Skip to content

Square Rigged!

September 10, 2010

Start of the Rolex Fastnet Race, 2009

Now this is a revelation! All sails have a fourth corner! There, I’ve said it. Triangular sails are square. So why, in my first 10,000nm of yachting, over half spent racing, has no one ever explained this to me? Probably because they’ve not had the benefit of half an hour with Adam Bowers, Yacht and Yachting’s crew of the decade and the RYA’s National Cadet coach!

Fig 1. Sail Controls: Mainsheet, Vang or 'kicker', Cunningham and Outhaul

The fourth corner controls the power in the rig. But where is this mystical fourth corner? Not a clew! Sorry, poor joke!

We’re all familiar with the head, the tack and the clew, the three points of a triangle, yes? (See fig 1). No! As a sailmaker will tell you, sails are cut to shape and the luff, leech and foot of the sail actually form curves. On a modern yacht the rig is tensioned to cause the mast to bend backwards slightly to match the curve of the sails’s luff, the leading edge of the sail. Lie on the deck near the mast and look up its length and it will be immediately obvious. Look more closely at the foot of the sail on a lazy afternoon when the outhaul is loosened off and you will see the foot sag beneath the boom.

So if the sail is not a triangle, what shape is it and where is this damnable fourth corner? Well, the leech, that third edge from which the air leaves the sail, also forms a curve, generally accentuated around the point of the top batten. In modern racing yachts this becomes so acute it forms a distinct angle ‘squared off’, stiffened by battens. This is really obvious in my photo of the Open 60’s at the start of last year’s Fastnet, and you see it more and more in modern dinghies, Olympic 49er’s for example. This is the fourth corner. You just have to imagine it exists in a more traditionally shaped mainsail or on a headsail.

To sail efficiently, in whatever wind strength prevails, it is this part of the sail that must be nurtured and optimised for the conditions.

Sail Control

There are three fundamentals of sail control.

  1. The way wind arrives, flows over and exits the sail
  2. To get power we must turn or bend the direction of the wind over the sail
  3. To release excess power, don’t turn the wind. Simples!


Concentrating on the mainsail for the moment, we can change the effect of the wind by adjusting boom angle or the direction of the yacht. Assuming we want to sail a course, we need to adjust the sail to its most optimum shape and position in relation to the apparent wind.


Angle of attack, luff faces directly into the wind

Air should flow evenly onto, over and off both surfaces of the sail, see figure 2. The luff of the sail should ‘cut’ directly into the apparent wind for even flow onto the sail (this applies when the sail is acting as an aerofoil i.e. when sailing a beat or a reach).

Air flowing evenly off the leech of the sail will cause the tell tails to stream. Streaming tell tails are good and most important of all, streaming at the fourth corner! Uneven flow, or turbulence, will cause the tell tails to flick backwards. Tell tails are an immediate indicator of what’s going on and the sail must be adjusted to best effect. It’s sometimes necessary to tighten the leech line if part of the leech starts to vibrate, further evidence of unwanted turbulence.

Mainsheet and Boom Vang (or Kicker)

Full set in heavy weather!

The top batten at the fourth corner, should be parallel to the air flow with streaming tell tails for maximum power. If it sags to leeward, tighten the boom vang (or kicker as it is often referred to by dinghy sailors). This pulls the boom down, which in turn straightens the leech and stiffens the sail, taking the arc out of the top of the leech. The sail will only work efficiently if there is downward tension on the boom and the mainsheet will not achieve this on its own without the help of the boom vang. It’s interesting to note a 4:1 ratio between mainsheet and sag. Loose off 1cm of mainsheet and this becomes 4cm of sag at the fourth corner. For racing, once the mainsail shape is set, fine control can be managed using the track rather than the mainsheet. This keeps the relationship between the mainsheet and the boom vang constant and maintains the most efficient shape in the sail. In gusty conditions, the mainsheet is brought into play as a secondary control to release power when necessary.

Outhaul and Main Halyard

The halyard and outhaul tension controls the draft of the sail, the depth of it’s ‘wing shape’ or bagginess. The outhaul should be tensioned for the beat and gradually loosened off for the reach or more for a run. Halyard tension can also affect draft shape but on a yacht, this is usually set for the conditions and is not generally adjusted so frequently (remember to loose off everything below the boom when adjusting the halyard otherwise one force counters the other and nothing is achieved except the possibility of a broken clutch).

The Cunningham

The cunningham is brought into play in stronger winds. The cunningham tensions the sail at an angle that forms an invisible link between the tack and the fourth corner of the sail, it flattens the draft, not just the luff tension as I wrongly assumed! So when we start to experience weather helm or the yacht luffs up in gusts, apply some cunningham, and normal service at the helm should be resumed!

