A mainsail cannot know whether it is hoisted on a monohull, a catamaran or a trimaran. The principles of sail trim are universal.

On all types of yacht the traveller, mainsheet, outhaul, halyard and the Cunningham can be used to control the sail, and the information from the telltales can be used to help judge how much power the sail is providing and whether it is well trimmed. But there are some important and some subtle differences between monohulls and multihulls.

Because the multihull is so much more stable at low heeling angles, the heel angle is harder to use as a reference for when the boat is overpowered. Rather like a car versus a motorbike; the car heels only a little when pressed, while on the motorbike it’s much more obvious when the limits are being approached.

So on the multihull the decision of when to depower and to reef has to be influenced more by other inputs rather than just heel angle.

Generally speaking, a multihull’s wide beam allows for a much longer traveller than on an equivalent monohull, so there is much better control of the boom position when reaching and downwind sailing. This can make a boom vang/kicking strap unnecessary.

A multihull does not roll downwind like traditional monohulls, so the risk of an accidental gybe is much reduced, though not eliminated. Additionally the boom is less of a danger to the crew as it is often high and above a long cabin top – though that can then mean that visibility to the mainsail is more restricted, and you may have to move to find the best spot to look at the mainsail trim.

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With the shrouds further outboard and often further aft than on a monohull, a multihull’s mast is well supported and permanent backstays and runners are not required. This allows for a bigger roach on the mainsail and a more powerful sail for the same mast height. The big roach on the main then necessitates full-length battens with cars on the mast, rather than short battens.

Inflexible mast

The high righting moment of the multihull combined with a lack of backstay or runners, means that the mast has to be strong and stiff and is not adjustable in bend under way, whether it is a fixed or wing mast. So it won’t be possible to flatten the mainsail with mast bend, as you might do in a monohull with a backstay.

And because of that wide shroud base, and consequent reduced mast compression, some performance multihulls can have rotating wing masts, to reduce the aero drag of the mast and clean up the airflow to the mainsail. This does not really change the mainsail trim but it is a slight performance gain.

But with the shrouds further aft and with full battens in the mainsail, it can be more of a challenge to reef downwind, as the battens and sail will chafe against the shrouds more.

Knowing when to reef

The fundamental control on the power of the mainsail is what size sail you have decided to set. As the wind increases it is very important to reef at the appropriate wind speed and not wait too long, otherwise the boat becomes overloaded.

For instance, when racing on a MOD 70 trimaran, we’d usually go to first reef at 20 knots true wind speed, upwind and downwind. Of course, you could temporarily carry full main to 25 knots, if you are careful on the helm and with sail trim, but the boat will be slower as the sails will be eased and creating more drag. It is also far more dangerous to overload a 7-tonne trimaran that can capsize.

However, on delivery with the same boat, the reefing wind speed would be 17 knots True, as we reef as early as we can without losing significant speed.

As a guide to boat speed on delivery, upwind we are happy to be doing 16 knots and downwind 25 knots, and we want to achieve those speeds with the minimum of sail. When racing of course we try to go faster with a 20 knots target speed upwind and 30 knots downwind!

As the boat is very powerful, we would go to second reef at 26 knots and third reef at about 33 knots on the MOD70, less on delivery of course.

When cruising or on delivery, I always like to learn the minimum wind speed that I can reef while maintaining a similar boat speed. That way there is less stress to the boat, rigging and crew. The centre of effort of the sails also goes down, so heel and pitching reduce. Downwind, the bow down trim will reduce and the boat will be safer and lighter on the helm.

If the wind is expected to increase, I’d confidently change down at these minimum wind speeds, but if the wind is oscillating up and down in strength I would make sure the wind was really holding above that minimum wind speed before reefing.

And if unsure of what is going to happen with the wind – when sailing into squalls, into nighttime, or into gusty offshore winds near high land and headlands – then I would always aim to err on the side of caution. When cruising, being set up for the higher wind speeds than expected, is an old axiom.

The MOD is a super high performance trimaran of course, but to find out what the limits should be on a popular cruising cat I spoke to Graham Laver at Ancasta, who sent me the sailing guide for the Lagoon 52.

Interestingly, it’s not that different. With the standard pinhead mainsail, the first reef is recommended to go in at 21 knots both upwind and downwind, and in less wind with the optional square-top main, or in a bad sea-state. The second reef is recommended at 33 knots and the third is at 40 knots. These are higher winds than the MOD70, but then it’s a less powerful, heavier boat with a shorter rig.

