Modern cruising catamarans and trimarans have unique requirements in terms of sail design and construction. The reason for this stems from the stability provided by their wide beams, which in conjunction with their lack of heeling can make it difficult to determine when to reef sails. It is therefore not uncommon to see cruising multihulls carry full sail much longer than monohulls of the same size. This, in turn, means that the loads on multihull sails can often be double that of the sails aboard a comparable monohull.

Of course, multihull sailmakers know this and make a point of selecting materials and construction that are up to the task. What follows are some of the design ideas and sail options I have come upon over the years that work best aboard boats with two or three hulls.

Mainsail Design

Many multihulls today are rigged with a simple tripod rig consisting of a forestay and single shroud to either side placed far enough aft so that together they double as backstays. One of the nice things about this arrangement is that without a backstay, there is no longer any restriction on the amount of roach (the sail area aft of a straight line drawn from the head to clew) a mainsail can carry.

Beyond that, with the modern large, mainsail rigs found on many multihulls, the design and construction of the main is one of the most important aspects of a boat’s overall sail wardrobe, because it often comprises the boat’s predominant driving force and is left up, either full-size or reefed, in almost all conditions.

Unfortunately, the production sails sold with all too many new boats are pretty basic and configured to be the most cost-effective for their manufacturers. So there is often plenty of room for improvement, whether it be in their materials, reduced number of full battens, or “cookie cutter” designs.

In terms of sail design the draft, or curvature, of a sail (including the amount as well as the fore-and-aft location of the point of maximum depth) is critical, as it directly influences a boat’s pointing ability. Sails for high-speed boats with daggerboards, for example, will be much different from heavy, shallow-keel cruising boats. Specifically, a sail that is too full will limit pointing ability and speed in most conditions, while those that are too flat will lack power, thereby reducing speed through a chop, for instance.

It’s important to note that there is a misconception among many monohull sailmakers that the draft on all multihull mainsails should be well forward, as is the case with beach cats. While this may be true with rotating masts, where the mast is part of the overall foil, with a non-rotating rig turbulence from the mast results in bad air at the sail’s leading edge, which brings the attached wind flow farther aft. Mainsails for these rigs should, therefore, have the point of maximum draft farther aft and closer to the midpoint of the sail. On fixed rigs, non-tapered battens will often perform better than tapered battens, for this same reason.

Many production mainsails also have “pinhead” profiles (in which the head of the sail is cut at an acute angle) with little roach. Therefore, increasing the roach in the mid- and upper-leech will improve performance, since it will add sail area and twist. In fact, for this very reason, more production cats are now following the lead of the racing set with square-top and elliptical head designs. When in use, the wide tops of such sails will twist off, moving the center of effort lower down in stronger gusts. This can help prevent a boat from becoming overpowered, and also allows crews to reef later than they would with a tight-leech pinhead main of the same area.

On the downside, this additional area in the upper roach makes the sail heavier and increases the batten compression on the luff slides. Stiffer top battens are therefore required. In addition, with a square-top main a diagonal top “gaff” batten must be dealt with, i.e, you need some way to lay it flat when the sail is lowered into a sail cover.

Aboard many boats, this can be done by simply removing the head of the main from the top slide assembly. In some cases, if the head is easy to reach, a quick-release pin can be used to facilitate removing the head from the top slides. Special headboards have also been designed that come away from the top mast slide when the halyard is eased, thereby making it easier to drop the sail neatly into its cover. It’s important to be aware, however, that while these work well, they can also be expensive.

As a result, while square-top mains may look sexy, for ease of handling I typically recommend an “elliptical” head for most cruisers: an approach that provides a similar total area as a square-top main, but replaces the top diagonal batten with a few closely spaced horizontal battens. With this profile, the advantages are much the same as with a square-top. However, with all battens kept parallel to the boom, the sail is much easier to stack.

In addition, with any large full-batten mainsail, offshore cruisers should have three sets of reefs installed to reduce area when needed. I normally design a mainsail with the first two reefs positioned so that they reduce sail area by 15 and 20 percent each. As the third reef will be the storm sail, I believe this should be proportionally larger than the other two.

