SailNet Community - View Single Post - Asymmetrical vs. Symmetrical Spinnakers
View Single Post
post #1 of Old 11-15-2000 Thread Starter
Dobbs Davis
Contributing Authors
Join Date: Jan 2000
Posts: 30
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 0
Asymmetrical vs. Symmetrical Spinnakers

When the wind gets light, some Mumm 30 sailors opt for the asymmetrical option off the wind.
The recent Mumm 30 World Championship regatta attracted 40 teams from seven countries to Miami Beach, FL, for four days of intense competition. In nine races, conditions varied from as little as seven to as much as 20 knots, with seas ranging from lumpy to flat. This variety of wind and wave conditions challenged all competitors to shift gears in their rig tune and sail selection in order to remain fast.

Accordingly, one of the hot topics of discussion at the regatta was the use of class-legal asymmetrical spinnakers. The Mumm 30s are relatively unique among the offshore one-design classes in allowing both symmetrical and asymmetrical spinnakers in either masthead or non-masthead configuration. Class rules specify the dimensional limits of these sails, both in the length of the luff, leech, and girth, and in total sail area. This gives sail designers parameters within which to find a solution for the critical trade-off between a sail's size and speed—large sails generally prove faster, except when the air gets very light and then smaller sails are usually better.

Asymmetrical spinnakers—commonly referred to as A-sails—have received tremendous attention over the last few years as wind-tunnel tests and events like the Whitbread and America's Cup have proven their use to be faster than conventional symmetrical spinnakers in many conditions. In fact, most easily-driven offshore yachts, such as Volvo 60s and turbo-sleds, only have A-sail spinnakers in their off-the-wind inventories. Because they’re also fast, easily-driven boats off the wind, Mumm 30s would also seem ripe for A-sail use.

The boat with the A-sail in the foreground chases those with symmetrical spinnakers.
While it's generally accepted that in light air at low true-wind angles, A-sails are faster, some sailors think the wind range may be quite larger. According to Quantum Sail Design Group's Dave Flynn: "Given these parameters, one could definitely design and build a VMG (velocity made good), asymmetrical that would in all probability test slightly faster than a symmetrical sail. Experimental testing and empirical observations in other classes would bear this out. The girth and foot restrictions allow a small enough and flat enough sail for these angles. This would likely be the case at true-wind angles as wide as 160 degrees."

Class rules for the Mumm 30, however, also specify that each competitor must declare before the regatta which sails he or she will use, forcing those with A-sails to effectively put their money where their mouth is by leaving their masthead symmetrical kites at the dock. (While class rules allow fractional spinnakers, it's acknowledged that the breeze must be really howling—25 knots or more—for their use to be competitive on windward-leeward courses.)

"If A-sails are so fast, why don't more sailors use them?"
So, the question remains, if A-sails are so fast, why don’t more sailors use them? The answer is simple: tactics. Because A-sails require somewhat tighter apparent wind angles, and it takes more mechanical coordination for a crew to pull off a good jibe with an A-sail, tacticians in big-fleet competitions ordinarily feel nervous about making the commitment to using just these sails. Having great boatspeed will help create gains downwind, but you must be able to jibe fast and well to keep those gains once you arrive at the leeward mark. Sometimes the trade-off just seems too risky.

Transferring the luff load to the tack line is the first step in successfully jibing the A-sail aboard a Mumm 30.
And then there's the fact that jibing an A-sail that is set on a conventional spinnaker—
as is the arrangement aboard Mumm30s—requires a particular technique. The essential elements of a successful A-sail jibe are as follows: first, the load from the afterguy must be transferred to the tack line by easing the guy as the tack line is tensioned. This eases the pole forward, lessening the angle of attack of the sail to the wind, so the sail produces less power in this mode. As the helmsman bears off into the turn, the trimmer eases the sheet out quickly, the spinnaker collapses as it rotates, and the trimmer then reels in the new sheet as quickly as possible. The latter is the most critical part of the jibe.

According to Jodi Davis, the bow person aboard Phil Garland and Rich Shulman's Mumm 30 Trouble, the North American Champion: "We also get someone to run aft with the clew of the sail on the new side, bringing as far back as the shrouds so that the sail snaps through the rotation. This allows the sail to fill on the new jibe faster." During this stage, Jodi releases the inboard end of the pole from the mast, and then outboard end from the old afterguy, and rotates the pole end-for-end, attaches the new afterguy, and pushes the pole forward before re-attaching the inboard end to the mast. All that time, the A-sail luff load is supported by the tack line, which is never looser than a foot or so off the bow. The tighter the tack line, the easier it is for the sail to rotate and avoid twisting into an hourglass.

Once on the new jibe, the tack line is eased as the afterguy is brought back to take the sail and pole to the new position. The topping lift and foreguy are never moved (except when needed to move the afterguy), and the pole remains about half as high off the deck as it would if it were supporting a symmetrical spinnaker. A variation on the end-for-end pole rotation is that used on larger boats where the pole is detached from the mast, shoved aft, the afterguys are switched, and then the pole is shoved forward and re-attached to the mast. This is particularly important when the pole length is long relative to the foredeck, as it is aboard most turbo-sleds, Volvo 60s, and America’s Cup boats.

Jibing an A-sail aboard a boat with an oversized pole—like the Mumm 30—takes practice.

While setting an A-sail isn’t significantly different from setting a symmetrical spinnaker, the takedowns require a little more coordination since there is no lazy guy on the clew to help the crew gather the sail. While a few variations are possible, in the Mumms, the best technique involves easing the sail to the pole while the tack line is tensioned, and their overtrimming the sheet so that the foot can be grabbed, and the halyard dropped as the sail is gathered under the jib into the boat. The so-called "Mexican" takedown is a common variation where the pole is removed first, the boat jibed, and the sail is then gathered on the bow as it backs into the jib as the boat turned upwind. In both cases, fast hands are needed to get the sail into the boat before heading upwind.

So, while there are many considerations involved in evaluating the efficiency of using A-sails, "it comes down to mechanics and possible tactical considerations in a large one-design fleet," says Flynn. He also points out that there can be other negative factors, such as "intangibles like the crew's psychology which might be overcome with better jibing and takedown techniques."

As it turned out, in Miami Beach Vincenzo Onorato's winning team aboard Mascalzone Latino never used an A-sail at the Worlds, and only two of the top-10 finishers –Trouble in seventh and Steadfast in eighth—used A-sails at all. Nonetheless, with the promise of better speed as an incentive, look for further developments to come with these sailsin the Mumm 30 Class and wherever else their use is allowed.

Suggested Reading List

  1. Using the Asymmetrical Spinnaker by Brian Hancock
  2. Spinnaker Fundamentals by Steve Colgate
  3. Spinnaker Trim for Performance by Rich Bowen
  4. SailNet Buying Guide - Spinnaker Poles

Dobbs Davis is offline  
For the best viewing experience please update your browser to Google Chrome