The Mainsail

By Dick Durham & Dick Beaumont

Treat your sails like aerofoils to make the best passage, says Dick Durham.

A yacht’s sails must ‘fit’ the wind for her to move effectively and their set is the first place to start. Get the halyards as tight as possible, as the luff – the leading edge of the sail – must cut cleanly into the apparent wind.

Please note the two grey shaded areas; roach and hollow. Roach should only be considered for slab and in boom furling systems. You will hear many people say that you can have full length vertical battens, and therefore roach, on an in mast reefing system, but I would strongly recommend against that idea. I have used vertical battens on an in mast furling system and tolerated them for 6 months, experiencing multiple sail jams in the mast. I returned to the sailmaker and told him to take them out of the sail, I didn’t have one sail jam in the following ten years and 100,000nm plus of sailing.

If an in mast system is used the sail should have some hollow at the leech of the sail to eliminate flapping.

For short handed sailing I am a big proponent of in mast furling systems despite the small loss of mainsail area, which of course, is in any case negated when you put in a furl.

A sagging, baggy luff breaks up the wind’s flow across the sail’s surface adding more ‘snags’ to the streamlining.

The leech – back edge of the mainsail – can be controlled with the mainsheet and boom vang to give the sail ‘twist’.

If you hold a piece of A4 paper in your hands – one atop the other – and turn the bottom half a little you will apply twist to the sheet.

It is this shape that determines how much power the sail contains. Too much twist and the drive is lost, too little and the sail is too flat and again power is lost. After twenty years or so you might begin to get it right, but every set of sails and rig behaves differently in every boat.

If you shorten the distance between the clew of the sail and the hoist you are making the leech ‘softer’ – you are ‘opening it up’ and increasing twist which will give a better shape for downwind sailing.

Doing the opposite by pulling the clew down with a tightened mainsheet and vang, ‘hardens’ the twist, makes the sail flatter and gives the boat more power to windward.

Getting the correct twist is a never-ending job, as wind conditions and angles change. There is no silver bullet to get the right twist using mainsheet, traveller and vang, that works the same for every boat, so continue with your own trial and error. Here are a few pointers for mainsail trim for the different points of sail:

  • Close-hauled: boom close to the centre-line of the boat and vang tight, sail quite flat.
  • Close-reach: boom towards the back corner of the cockpit, vang loosened to allow the boom to lift a little.
  • Beam-reach: boom over the primary winch, vang loosened further still but not to the point where the boom lifts with each gust.
  • Broad-reach and run: ease the boom out towards the shrouds. Although the sail will inevitably lay against the spreaders to some extent, that’s not a problem if the spreaders are smooth. Use the vang to bring the boom down to lift the sail off them a little and ensure the boom and sail is not riding with each wave and gust, as this will cause a lot of chafe and will also reduce the life of the sails dramatically.

This is the sequence I apply to each change of point of sail.

  1. First position the car on the mainsheet traveller:
    Close hauled:
    Position the car on the windward side of the traveller
    Reaching:
    Position the car amidships
    Running:
    Position the car on the windward side of the traveller

  2. Trim the mainsheet to the required angle of the sail to wind.
  3. Adjust the vang to position the boom to induce the correct degree of twist.
    Note: Release the vang before you start to pay out the mainsheet or you will induce very heavy loads on the vang and boom.


Make sure the boom is never pulled upwind of the centreline. Most cruising yachts mainsails will respond better if the boom is a little to leeward of the centre line.

The Telltale Signs

Look constantly at the telltales on the back edge of the sail, the leech, they should stream out at the same angle as the sail. If you are on starboard tack (wind coming over the starboard side) and the telltales point towards the port side, your mainsail is too hard and you want to ease your mainsheet to ease the sail out to port. If the telltales are breaking in or pulling towards your starboard side, you need to tighten your mainsheet. The topmost telltale will often flutter occasionally due to the rocker motion of the yacht. Don’t worry too much about this, but if it’s constantly fluttering, harden the vang a little to decrease the twist.

Reading the Telltale Signs

Once a yacht’s sails are set and trimmed perfectly the yacht comes alive, the speed increases and the motion improves. Life underway turns from a chore to a complete pleasure. 

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