Here we are talking all the time about wind speed in True, which requires a reasonably accurate instrument system. This is now common on most boats, but it’s very worthwhile to put in a little time to calibrate this as well as possible.

Not all about numbers

And this opens up the whole subject of instrument calibration. In essence the wind cups will, out of the factory, read the correct upwind and reaching apparent wind speed, but to get accurate true wind speed, the calibration of wind vane angle to the centreline, the boat speed and the compass are all very important so that the wind speed reads the same, tack to tack.

Downwind, the windspeed will often overread due to the upwash of wind from the square-top mainsail at full hoist, or a spinnaker, so be aware this may add about 2 knots to the windspeed. This can be calibrated out with advanced instrument systems

Even if the instruments are really well calibrated I like to always imagine how I would sail if they suddenly stopped working. So what are the other clues to use to estimate the wind speed?

Very important is the look of the wind on the water, the amount of white caps, as in the classic Beaufort scale, and also the feel of the boat: the speed through the water, the subtle heel angle changes, the amount of helm that is needed, and the balance of the boat.

For instance, how much you have to depower the boat to achieve a certain boat speed is a good guide. If I have eased quite a lot of mainsheet and traveller and the boat speed is as desired, and the front 20% of the main is luffing, then its probably past time to reef, even without looking at the true wind speed numbers.

In 2001, I sailed in the Mini Transat Race. At that time those boats had no wind instruments and it soon became second nature to know wind speed and direction, day or night. It’s a good exercise to always check your estimate of the wind speed against the instruments.

Of course you want your instrument and power system to be super reliable and well calibrated, so you don’t lose wind data on a dark night, but also you want to blend that information with your own judgement of the situation and have confidence in that judgement/intuition. The wind number is not an absolute guide to the forces on the boat anyway.

For instance, the same 20 knots wind speed is less powerful in the warm tropics than in the cold Southern Ocean. And on a foggy day with cold water and warm damp air, the wind is less powerful as there will be a lot of wind shear and the wind at the deck level is much less than the wind at the masthead.

So on a day when the wind is better mixed, the same 20 knots at the masthead is more powerful than that foggy day with laminar flow wind.

To summarise the discussion on reefing limits: it’s not just about the digital TWS number, but also what is right for the conditions you feel, the feedback from the behaviour of the boat, your predictions about what is going to happen in the future, the goal of the sailing and the abilities of the crew.

Reefing tips

Reefing is easy to do upwind or reaching, and I often slow right down to reef, and then get going again when the reef is finished. This keeps the apparent wind speed down, and makes for a safer platform for the crew. Downwind it can be trickier, with the friction of the sail and full length battens on the shrouds. If it’s not possible to head up, then winching the luff down at the same time as the leech should get the sail down slowly.

On very fast boats such as MOD70s you can keep going fast enough downwind to keep the apparent wind angle at 50° just with the gennaker, and then the mainsail can flap without hitting the shrouds when the traveller and mainsheet are eased. Then, once you are reefed, you can get back to your 37° to 40° of apparent wind angle and 30 knots of boat speed!

Mainsail trim

So now we have the right size mainsail for the situation, how are we best going to trim that sail?

The big controls are the mainsheet and the traveller, and the minor controls are the outhaul, halyard and Cunningham (and on some boats some of these last three are not adjustable).

So let’s talk first about upwind and reaching in moderate, flat water conditions, when the mainsail is trimmed the tightest – let’s say in 12 knots of wind.

For the mainsheet, you can keep trimming until the top telltale is folded to leeward of the leech about 20% of the time, and the other 80% of the time it, and all the other leech telltales, are flying. That is max main trim, giving the straightest leech and minimum twist. I like to think of this as 1 out of 10, or ‘minimum twist’. I like to look up the back of the leech sometimes to assess the amount of twist.

In fact, if you can then mark the mainsheet at this position along a scale on the boom, you could have marks from 1 to 10, with 1 being the most trimmed in you can go, and with 10 being the most eased. This is ‘maximum twist’, perhaps used when you are reefing.

I really like to have marks on the mainsheet (and everywhere else it might apply), as it’s easy to then repeat your settings, and also to pass on information to the rest of the crew in a concise form. In these upwind conditions, position the traveller so the clew of the mainsail is on the centreline.

