When we purchased Wildling, I didn’t order a spinnaker because I wasn’t sure if we would need one, and if we did, what type and size we should get. I’m glad I waited, because now that we have sailed our boat for a year, I have a better idea of what I want a spinnaker to do for us. So this month I went through the process of selecting our spinnaker.

Although Robin and I have sailed many miles with a spinnaker on our Catana 471, I really enjoyed learning more about spinnaker design during this process, and I thought I would share some of that information in this post. I would like to thank Jean-Pierre at Outremer (designer, test pilot, and catamaran guru), Nick at Doyle Sails UK, and Volker (who’s 5X is now under construction and will be launched later this year) for sharing their experience and advice.

First a bit of theory: When sailing to a downwind destination in a catamaran, particularly a fast one, we really want to sail on a reach (90-130 degrees AWA), because it’s much faster than sailing deep downwind. Once the angle starts to get more than 130, things begin to slow down to a point where at 150-160, a reaching sail isn’t much use. On a performance boat, as we sail higher (further towards the wind), the boat accelerates and the apparent wind angle moves further forward, which makes sailing a reach very effective, and even though it might be at an angle to the destination, requiring us to travel further by gybing back and forth to stay on course, the extra speed means it’s still the fastest way to get there.

Reaching under Code-D. Notice the luff of the code-D is quite straight, which makes it good for reaching but limits it’s abilities deep downwind. The straight luff also makes it easy to furl.

On a technical note: this technique of creating apparent wind is what allows true performance catamarans to move the apparent wind forward of the beam and create speeds well in excess of the true wind speed. take a look at the Americas Cup multihulls that sail at boat speeds close to 40 knots for a demonstration of this.

So every boat should have a downwind reaching sail, which for us, is our Delta Voiles, Code-D gennaker, and you would think that since reaching is the fastest way to get downwind, then that’s the only downwind power sail you need! Well, perhaps… if you’re a racing sailor, then yes, reaching is the way to go. But it takes a lot more work and attention from the crew. You have to trim the angles just right, compensate for wind shifts and watch things carefully. But when cruising, our objectives are different to a racing sailor. What we want is low-stress, comfortable, and fast enough.

So what’s fast enough? I’m sure this is different for everybody, but on Wildling, our target speed range is 9-12 knots. If we’re under 9, I’m working to get us more speed, either by sailing different angles, changing sails, or dialing in the trim. If we’re over 12, then I’m going to start reducing sail to slow us down. There’s no extra benefit to us from sailing a reach and doing a bunch of gybing if we could go fast enough, and more comfortably, on a direct course.

We’re going to be sailing Wildling back home to Australia, a voyage that involves may thousands of miles downwind in the tradewinds, that are often blowing at 15 to 25 knots. It would be ideal if we could turn the boat downwind, set our sails for our “fast enough” speed and not have to constantly mess with the sail trim. That’s where the spinnaker comes in. With a spinnaker, we can sail deeper downwind angles than the limit of our Code-D. But which type of spinnaker should we buy?

There are two types of spinnaker: asymmetric and symmetric, with the asymmetric being the most common spinnaker on a cruising catamaran. Asymmetrics are more like a gennaker, in that they have a different length luff, and leech, but they have a lot more curve in the luff than a gennaker, which allows the luff to twist around to windward and generate power at a deeper angle. They are good for deep reaching, usually up to about 150 degrees, and they can get deeper still if the tack is moved to the windward hull. They’re good because they are versatile, and if you don’t have a gennaker, and want only one downwind power sail, then the asymmetric is the way to go. There are a couple of problems with running deep angles on an asymmetric though. Running deep is not their optimum power zone, and in order to gybe, you have to first douse the sail, move the tack to the other bow, gybe the boat and then rehoist the sail. Some folks rig a system of lines to move the position of the tack between the bows, which saves some of the work when gybing, but depending on the amount of wind, it’s usually safest to douse the sail and rehoist.

Doyle asymmetric spinnaker

Doyle symmetric spinnaker

Asymmetric spinnaker reaching downwind. Notice the much more curved luff than a reaching gennaker (e.g. code-D) allowing it to twist to windward and generate more power at deeper angles.

Since we have a Code-D, we’re looking for our spinnaker to fill in the 140 to 180 degree angle range, and that is the domain of the symmetric spinnaker. Symmetrics are made to run deep, and gybing is essentially automatic, because they just float around the front of the boat onto the new point of sail, with a few tweaks on the control lines. No need to douse and rehoist.

We decided to get a symmetric spinnaker for Wildling, so the next step was to figure out the size, cut and type of sailcloth for the sail. For our tradewind ocean sailing we want to be able to handle situations where the wind builds more quickly than we expected, and we don’t want the spinnaker to tear if it gets hit by a 25 knot gust. We also don’t want it so big that we can’t get it down in a breeze.

There are many excellent sailmakers that can make a great spinnaker. We chose Doyle sails, mainly because I’ve used their sails before and know they are well made, and Doyle was willing to listen and understand our needs and design a specific sail that will work for our boat and our cruising plans. They are making us a symmetric spinnaker with a fairly wide and deep cut in the top section of the sail which will allow it to lift and fly out in front of our mainsail. We sized the spinnaker to be 30% smaller than our sail plan specifies with a heavier 1.5oz fabric that will handle 25+ knots of breeze. This should give us a strong sail that will hold it’s shape well and be fairly easy to control and douse with a spinnaker sock.

Deep running with a symmetric spinnaker

It’s also worth mentioning that there is another symmetric sail that is becoming more common, and that’s the Parasailor. It’s essentially a symmetric spinnaker with a parafoil wing in the center. The wing provides some lift and also can spill the power from gusts. It’s a great idea, but we decided against this sail for two reasons: It’s more difficult to get the sock down over the parafoil wing in stronger winds; and we know of another 5X that tried the sail and found it didn’t work very well. The 5X has a lot of acceleration and it tended to collapse the sail too much when they sped up on waves (although it seems that could be solved by experimenting with the size of the sail).

Parasailor spinnaker

We don’t have the all the deck fittings and control lines needed to fly a symmetric spinnaker on Wildling, so now that the sail is ordered, the next project is to prepare the boat to be able to use it.