Reef patches, of course, should be strongly built with large rings, and well reinforced with multiple layers of material and additional straps of webbing. If reef blocks are used, I prefer to have rings installed first with the reef blocks shackled or webbed onto these rings. The reason for this is that if a reef block fails, the ring will still be there as a back up.

Headsails

Multihulls are often rigged with either a furling self-tacking jib or a furling genoa. Unfortunately, while a self-tacking jib is a very convenient sail to use, it is not without its drawbacks. In order for a self-tacker to work, there must be a track or sheet leads in front of the mast, which will restrict foot length to less than the distance from the forestay to the partners. The resulting sail is often very tall and thin, leaving the top portions fairly aerodynamically ineffective (although adding vertical leech battens and some roach can improve the situation by increasing area and power).

Another problem with self-tacking jibs is an excessive twist in the top when sheets are eased, although curved tracks and special spars can help.

For these reasons many cruising cats and tris employ furling genoas that can be reefed. Because most cruising multihulls are under-rigged, such larger headsails can be a real help in terms of performance. In fact, a well-designed and built genoa can provide an effective driving force, and I often see large differences in speed resulting from the fine tuning of a genoa compared to the mainsail.

As is the case with draft location in mains, there is a general misconception that a partially rolled-up genoa is inefficient upwind. However, while this may be the case with stretchy Dacron genoas without luff-flattening additions, it is most definitely not the case with a modern well-built furling sail. In fact, if a genoa that is built with good low-stretch materials and a foam luff or ropes sewn into the luff area to remove depth as sail area is reduced, can be rolled in part way and still maintain a good upwind shape.

This becomes especially important as the large genoas on cruising multihulls typically are the primary working headsail, in which case they are used either full-size or reefed down along with a reefed mainsail. Multihull genoas should be built with similar material and construction as the mainsails. By using single-line mainsail reefing and a well-built furling genoa, sail handling overall becomes greatly simplified.

Downwind Sails

The average cruising cat with a stock main and jib will suffer from being underpowered in light air and sailing off the wind. The most effective way to increase sailing performance in these conditions is to add one or more specialty sails to the inventory. This, in turn, will allow you to save time on passages, reducing the risk of bad weather and minimize motoring time.

Screechers, reachers and Code 0s are all names used for any loose-luff furling sail that flies from a bowsprit. Be warned: these kinds of sails also sometimes go by different names in different countries. In the U.S., for example, the old wire-luff reachers made of nylon have been replaced by screechers or Code 0s—large, loose-luffed sails typically used with an anti-torque luff rope and furler. However, in Europe, these same sails may be referred to as gennakers.

In fact, the term “screecher” was coined in the early days of Corsair F-27 racing in South Florida, in an attempt to get a reacher classified as a small spinnaker to avoid a handicap hit for an oversized working sail. This didn’t last long, as race committees soon saw it for what it was. Nonetheless, the end result was a low-stretch sail that can be used upwind in light air. To function correctly, a screecher needs to be sheeted inside the cap shrouds, where it can be trimmed to an inboard position of 10-12 degrees off centerline. Screechers work best with low-stretch, anti-torque luff ropes and two-to-one halyards highly tensioned.

Unlike the reachers of the past, screechers need to be designed and built to take the considerable loads of high apparent-wind angles and speeds, while maintaining a flat upwind shape. If used upwind in true winds of as little as 10 knots, for example, it is not unusual to experience apparent winds of 17 knots or more. Properly designed and trimmed, a screecher can be a real workhorse, especially in light air. This, in turn, can greatly add to sailing pleasure, whether you’re daysailing or off soundings.