As it gets windier, the first thing I would do is ease just a little mainsheet, to mark 2 or 3 on your scale so that the top of the main opens a little to reduce the power up top, and to reduce all of the loads. All the leech telltales will be flying now.

After that start easing the traveller down a little to depower further. Then, when you have 20% of the front of the sail luffing, its certainly time to reef, which will provide the same power, with less drag, so will be faster and safer.

In lighter winds you’d again have slightly less leech tension, perhaps also at twist mark 2 or 3, and the traveller slightly higher up the track so the clew still stays on the centreline. In light conditions you might use a little lazyjack or topping lift to stop the weight of the boom closing the leech. But by then most people will be motorsailing!

Use those leech telltales again when sailing downwind and reaching to set the correct twist through the mainsheet, and use the traveller to set the correct angle of the whole sail to the wind.

In waves you need to have more twist than in flat water as the apparent wind angle changes much more with all the heeling, pitching, and course changes, so you need a more tolerant setting.

Tweaking minor controls

The use of the minor sail controls can depend on the boat set up. Most boats will have an outhaul, to control the foot of the mainsail, and this will be tighter for strong winds and eased a little in the moderate, downwind and light conditions.

You can use either the halyard or Cunningham to control the luff of the sail. The goal is to barely eliminate the wrinkles in the luff and the sag between the batten and intermediate cars, but no more than that. Downwind you can ease the luff tension but still get rid of the wrinkles. On a bigger multihull with full battens, you can’t significantly move the draft forward of the sail with more luff tension or bend the mast like you can in a smaller cat.

The lazyjacks or topping lift should be left loose enough not to interfere with the mainsail but tight enough to hold the boom up if you forget to tighten them up before reefing or dropping the main.

Setting up your mainsail

When you first put on a fully battened mainsail on a multihull, the first thing you’ll notice is that it’s heavier than on an equivalent length monohull: the area is higher with the big roach, the full-length battens heavier, plus the weight of the mast track cars that go with the battens. The sail is typically made in a tougher cloth because the righting moment of the boat is greater than a monohull. So once you have that heavier sail into the lazyjacks, here are some tips for setting it up properly.

• Head: Set the correct distance between the head ring and the mast. If it’s adjustable you’ll find that bringing the head ring closer to the mast will put more fullness in the top of the main and make the leech tighter.
• Battens: Check the batten tension: ideally you want to be just getting rid of any vertical creases around the batten pockets. Too tight and the sail will be overly stressed and you can see the batten making the sail locally deeper.
• Depth: The mast is not tuneable in bend without a backstay, but one trick with full batten mains is to tune the thread that attaches the batten car to its mainsail track car. If you have more thread showing then the mainsail will get locally deeper as it is the equivalent of straightening the mast.
• Foot: Make sure the tack is lashed with the appropriate distance to the mast track so that it follows the luff of the sail. Ensure the clew is lashed down close to the boom and the outboard reef lines are run in the correct position.
• Reef lines: Due to the righting moment of the boat, the loads on the reef lines are higher than a monohull. Bigger multihulls will benefit from a hook arrangement for the reef so that it engages with a loop or a shackle on the leech of the main. This can help with chafe problems on lines and lessen compression on the boom.
• Telltales and stripes: Cut strips of red spinnaker cloth to use as telltales on the leech end of each batten. Big is better. The top one is particularly critical to make sure the mainsail is not over-trimmed. One trick for night sailing is to sew a little retroreflective tape to the back end of the telltales so they show up well when a flashlight is shone on them.
  Draft stripes are useful on the mainsail to look at the shape at three different heights, and these can also have retroreflective alongside them for nighttime sailing.

About the author

Brian Thompson is one of the world’s foremost multihull sailors and has sailed more miles in multihulls than any other British sailor. Thompson, 58, has been sailing since he was three years old. He has competed in virtually every major offshore race, including the Mini Transat Race, the Vendée Globe, round the world records for the Jules Verne Trophy, the Route du Rhum and the Volvo Ocean Race.

Thompson has held most offshore records over the last three decades, including spending 11 years sailing with the American adventurer Steve Fossett. He has become the go-to helmsman for multihull sailing records and during his more recent time aboard the MOD70 Phaedo3, helped set ten world records. Thompson is currently campaigning the MOD70 Argo and is one of the directors of Doyle Sails UK.

First published in the September 2020 issue of Yachting World.