International One Metre
Irish National Class Association

Main Sail the A-Z

The mainsail is unique. unlike other sails the luff is attached to the mast and so its tuning is rather particular. Through this article I hope to clarify how to tune it well :

(Please bear in mind that it is based on my own experience and findings, should you have any comments please forward them to my email address)

Drawing from my years of sailing single handed I quickly found out that the marriage main/mast plays a crucial part in optimum boat performance. I mean by that the curve of the luff must follow the bend of the mast. It is really easy to see whether the main fit the mast it is rigged on as when sheeted in the main must look good (ie no creases or tension lines across). Then as you play with the different adjustments the main must react properly.

I mentioned earlier that the main is unique as it is attached to a mast. Unlike a jib where the tale tells are at the front end to show the proper air flow, the air directly behind the main is disturbed by the mast and so one of the best way to see its shape is through the leach. Tight and closed, the leach will create a depression behind the back of the sail and helps pointing closer to the wind (as long as that depression is controlled otherwise you will loose drive like a plane staling). An open leach will favour speed over pointing ability. Another way to quickly see the main setup is through lines marked on the sail. Usually 3 lines at regular interval will do the trick. These lines will help you see where the depth of the sail is. The trick is to set the twist according to the sailing conditions (eg. Water and wind conditions) like a wing of a plane. Next we will go through each elements available to us to control that twist.

1. The Backstay

It is multi-functionals : tension the jib forestay, bend the mast and open the main leach. The more you tension the backstay the more do just that. I personally set it first to primarily set the tension on the jib forestay. I tend to use the mast ram and shroud to control mast bend.

2. The Mast-ram

A small but rather important tuning part. As you tight it up, you put pressure against the mast and so reduce mast bend. I reckon that it impacts on the lower 2/3 of the mast the remaining top 1/3 is influenced by spreader angle and backstay tension. As you loosen the ram you help in bending the mast. Bending the mast means flatter main as you increase the distance between the leach and the luff. Ideal for relative speed in flat water. In shoppy condition you want more power to go through the waves and so a fuller main. Achieved partially by straitening the mast (tight up the ram.)

3. Shrouds

Shrouds tension and spreader angle mainly affect the lateral bend but also the mast pre-bend. I found that you can sail with relatively low tension on the spreader in light wind and tensioning them as the wind increase.

4. Cunningham

Pulls the main down at the luff and so tensioning it. Its main effect is to pull forward the draft of the main and open the top of the leach. I usually leave it loose until I become overpowered and then I pull it progressively until the boat is back under control.

5. The Kicker or vang

Acts against the mast ram. It puts pressure on the mast to bend and tension the leach. To be used extremely carefully. I quickly noticed that one single full turn on the treads of the kicker affects the shape of the main quite a lot.

6. The Outhall

It mainly control the bottom 1/3 of the main. Tight it will flatten it and open the leach (just a little bit). Again tight is good for light or heavy conditions in flat water. Loosen up in choppy conditions to power through the waves.

7. The Mainsheet

Most IOM boats are equipped with a mainsheet post which allows you to sheet horizontally on the main instead of pulling it down too if the main sheet was pulling from the bottom of the boat. Let it out a bit in light wind and bring it closer to the centre line in other conditions.

The leach of the main is paramount to optimum boat performance. I like having bands on the main as the main shape becomes more visual. They allow you to see where the main draft of the main is and how much twist is on the main.  Remember that as you adjust your shape ashore it may not be the same on the water and so it is important to test it out before racing.

Each of the above controls impact on their own way on the shape of the main. The art is to adjust properly each of them to create the optimum main shape. This is a learning curve. You need to know what to look for and what control line to adjust to get the shape you want. I hope that my article has been helpful and demystify some of the control on our IOM.

More on main tuning at Blackmagic Sails.

Gilbert Louis

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