck. The mast compresses under this tension. Imagine a straw that you set against a table and push down on. This is basically what is happening to your mast as you add backstay tension. Assuming you start with a little bit of pre-bend (mast bending forward at the middle), the compression induced by the backstay will increase the bend forward at the middle of the mast. This has a few of effects.

First, as the middle of the mast moves forward, the luff of the mainsail is pulled away from the leech. This flattens the sail just as pulling on the outhaul and moving the clew away from the tack flattens the foot of the sail. It just does it over a much larger area since the whole mast is moving forward to some extent. The more bendable the mast, the easier it is to add backstay tension and flatten (de-power) the sail. On many small boats which don’t have a backstay, pulling on the mainsheet has the same effect. Tensioning the mainsheet tensions the mainsail leech. The leech acts like a backstay to induce compression and mast bend. The boom vang can also help. Pulling on the vang pulls down on the boom to control leech tension, but it also forces the boom forward, pushing bend into the lower sections of the mast.

Second, when you compress the mast with the backstay, the head of the sail gets closer to the clew. This opens the leech of the mainsail which induces twist. Normally, whenever you add backstay you need to add mainsheet (or vang) to keep the twist the same, unless of course you want to depower even more by adding twist.

Finally, mast bend also shortens the distance between head and tack, making the luff go soft. You will see the telltale sign of horizontal wrinkles perpendicular to the mast appear as you add backstay. To compensate, add luff tension with either the halyard or cunningham. Conversely, when you ease the backstay, you need to remember to ease luff tension and ease the mainsheet or vang. It all goes together in an endless loop! 

Make a point to study the cause and effect of the backstay and experiment with it in different conditions. As you master the dance, you will have a valuable tool to control mainsail shape.

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