Multihull sails for the modern cruising catamaran
Dave Calvert

I would like to focus on performance sails for cruising catamarans.
As most of these boats are purchased with intentions of coastal cruising and offshore voyaging, the sails for these boats need to be designed and constructed accordingly.
The modern cruising catamaran has been intentionally designed with a conservative rig and sail plan.
Yacht designers have done this to build in certain safety factors. This reduces the risk of being overpowered in sudden squalls and large seas found in offshore conditions.
With most cats cruising shorthanded, often with a husband and wife crew using auto pilot, response time from increasing winds is reduced. Forgiving sail plans are needed.
In addition to conservative sail plans, other safety features include, mainsails with 3 reefs, single line reefing, and furling headsails that can be reefed from the cockpit.
The risk of capsizing has been greatly reduced with modern boat designs and rigs, to the point that this is a very rare occurrence indeed. Monohull advocates are losing their main arguments as capsizes become more rare. The downside to all this is, less power under sail.
I have been researching used catamarans while looking for our next cruising cat. One thing I have noticed is, diesel engines with high hours. This is true even on boats that have not been chartered. Often the sail inventories for these boats only include a main and a jib, or genoa.
I have to assume these boats are motoring more than sailing.
The performance of an average production cat, with basic mains and jibs, will suffer upwind in light air and on reaches and downwind, even when there is a breeze.
These boats will only sail well in moderate winds upwind and close reaching. To achieve good top end speeds on low sailing angles, strong breezes are required..
A large part of job as a sail designer is to maximize a boat's performance. In addition to providing the optimal sail shape, choosing correct material and construction for the intended sailing, is of equal importance. This is the case with cruisers as well as racers.
For racers, I look for loop holes in the rules to maximize sail area, as well as optimizing shapes, and reducing weight, to get that extra speed to win a race.
For cruisers, I look for ways to improve durability, ease of handling, and sailing performance to minimize the motoring time. After all, we did but a SAIL BOAT!
The first part of my discussion will be dedicated on ways to make the sails easier to handle with a minimum crew. Then, I will go to options for increasing performance under sail to reduce engine time.
The following are key points for ease of handling on the main and headsail.

Attributes for ease of handling and safety, what features do you want in your mainsail?

LIGHT WEIGHT. Sail construction, sail material,and battens.
These are main contributors to the weight of a sail. The cloth must be chosen to be up to the job but, without excess weight. Sail construction, well done, minimizes unneeded weight. The number of battens are dependent on the amount of roach and the stiffness of the battens. These full battens should be engineered to support the sail's profile efficiently with minimal weight.

GOOD DESIGN. Too full will increase effort to raise, weather helm, and be inefficient upwind. Too flat will reduce power. Sails should be designed for each boat's predicted windward ability and intended areas of sailing.

MODERATE ROACH. This allows for less battens. 5 to 6 stiff battens are best for cruising. More roach increases batten compression and friction raising and lowering. Although, often production mainsails, are made with very little roach. This leaves room for improvement.

BATTENS PARALLEL TO BOOM FOR GOOD STACKING. This is important to have the main stack compactly for ease of installing a sail cover. Less batten and intermediate slides also reduce stack height.

GOOD TRACK AND CAR COMBINATION. Roller bearing cars offer less friction, but there are many good options for sail luff hardware. This depends on boat size, sail size, and sail loads. Ask your sailmaker for recommendations.

LAZY JACKS AND BUILT IN SAIL COVER several good choices on zipper covers. I feel that this is the next best thing to a boom furler.The sail covers that are supported by lazy jacks and battens and use a zipper on top, seem to be the most conveyance to use.

3 REEFS WITH 2 SINGLE LINE REEFING In these cases, the first and second reefs can be put in with the use of a single line run to the helm station. This effectively brings both the new tack and clew down to the top of the boom very easily. I recommend a third reef that is much higher, proportionately, than the first two. This is the "Storm Sail". The height of this third reef is too high to have lines run thru it. Best to simply drop the main, tie in the third reef, and re tighten the halyard, for this storm reef.

ELECTRIC HALYARD WINCH. This is especially nice on boats with large mainsails over 600 sq. ft. and short handed crews. The weight of large mains make raising them a real chore, even with good luff hardware.

Attributes for jibs and genoas, what to look for in efficiency and ease of handling.

FURLING JIBS, Furling systems are great to deploy and douse a self tacking jib or genoa. Genoas can be reefed to reduce sail area from the cockpit.

SELF TACKING JIBS, Prevents 2 steps when tacking. No easing and trimming on sheets. Usually too small with tall thin profiles to reef, Best to use all or nothing. Although a self tacking jib is very convenient, it often is less efficient than a jib or genoa with sheet track on each side.

