Do Twin Sheets Better Control the Mainsail?

The main traveler is one of those love-hate pieces of gear on a boat. We love that it separates control of twist and angle of attack into a quick and instinctive process. We hate it when it clutters up a cockpit and fear it when it slashes across the deck during a breezy weather jibe. A traditional alternative is a split- or twin-mainsheet system. We thought this setup had become extinct, but in fact we’re seeing it on a few of today’s high-performance multihulls, which suggests it still has merit. Every user we interviewed loved it. And so some testing was in order.

There are three main arguments in favor of a split mainsheet.

• Convenience. The traveler track is in the way. Often a twin mainsheet system can be designed to span a cockpit or cabin.
• Simplicity. Travelers are expensive, wear over time, and occasionally fail spectacularly.
• Better jibe control. Because the two lines create a triangle, the boom can be fully controlled through the jibe. Countless MOBs and notable fatalities have resulted from accidental jibes, (see “Risk Management and Renting Adventure,” PS January 2017).

Even if you have no interest in converting your system or buying a boat with twin mainsheets, it can be a robust jury rig that every cruising sailor should understand. As long as you have a pair of unused winches, a twin-sheet system can be rigged from gear on hand in a matter of minutes.

How We Tested

It was a simple matter to rig our test boat for both traveler sheeting and twin-tackle sheeting. We don’t use our secondary winches under working sail, so we installed a pair of turning blocks where the rail of a monohull would be and set up a simple 2:1 block and tackle led to winches. We equipped both systems with a quick connect at the boom end so that we could easily switch back and forth between sheeting systems.

Observations

Retraining is required. Instead of adjusting the twist with the sheet and angle of attack with the traveler, the adjustments are combined. When sailing to windward the sheets are each about the same length, both highly loaded and most adjustments require pulling two strings. When the leeward sheet is trimmed, the leach closes and the boom moves to leeward. If the windward sheet is trimmed, the leach closes and the boom moves to windward. Perfecting boom position and twist requires adjusting both in a trial and error process. Dumping a sheet in a gust increases twist, but not as quickly or dramatically as releasing a single sheet. You can’t play the traveler in a breeze, the way many racers do, varying the angle of attack without increasing twist.

Marking the sheets at the centered, windward position can be handy during the learning phase. Mark both the low-tension setting, used during jibes and when the main is furled, and the fully loaded windward position. This will reduce trial and error.

Dig into a Dual-sheet Debate

Like many running rigging modifications, switching to a twin-mainsheet arrangement has advantages and disadvantages. One of the most rewarding benefits has been the ability to prevent unnecessary movement of the boom.

Pros

• Jibing. Better control and less traveler hazard in heavy air.
• Boom does not bounce around as much in lumpy conditions.
• No vang. (Actually, the need for a vang is about equal in both systems.)
• May better accommodate cabins and dodgers.
• Better stabilizes boom at anchor, but may block cockpit.
• From 5 to 35 percent less sheet purchase is required, depending on the width of the sheeting base. (Typically, a dual-sheet system offers little or no advantage for multihulls.)
• Cheaper. However good block and tackle setups still aren’t cheap.

Cons

• Jibing. More trimming in light to moderate conditions.
• Sail can back and fill violently in certain conditions, since the boom cannot move.
• More complex sheeting. Unlike traveler systems, twist and angle of attack are not separate adjustments.
• More weight on the boom.
• Tackle can block cockpit when reaching.
• Nearly double sheet length. Tripping and entanglement hazard.
• More expensive if you need two additional winches. Slightly cheaper if tackles will work, though a pair of good tackles still aren’t cheap.

There is a lot to like about twin sheets when jibing in a breeze. First, pull in the leeward sheet to the center position mark. Then pull the windward sheet to same length, centering the boom and securing it. Once through the eye, release the new windward sheet followed by easing the leeward sheet as needed. The process is very smooth and the boom is fully controlled throughout the jib. The boom will lift a little more than with a traveler, but not excessively so.

Using a traveler, you first sheet the main in fairly tight, then center the traveler or even bring it a bit to windward. The boom will thump a short distance when the wind moves from one side to the other, but only a little if the mainsheet was snug, after which the traveler is eased all the way down, followed by the easing the sheet as required. If the sheeting base of the twin mainsheet system is the same as the traveler length (both should be as wide as practical) the amount of mainsheet out during the jibe is the same in both cases.

When jibing in light winds, the advantage is goes to the traveler system, which can be left to run free. In moderate winds jibing safety and ease
becomes roughly equal, with the edge on trimming speed going to the traveler, and often, the edge on trimming range, to the twin sheet system.

Some say twin sheets work like a vang. Well no, not really, no more than a traveler system does. Most multihulls dispense with a vang entirely for this reason; a full width traveler provides sufficient twist control. Neither functions exactly like a vang. However, using a vang with twin mainsheets is a bit like scratching your head and patting your tummy at the same time. We found it hard to keep straight and would avoid it. With a traveler system, it is simple to switch between vang sheeting and traveler sheeting, and they can even be combined without complication.

