Leech Tensioning – How do you know when it is right?

by Hamish Ferguson

The leech of a sail is its back edge, and how it is adjusted and set is a very important part of sailing fast.  Leech tension applies to both the main and jib sails.  It has to be set correctly for sailing both up and downwind.  It also has to be adjusted for different wind strengths and wave conditions.  All that!  Now you know why I have so many strings on my boat.

When the leeches of both the main and jib are set properly, the air flows across the whole sail from the front to the back and also from the bottom to the top.  The most difficult question to answer is:  “How do you know when you have it right?”

Most sails come from the sailmaker with tell-tales or ribbons attached to the leeches on both the main and the jib.  On the main sail, there are usually tell-tales on the ends of the two top battens and the jib has one in the upper 25% of the leech.

For the main sail, the objective is to have the top batten tell-tale streaming out the back of the sail for 75% of the time.  This is easy to achieve going upwind because the main sheet is pulling down on the boom, thereby pulling tension into the leech.  When going off the wind, it is much more difficult to achieve good leech tension on a Sea Spray and the only way I have found is a combination of manipulating the shape of the top batten and playing with the mainsheet traveller.

By putting the top batten into the pocket with a great amount of tension, the sail is held in a nice curve.  The only problem is that the curve requires to be actually pushed somehow from tack to tack.  On a sock mast, the top batten is flipped across by the batten holder which is attached to the back of the mast and the action of rotating the mast when tacking or gybing flips the batten.  On the halyard mast, I’ve had to make what I call a camber inducer and fit it on the mast at the top batten area.  It consists of two pieces of aluminum plate attached to the side of the mast, coming back on either side of the sail so that when the mast rotates the top batten is pushed over to the other side. The top batten tension and the shape it creates at the top of the mainsail are absolutely critical for fast sailing.

 The other part of the equation is the mainsheet traveller.  The mainsheet has to pull the boom down as much as it can and this can be achieved by letting the mainsheet traveller down as much as possible.

It’s important to note that if the leech is too tight or closed, the top tell-tale will start to flutter erratically and will go round the back of the sail.  This means that air is not flowing smoothly off the main sail and the boat will not be as fast.

 The jib works the same in principal, but before going into details I should explain that the jib and the main sails work in combination with each other and the “slot,” which is the space between the back of the main and the leech of the jib, has to be correct.  A too tight or a “closed” slot will result in back winding the luff (front) of the main.  And while the boat may point high, it will not be fast.  If the boat feels slow, let the jib sheet out a little and see what happens.  Too open a slot, with space between the main and the jib leech too wide, will allow to boat to sail more easily but it too will not be as fast as the correct setting.

Now to discuss jib leech tension.  When you look at Sea Sprays sailing, it becomes apparent that the leech tension is usually too little and the top part the leech is open and even flapping.  This is indication that the jib is fully contributing to speed and this applies to both upwind and downwind sailing.

 Start considering upwind sailing and one of the problems to be solved is where to position the turning point of the jib sheet.  What I suggest is a bit of competitive espionage!

 Take a look at the faster boats and measure the distance from the inside of the lee hull to the turning point, then measure the distance from the rear edge of the mast cross tube to the turning point, and thirdly measure the distance up from the lee hull deck joint to the turning point.  Once the turning point is established, then the sheet tension is applied such that the top tell-tale on the leech should be streaming straight back with the occasional flutter behind the sail.

 I think you will be surprised how far forward you can have the upwind sheet turning point for good speed.  Also bear in mind that the further forward the turning point, the fuller the bottom part of the jib becomes and the tighter the leech; the further back the turning point, the flatter the bottom part of the jib becomes and the looser the leech.  Also be aware that as the wind speed increases, the turning point should be adjusted further out toward the lee hull in order to open the slot a little.

 As for adjusting the jib leech tension downwind, most of the fast boats have a barber hauler system to pull the sheet turning point to the top of the lee doghouse.  In other words, move the turning point out and forward.  This can be done with a pulley system or the Osoyoos bungee and rope bridle system.  As far as setting the sail is concerned, the leech should be as closed as possible so that power is not spilled out of the top.  Once again, look at the leech tell-tale and try to keep it streaming.

 Good sailing in the coming season!