Do you pre-bend?

Some sailors put a mast chock in front of the mast. As you pull more kicker on, the mast bends and that causes it to move forward in the mast gate until it either reaches the front of the gate or a chock. So a mast chock reduces the amount of mast bend in the lower half of the mast and as discussed in "Different Sails - Different Settings" what size of chock you use depends on where the mast starts from and the shape of your sail.

But what happens to the sail shape when you don't pull the kicker on? Obviously the mast stays put and the sail is not flattened in the luff area by any extra mast bend.

Now this is fine when you are going downwind - you want a fuller sail and you want the maximum depth in the sail to be further forward. That all helps you go quickly.

But the problem is what happens to the sail shape in light winds. If you need the kicker on to get your sail to set nicely upwind, with a fine entry in the luff area so you can point, then you have an issue.

Pulling on the kicker when the wind is light reduces the twist in the sail. Unfortunately you need some twist to make the air flow over the sail well. Too much twist means the top of the sail is not giving you any power, but too little twist - or a leach that is too tight if you want to think of it that way - greatly increases drag and reduces lift. And you don't want that.

Now one way - I learnt recently - to increase twist is to pull on the cunningham. Unfortunately that will wreck your sail shape in light winds as it flattens the sail and pulls the maximum depth of the sail forward, stopping you pointing and reducing lift. You should only use the cunningham when you start to get over-powered in a breeze and are sitting out hard.

But pre-bend is a better way to get both the right amount of twist and enough mast bend to create an efficient sail shape upwind. And all you need for pre-bend is to put a chock *behind* your mast when the wind is light. This bends the mast without you needing any kicker at all.

 

How big a chock should you use for pre-bend? As usual, there is no one answer to suit all. To figure it out, sail your boat upwind in light winds and apply some kicker until the mast is bending and you have the best sail shape at the front (don't worry if the leach is too tight at this point). It needs to be a fairly flat luff area, but not so much as to create "over-bend" creases where the designed luff shape of the sail is less than the mast bend. Then add different chocks until that space behind the mast is filled.

Now release the kicker until you have a nice leach shape too - which might mean no kicker at all if the wind is light. Hopefully you now have enough mast bend to create the right luff shape upwind without needing to over-tighten the leach.

Don't forget - this is a light wind technique - you don't need pre-bend once the wind is over about force 2.

Let me know how you get on! ⛵