For long reaching or downwind passages, on the other hand, a Code 0 or asymmetrical spinnaker is your best bet for generating the necessary horsepower to achieve good average speeds. The original Code 0s were designed for the Volvo Ocean Race as super-flat spinnakers and were made of heavy nylon with wider mid-girths. Since then, because of their popularity, sailcloth manufacturers have created fabrics specifically for Code 0 sails, including laminated fabrics with Mylar film on one side, strong warp fibers in the center and a light taffeta on the other. Even heavier versions include Aramid fibers in the warp for added strength. The end product is a light, low-stretch, fabric with a “soft hand.” This can be stuffed into a bag, doused with a sleeve or flown with a furler.

The Code 0s I build for multihulls use the same attachment points as a spinnaker, i.e., the end of the bowsprit, the spinnaker halyard and the sheet blocks at the back of the hulls. However, because the foot is longer than on a screecher, Code 0s are trimmed outside the shrouds, which means wider sheeting angles, so that the highest these sails can be used is 60-70 degrees off the true wind.

When in use, the additional area compared to a screecher creates a powerful reaching sail as well as an easy-to-use sail for downwind use. Because Code 0s are designed with their draft forward and a flat leech, like a large genoa, they can also create more power per square foot than an asymmetrical spinnaker. If made with laminate Code 0 cloths or light Dacron, these sails can have a light-weight UV strip added, allowing them to be left furled on the bowsprit when not in use.

In terms of spinnakers, both symmetrical and asymmetrical configurations can be found on cruising multihulls. However, while symmetrical spinnakers can have their place for long downwind passages, I feel if one spinnaker is to be bought, it should be the more versatile asymmetrical.

When designing an asymmetrical spinnaker, I like a draft-forward camber and fairly flat leech, which creates a true foil shape that produces lift, as opposed to a semi-circular symmetrical spinnaker that only presents area to the wind. Granted, the area of an asymmetrical spinnaker is a bit less than a symmetrical sail, but the asymmetrical will have a wider range of use and produce more power per square foot.

At sea, asymmetrical spinnakers work well either tacked to a bowsprit or to blocks on the bows. A simple way to rig the latter is to install two stand-up bow blocks through which you can lead a pair of tack lines, with the sheets leading to a pair of spinnaker blocks aft. When sailing at low angles, bring the spinnaker tack to the windward bow to avoid the wind shadow from the mainsail. On close reaches, drop the tack to leeward.

Of course, if you can it’s still best to use a bowsprit, since you need fewer lines and a sprit will bring the tack lower to the deck, allowing for a longer luff and more area. Gybing is also easy: just bear away, easing the sheet as you do so until the sail is forward of the forestay, at which point trim in on the new sheet. Note that a spinnaker on a bowsprit can also still be tacked to a windward bow block, for downwind sailing.

I like to design my asymmetrical spinnakers with a full triradial cut, which reduces stretch and helps maintain a draft-forward curvature when used with higher-weight nylons. If well-designed and built, an asymmetrical will provide a wide range of use, with close reaching true wind angles of 70 to 80 degrees possible in lighter breezes, and low angles as great as 150 degrees. These sails work best with a spinnaker dousing sleeve that incorporates a molded “bell” shaped mouth, like the products from ATN (atninc.com).

Ultimately, when it comes to finding the right downwind sail for you, the answer depends on the type of sailing you do. For example, will you be daysailing or coastal cruising? Will you be embarking on an occasional short offshore cruise or heading off on long ocean passages?

For daysailing and coastal cruising, a screecher would be a good first step, since this will improve your light-air performance upwind and reaching. For long-range tradewind passagemaking, on the other hand, the addition of a good downwind sail will be well worth the expense.

Beyond that, for those on a budget, a Code 0 can be an alternative to having both a screecher and spinnaker, since the size of a Code 0 falls in between. Be warned, though, that it will not go as close to the wind as a screecher or offer the downwind speed of a full-sized spinnaker. Nonetheless, it will be a good all-around reaching sail that can be rolled up from the cockpit. Indeed, for light offshore cruising with husband and wife teams, this may be the ideal third sail. 

David Calvert has been sailing and racing catamarans and trimarans for years and is the owner of Calvert Sailmakers LLC, which specializes in building sails for multihulls of all sizes.

MHS Fall 2017