GENOAS, These sails can add more sail area for light air conditions than, be reefed to predetermined locations. The best reefing arrangements will include a foam or multi rope luff along with reefing strips at pre determined locations. The foam luff helps to remove shape in the genoa's mid section by rolling up more material in the mid luff. Without this, the partially furled sail would become much fuller when reefed. This is the opposite of the desired shape in stronger winds.

LARGE SELF TAILING JIB WINCHES. These should be large enough for the smallest crew to handle. This is especially important for husband and wife teams. Too often, genoa sheet winches are undersized on production boats. This is something to make note of when shopping for a cat. I have been on mid sized catamarans and found it a real workout to trim the jibs for upwind in moderate winds. This would be even more difficult for smaller people.

WINCH LOCATION, It is best to have both main sheet and jib sheet close to the helm station. With this arrangement, the sails will be trimmed more often and the boat sailed more efficiently. Having the single line reef controls close to the helm, is a big plus as well.

Here are some ways to increase sailing efficiently over the production main and jib rigs. I often am asked when it is time for sail replacements, "What can be done to improve performance on the next set of sails? What can be added to the sail inventory to improve light air and downwind performance?"
Here are some key features and sail options to consider.

Custom sail designs for the boat can be a big improvement. Some production sails are basic designs and can often be either too full or too flat. Draft location is also critical. There is a misconception with monohull sail makers that the draft on all catamaran mainsails should be forward like beach cats.
In reality, because the great majority of cruisers use non rotating masts, this is not the case. Turbulence from the fixed mast, results in bad air in the forward parts of the mainsail's leading edge. This causes the attached flow, on the mainsail, to be farther aft. Because of this, mainsails should have the maximum draft closer to the center. Non tapered battens will actually perform better in most cases than tapered battens, on sails with fixed rigs.
Adding area with more roach is a typical improvement.
This is a good way to improve light air performance. I see several production main sails with very little roach. Increased roach up high will improve twist in the upper leach as well as add area. This will help in the light and moderate conditions. The trade off is, sails that are a bit heavier with increased batten compression on the luff slides. I feel that a moderate, "performance" roach will be a good compromise in performance and ease of handling. (see diagram 1)
Reducing weight and stretch are areas where large improvements can be made. A very important consideration in choosing sailcloth and sail construction options is maintaining the designed shape over wide range. There are two lifes of a sail. The life of the sailcloth, and the performance life. The performance life relates to how long a sail will maintain it's designed shape. The best designed sail will still not be a good one if inadequate cloth or construction is used. In the larger size multihull sails, dacron may not be up to the job. In these cases, other material and construction options should be considered.
Some of these other options include, triradial cut cruise laminates, D4, FST, 3DL, Load Path, and other "string sails" with taffeta.

Furling self tacking jibs and genoas;
The small self tacking jibs often have no battens. This results in a hollow leech, to reduce leech flutter. When these tall, thin, sails have hollow leaches, the upper sections are too thin to generate much power.
Adding vertical leech battens with some roach (convex shape instead of concave), can enhance performance. This improves the tip effect, adds more sail area, keeps the exit flat on the leech, and reduces leech flutter. These vertical battens must be parallel to the luff for the sail to furl tightly.
Use of a multi hole clew board can improve fine tuning and reduce twist. The jib sheet can be changed to the various hole locations, to improve trim.
Problems with self tacking jibs include too much twist in the top, on all but close hulled settings. Once the boat is on a reach the lower section, of the jib,will have to be over trimmed to obtain correct upper trim. With an athwartship track only, it is difficult to get good all over trim while sheets are eased out.
Innovations like camber spars help here but, loss of furling is the trade off.

As I mentioned before, the average cruising cat, with stock main and jib, will suffer from being under powered in light air and off the wind.
The most effective way to increase sailing performance in these conditions is to add specialty sails to the inventory. By doing this, much time can be saved on passages and motoring time can be greatly reduced. These are some of the options on additional sails for a cruiser's inventory.

These large headsail's names are typically descriptions for the same sail. The terms may be different throughout various parts of the world. In the US the old loose luff reachers made of nylon, have been replaced by "screachers". These are large, loose luff sails, set on short bow sprits, typically with a furler. Europeans sometimes refer to this as a gennaker.
The term "Screacher" was coined in the early days of the Corsair F 27 racing in South Florida.
It was in an attempt to get a reacher classified as a small spinnaker to avoid a handicap hit for an over sized headsail. This didn't last long as race committees saw it for what it is.
The evolution of the screacher, has produced a low stretch sail that can be used effectively upwind in light air. For effective upwind use, it will need to be designed to be sheeted inside the cap shrouds and trimmed to an inboard position at 10 to 12 degrees off centerline.
The sail also requires working around the considerable luff sag from a loose luff attachment.
Unlike the nylon reachers of the past, these sails must be designed and built to take the considerably high apparent wind angles and speeds from upwind sailing on modern multihulls.
If used upwind in true winds of 10 to 12 kts., it is not unusual to have apparent winds in the 17 + knots range. A properly designed and trimmed screacher can be a real work horse upwind and close reaching in the light breezes. This can greatly reduce motoring time. Other advantages are seen while reaching and even running in higher winds.
If used on a furler, the screacher furling line can be lead to a position close to the furling line for the jib. It will than be a simple procedure to simply roll up the jib, and unroll the screacher, as the wind gets light, or reverse this, as the breeze increases.
(See diagram 2)
For long downwind passages, often the main, jib, and even screacher, lacks the horse power to achieve good speed averages. The solution here is either a code 0 or an asymmetrical spinnaker.
This is also a bit of a vague term. The original code 0s were designed for the Volvo round the world racers, as their flattest spinnaker. These early sails were heavy nylon. Because of the popularity of this trend, sail cloth manufacturers, designed and produced fabrics specifically for code 0 sails.
This code 0 cloth, is a laminated fabric with mylar film on one side and a light woven taffeta on the other. Heavier versions include Aramid (Kevlar) fibers in the warp for added strength.
The end product is a light, low stretch fabric with a "soft hand." These sails can be stuffed in a bag, doused with a sleeve, or used with a furler.
We design these, as well as the screachers, with an adjustable Spectra, or Aramid luff rope that furls well.
The code 0 that we build for multihulls, uses the same attachment points as the spinnaker, the end of the bow sprint, spinnaker halyard, and sheet blocks in the back of the hulls. The foots are longer than the screacher and are trimmed outside the shrouds.
Because of this, the highest angles the sail can be used are about 60 to 70 degrees off the true wind. But, the extra size creates a powerful reaching sail as well as an easy to use downwind sail.
These code 0 sails are designed with the draft well forward and flat leeches. This creates good top end speeds for the reaches, often giving much more power per square foot than the spinnakers.
Because of the light weight cloth typically used, it is not the best candidate for a UV cover strip. This means that the sail should be deployed than, stowed in a bag after use.
(See diagram 3)
We design multihull spinnakers with a draft forward camber, and fairly flat leeches. These designs work well on reaches as well as downwind.
In the case of these "all purpose designs" , the upper girths are less than those of a downwind spinnaker. Because of the extra area outside of the straight line leech and luffs, the total sail area can be much more than the comparable code 0. These sails, with their larger areas, will optimize downwind sailing.
Typically, cruising cats will use the spinnakers tacked to twin lines from the two bows. This allows a wide range of adjustment. For sailing downwind, the tack can be brought to the windward bow.
For close reaching, simply ease the windward tack line and trim the leeward one. This brings the tack to leeward, increasing close reaching performance.
I feel that the best case still, is to use a bow sprint. A bow sprit brings the tack, of the spinnaker, lower to the deck, allowing for a longer luff and larger area.
Gibing is easy. Just bear off easing the sheet, than, once the sail is forward of the forestay, trim in on the new sheet.
The spinnaker on a bowsprit can still be trimmed to the windward bow for downwind sailing.
We design the asymmetrical with a full triradial Matrix cut. This along with 1.5 oz. nylon, reduces stretch and helps maintain the draft forward shape. A well designed and built asymmetrical spinnaker will give a wide range of use. Close reaching in true winds up to 70 degrees are possible, in lighter breezes.
These sails work well with a dousing sleeve like the ATN product.
So, with all of these extra sail options the question is, "What sail is best for me?"
This depends a lot on the sailing in mind. Will you be;
Day sailing and coastal cruising, short offshore runs, or long offshore passages?
For day sailing and coastal cruising, a screacher would be a good first step. This will improve the light air upwind and close reaching, greatly increasing sailing pleasure in these conditions. Long downwind runs will not be an issue. For short and long range passage making, the addition of a good downwind sail will be well worth the expense. This will greatly reduce hours of motoring time, as well as time spent offshore on passages.With less time spent on passages the risk of encountering bad weather is also reduced.
A code 0 can be an alternative to having both a screacher and spinnaker.
The size of a code 0 falls in between the screacher and asymmetrical spinnaker.
It will not sail as close to the wind as a screacher or offer the downwind speed of a full sized spinnaker. It will be a good all around reaching and running sail than can be rolled up from the cockpit, making it easy to deploy and douse. (See diagram 4)
Best to evaluate your needs and talk to a multihull sailmaker for a custom evaluation of what sail inventory will be best for you. Than, go use the sails more and the engines less!