A single continuous sheet was simpler to manage with twin sheets. It saves some length, since the leeward tackle is always shorter than windward tackle. It also reduces tangling, because there is no free end to form knots. You can always bring both ends to the windward rail. It does leave a lot of line on the floor. Remember the old saying, “Mainsheet men hop on one leg?” You’ll need to watch your footing and learn some boat-specific line management.

No traveler equals less cockpit clutter. Yes, a traveller divides the cockpit in two. But the twin-sheets interfere in a similar way. We really dislike the mid-cockpit traveler on the F-24. If you want to go aft you have to step over it; with twin sheet systems through-transom access is clear. But with the twin mainsheet system, we had to duck under the windward sheet, especially when off the wind, a time when the cockpit is normally free.

The same argument can be made for securing the boom at anchor. On one hand this is dead easy with two sheets. On the other hand, even if you secure it off to one side, one of the sheets still crossed the cockpit. On our PDQ we would often move the traveler to one side to clear the aft cockpit walk-trough and secure the boom with a short line to a cleat, creating the same sort of triangle. The overall advantage or disadvantage is boat specific.

Safety

There is no traveler car potentially zipping from side to side. On the other hand, the extra rope poses a tripping and entanglement hazard. Both can be managed.

We’ve heard it said a twin sheet system can serve as a preventer of sorts, because if the sheet is kept short, the boom cannot fly across the cockpit center line.

We reject this as an advantage, because exactly the same can be said of a properly used traveler. With the traveler locked down at the end of its travel, and with each sheet in a dual-sheet system kept the same length, the swing of the boom is the same.

However, if you run downwind with the traveler centered, the boom held down with the vang, and a lot of sheet out, or you do not lock the traveler car down at the end, the boom can move violently, but only because you are not using the equipment correctly.

Multihulls

Extreme beam of multihulls affects how traveler and twin sheet systems are used. Travelers are typically full width and aft swept shrouds limit easing the boom to about 50 degrees from the centerline. As a result, there is little sheet out when the traveler is fully eased, even running. Likewise, twin-sheet systems have only a little sheet out to leeward. The downside is that widely spaced, twin-sheet tackle anchor points result in an obtuse
angle between the tackles and very high sheet loads when sailing upwind.

Less banging in sloppy light airs. With two sheets, the boom can be secured. But we very seldom had this problem, and we could always rectify it with a line to a stern cleat. On the other hand, in light moderate conditions, instead of the boom being able to move a bit and the sail flexing with the wind, the fixed boom made for some atrocious “booms” as the sail filled and backed when rolling. When it became a problem, there was no easy solution, as there would be with a traveler system. In the end, we decided this over-control of the boom was more minus than plus, but this probably varies with the motion of the boat and others may feel differently. Let’s call it a wash.

More complex sheeting. With a traveler and mainsheet system, the sheet primarily controls twist and the traveler controls the angle of attack. With two sheets they work together to form a triangle that controls the position of the boom. To windward, most adjustment require pulling two strings. When the leeward sheet is trimmed, the leach closes and the boom moves to leeward. If the windward sheet is trimmed, the leach closes and the boom moves to windward. Often both must be moved, in a sort of trial-and-error process. All of the same boom positions are possible, but it requires more thought and fiddling.

Less sheet purchase. It’s true that if the sheeting base is narrow, the tackles can be smaller, since they share the load when sailing to windward. On the other hand, with the wide base typical of a multihull (base 16 feet, boom height 5 feet), the angle is disadvantageous and the sheet load is not appreciably reduced. In both cases, there are now two tackles to be tensioned instead of one. Because our sheeting systems were different in each mode, we can only comment on the obvious math.

Load distribution. A traveler track is often a structural member of the boat. When the boat jibes, the stress is spread over a span of deck. With a twin sheet system, the loads, both vertical and transverse, are point loads, and either hard on the wind or during a jibe they can be massive. If converting from traveler to twin system, remember that managing the loads is more complicated than just adding a big backing plate; the cockpit coamings or cabin must be able to transfer potentially enormous shock loads safely to the hull. We’ve heard of cockpits crushed when a sailor removed a structurally necessary traveler bridge.

Conclusions

After weeks of playing with twin sheets, we went back to the traveler and single-sheet system. We enjoyed sailing with twin sheets and felt that a well engineered twin-sheet system will suit some boats and some sailors. We like twin sheets for cruising and even reefed down in heavy weather. Controlled jibing is nice. But racing or powered-up in a multihull or sport boat, rapid, simple adjustment is vital. In the end, we understand the overwhelming popularity of traveler-based systems on both cruising and race boats. Sailing is simpler.