If increased wind strength makes it necessary to reef a sail, we lose the use of the cunningham. So we then rely more on halyard tension, boom vang and mainsheet controls to keep the sail flat. This is an inevitable compromise, which is why racers prefer to spill wind from an overpowered rig and cope with the conditions rather than immediately reef, which would be the more sensible and comfortable when cruising. To mix up an old sailors adage, racers tend to, ‘Think its time to reef, wait five minutes. Think its time to shake it out, do it!”


505 World's

All the principles affecting the mainsail also apply to the headsail, particularly that all important but imaginary fourth corner. Of course, the headsail has no boom, boom vang or cunningham but we still have halyard and sheet tension available as adjusters. Halyard tension can be adjusted using the backstay adjuster. We are aiming to achieve a clean luff that cuts into the apparent wind and creates an even flow over its height.

Headsails are generally fitted with luff tell tails. They should be fluttering evenly on both the windward and leeward face or, if you can achieve it, the leeward side tell tails should be just starting to lift. Adam Bowers refers to this as ‘disco dancing’. Tell tails are fitted at bottom, middle and at the all important top of the headsail, near the fourth corner. The sail should be adjusted so that all tell tails are behaving in the same way.

The shape of the leech is affected by the angle of sheet tension at the clew. The general rule is that the headsail sheet should evenly dissect the angle of the luff and foot of the sail running away from the cringle. The sheet angle is adjusted to its optimum angle by moving the car forward or back.

The headsail sheet can be winched in quite tight but care should be taken to keep its trailing edge off the most spreaders. Poor adjustment will cause too much draft or bagginess along the foot or too much bow in its height, particularly at the fourth corner.

Sail trimming in practice

Crew Alert! Change of course.

After any change of course, usually preceded by the commands ‘ready about’, ‘ready to gybe’, ‘harden up’ or ‘ease sheets’, all sail trimmers (i.e. you’re not the skipper, navigator or helm!) should be alert to the need to alter sail shape to optimise performance and speed. This is a very important aspect of race crewing and develops with experience and tuition. Trimming is normally done quite quickly once the helm has settled onto the new course and requires communication between foredeck, who have full visibility of the foresail, and pit who have access to winches and control lines.

It’s best to start the trimming cycle from the bow and work aft, since the headsail’s shape has a direct bearing on wind flow across the mainsail. It should be possible to create an even slot between the leech of the headsail and the luff ‘area’ of the mainsail. If you can see the affect of air pressure flowing off the headsail onto the leeward side of the mainsail luff then further adjustment is required to balance the ‘slot’. Sailors at the top of their game have beautifully trimmed sails as the above picture of 505’s at their World Championships shows.

Putting it all into practice

Racesail is the racing section of the West Midlands based Heart of England Offshore Cruising Association. HOEOCA organises a range social, onshore and offshore yachting activities and training, and welcomes both yacht owners and budding crew with a keen interest in yachting. We’ re an enthusiastic bunch catering for all levels of experience. Association membership is open to adults and costs just £27.50 per year. Our Racesail objectives are quite modest:

  • Safety first
  • Developing Skills
  • Fun for all
  • Never come last!

Next HOEOCA Racesail, Sunsail Regatta 8, 15-17 October.

5 Comments leave one →
  1. racesail permalink*
    September 10, 2010 10:38 pm

    I’m inviting comments because peeps may not agree with what I say here and I would be interested to get other’s views.

    Richard – Racesail

  2. Mike T permalink
    September 11, 2010 7:54 am

    We’re you taking notes during the video!!!

    I agree with what you are saying, I’ve never been told about the fourth corner before but it seems to make sense, to control the curve of the top quarter of the sail because this is where the clean air is, but it’s never been explained to me to think of it being a fourth corner. This does make sail trimming clearer, well it does to me!!!!

    • racesail permalink*
      September 11, 2010 3:22 pm

      Yep, took some notes and replayed.

      It goes in eventually!

      More focus on sail trim after transitions will move us further up the leader board! 😉

  3. Topperman permalink
    September 12, 2010 7:56 pm

    Richard, very interesting, 3D sailing, I can now understand how to adjust my top tell tail
    with ease, no more coming 2nd for me !!
    at this stage can I plug a new book – RYA sail trim for Cruisers – intersting video clip from Author

    Happy Sailing

    • racesail permalink*
      September 13, 2010 9:29 am

      Ah yes, RYA Sail trim for cruisers….will send me into a bogging frenzy!

      See you Thursday.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: