Crossing the Atlantic from Cape Verde to Barbados took us just 13 days in our 1998 Lagoon 410, Cushla Na Mara. In preparation for this trip, I spent time experimenting with the best downwind set-up for tradewind sailing, including using twin headsails, a popular option for good reason.

Although much of this learning was with the ultimate plan to cross oceans, it can be adapted and is equally useful for coastal passages of any length.

It became clear soon after we bought Cushla that she was not very fast downwind. The genoa, only 38m2, was not powerful enough to drag the eight tonnes of Cushla’s twin 41ft hulls through the water at much more than 5 knots even as the wind picked up to over 15 knots.

With days of down-winding on our Atlantic crossing looming I turned my attention to the various options. The obvious answer, a spinnaker, felt like it came with too many constraints. The opportunities to use it are limited by the wind angle and windspeed.

With two sheets and two guys to rig, managing the sail is no easy job when shorthanded or night sailing, so I could imagine that between the daylight hours and wind conditions we may not be able to fly it very often. The cost was going to be well over £3,000, including a sock or furler and tackle, and this seemed like very poor value.

A Parasailor, essentially a spinnaker with a slot in it that houses a wing to stabilise the sail, has a bit more flexibility, able to fly in a wider range of points of sail and windspeeds.

A few owners I spoke to have confidently kept them up overnight on passage, but like a spinnaker, the time it takes to drop means squalls present a real risk. They also cost more than a spinnaker, don’t pull you along as fast, and they’re a shade more finicky to fly – friends of ours shredded their Parasailor after accidentally hoisting it inside out in the dark.

Like most cats, we don’t have a backstay, so our shrouds sweep quite far aft, and using the main downwind gives limited power unless you’re willing to accept the fabric damage caused by letting the main rub on the rigging. It’s bad for the sail and not great for the rig either.

Using the main also moves the centre of effort further aft and makes the boat a bit more skittish, like pushing a shopping trolley instead of pulling it. It’s especially noticeable if following waves are rolling in at an angle.

I had my heart set on an asymmetric spinnaker. It’s easy to manage with just two sheets. On a cat you can use the windward bow for the tack, removing the need for a bowsprit or a pole, and all the hardware installation and line management that comes with them. They also offer great versatility for wind angles, too, making it a sail that will actually get a lot of air time.

But then I saw the price… For a 40ft boat I was looking at a bill of close to £5,000 for a new sail and the necessary hardware.

I searched for a used one but, perhaps testament to how practical they are, second-hand ones are rarer than hen’s teeth.

Twin headsails

Instead, I opted for a twin headsail setup. Like most headsail furlers, the one on Cushla has two luff grooves in the foil meaning you can have two sails up simultaneously, and downwind they open up like a butterfly. The big advantage is that it gives you a large sail area but it can be reefed very easily from the cockpit as both sails wrap around the furler.

I found a cheap used sail that had a slightly shorter luff and would fit on the foil.

It took a few attempts to get the new sail up the groove which had some dirt in it and had probably never seen fabric in its 20 years of service. Threading two sails at once takes a bit of patience and there was a lot of halyard tension with the weight of both sails on it.

I eventually upgraded it to dyneema for peace of mind, and kept it taut so the tension was running through the sails and not the steelwork. I had to use a length of rope from the tack of the new sail to tension the shorter luff equally with the original genoa. Finally I had to re-stow the furling line on the drum anticlockwise as the new sail had the sacrificial strip on the opposite side which was fortunately wider and big enough to cover both sails when furled.

The biggest challenge was running four sheets on two winches. An advantage of this setup is you can still beat upwind without a sail change, by laying one sail over the other, but that means both sheets are looking for a winch on the same side of the boat.

The problem was exacerbated because the two sails weren’t the same size and needed separate barber haulers to pull tension in each leech at different angles.

Without good tension in both sails the fabric was prone to fluttering and chafing against each other, a form of abuse they aren’t designed to withstand, and this was my biggest concern with the setup.

Furling solution

Some furling systems, notably on larger Amels, have a remedy for this. A slightly convoluted arrangement of lines allows you to raise and lower a second sail in the furler foil without dropping the genoa, so you can add or remove the second headsail while underway if you find yourself changing point of sail. It’s typically used with a lighter-weight larger headsail nicknamed a ‘ballooner’, which is a bit flatter than a spinnaker so it can be sailed while reefed.

Up to 2006 Amel rigs were set up to run two poles simultaneously with attachment points outside the rigging allowing the poles to move unencumbered by each other or the stays. In a squall you can turn the boat to gybe the lighter sail on top of the thicker one, protecting it from forces capable of ripping it.

On catamarans it’s fairly common to use the midships cleats instead of a pole to hold out the headsail’s clew.

In 4m waves coming at 45° to the stern we were able to steer a downwind angle up to 150º before the upwind one risked collapsing. Flatter seas meant we could push it another 20º to around 130º. Close to this limit I found it helped to route the lazy sheet forward through a bow cleat to hold the clew down and forward, and pull in the opposite headsail to create more pressure inside the sail balloon.

This helped prevent a few collapses but when it did inevitably occasionally collapse, the reopening was more of a shock to the rig than the collapse. It’s an unnerving feeling to feel the rig shudder far from land.

With the mainsail stowed, I kept the mainsheet and an oversized topping lift taut to act as a backstay to support the mast. We used the Lagoon manual’s reefing guide to dictate how much sail area to use, estimating how much headsail was equivalent to the recommended size of reefed main.

We reefed for the gusts, rather than the average windspeed, and kept to one reef below the guide’s recommendation to mitigate for the shock loading of a reinflating collapse.

Dead downwind Cushla almost doubled her speed from sailing at 30% of windspeed to 50% with the twin setup. But in light winds any chop would cause the headsails to flap. Their weighty fabric needed more wind to fill them.

Using the slot effect with twin headsails

I considered free-flying the second jib from the spinnaker halyard without threading it in the furling foil. Theoretically this could provide a bit more speed because it creates an aerodynamic effect.

Downwind, the sails create force by acting as an obstacle to the wind, and they don’t work aerodynamically. But having one sail looser than the other creates a gap that air can accelerate through. The higher airspeed creates a low pressure in front of the other sail, which according to the physics books should create more forward force.

But in a cruising situation I wasn’t convinced this would make a difference. Weighed against the risk of having to wrestle down a large sail on a bouncing bow as a squall approaches, I dismissed it as an option.

Our boat doesn’t have a second forestay, but this aerodynamic boost can be achieved on a cutter rig, goose-winging the headsails. Both sails can be on furlers so can be reefed more easily in tough conditions.

With wind off the centreline, the larger of the two sails needs to go on the downwind side to get the best wind angle range. Having different-sized sails can affect the trim in gusty winds. As the wind ramps up the larger headsail pushes the boat more and turns it, risking a collapse of the other sail if you’re close to the limit, but a cutter rig allows you to reef each headsail independently to balance the sail forces.

Key considerations

The conclusion I’ve drawn is that there are two key criteria for a high-performing downwind cruising setup.

First is the reefability of the setup. This is really paramount. If your setup is easy and safe to reef you’ll be confident to leave more sail area up, making the best miles you can with the prevailing conditions, rather than limping along, under-powered and cautiously prepared for worst case scenarios that may or may not materialise.

This is particularly true when reefing down overnight, which could lose you as much as a quarter of your daily mileage. An easy reefing setup that can be done singlehanded, in the dark, quickly and from the cockpit is probably the single best improvement cruisers can make to their boat to increase passage speed.

It may seem counterintuitive but being able to quickly and easily slow the boat down will help you go faster.

Secondly, the versatility of the sail plan really helps. Tradewind sailing isn’t always dead downwind, so a sail plan that comfortably takes in a variety of points of sail allows you to trim and stay on course, rather than alter course or embark on a sail change.

And while it’s nice to think your boat’s optimum downwind course is fixed, the bigger the waves the more your boat will yaw around, so a sail plan that can cope with 120º off the wind in flat water, might only be reliable to use at 150º to the wind in big swell where a wave can occasionally kick the boat 30º off course.

The same is true for windspeed range. YouTubers Ryan and Sophie of Polar Seal kindly passed on an old asymmetric spinnaker they no longer needed. ‘Don’t use it over 10 knots!’ they warned us as they handed over the sun-aged sail.

I was so excited to receive my dream sail I didn’t think to ask if they meant 10 knots of true wind, apparent wind, or boat speed. It propelled the boat nicely with wind from dead astern all the way round to beam on, so it was a fantastically versatile sail. But having to take it down as the wind got into double figures made it practically useless for passage-making in the Trades.

In the end, partly through scientific endeavour and partly through impatience, I pushed it during day sails up and up the wind range, in what unwittingly became destructive testing. After ripping and repairing a seam I settled on 18 knots of true wind as a limit.

On our crossing the wind was rarely below 20 knots and when it was, it constantly threatened to increase, so the asymmetric hardly got used. Cruising long distances is very different to racing, but speed is still important because it translates to faster passage times.

On a 2,800-mile Atlantic crossing the difference between averaging 7 knots and 6 knots is three days less at sea. Faster passage times translate to less food and water to weigh the boat down and a more rested crew, all of which makes for a safer passage.

A reefable sail plan that is versatile enough to handle up to 25 or even 30 knots of squall gusts and a wind angle of at least 120º will get you there faster – even in big swell.

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