pin-to-pin measurement. 112% would be for a boat like a J/120 where projection is important. The longer luff helps to increase projection when sailing at the deeper angles. Tweakers are another tool that
can be used to improve your sail’s performance. As the sheet is eased the active tweaker should be tensioned, effectively moving the sheet lead forward, this controls twist and allows the designed depth to be maintained, so that the head doesn’t flip open as the sail is eased. All these contributing factors that
make displacement boats’ VMG quick downwind are your enemies when sailing a performance boat offshore in waves. Unlike displacement boats, performance boats generate tremendous amounts of apparent wind.With the apparent wind angle rarely aft of 110 degrees, the helmsperson will drive down to surf waves then turn up to regain speed and height. Displacement boats alsomay surf down a wave moving the apparent wind forward but when they put the bow up to dig out of the trough, they won’t respond as quickly. Also, the apparent wind shifts relativelymore than on a performance boat. Offshore and coastal running sails
generally have the same depth, but what differs would be the luff length defining the luff round, and the overall projection of the luff. This is achieved by designing the sail’s optimal flying shape and then engineering the panel layout to handle the loads and maintain stability. Sailing in waves you “drive” the
boatmore aggressivelymoving the apparent windmore dynamically. If conditions allow the boat to be stable, it’s great to have a super long luff; but when the boat starts tomove around under the sail, oscillation increases as do the chances of collapse or wrapping the sail around the forestay. There is also the physical limitation on how long the crew can keep up aggressively trimming the sheet in a distance race.
Above: the seven key sail measurement points for an asymmetric spinnaker. It’s crucial to understand how these parameters affect a sail’s performance when asking a sailmaker to design a new A-sail. Right: an asymmetric spinnaker rotates as the sheet is eased. A wider SMG (mid-girth measurement) allows it to rotate further in front and to windward, which is ideal for inshore racing in a displacement boat. Sails for offshore use and for fast planing boats tend to have a narrower SMG which is more efficient at tighter wind angles
Because of the relatively smaller
shifts in apparent wind angle in performance boats, their offshore spinnakers are cut flatter with a tighter luff. This design will bemore resistant to collapsing froma serpentine steering pattern through waves. By adjusting the tackline you havemore range with this sail.
Mid-girth measurement (SMG) A big difference between one-design or coastal and offshore spinnakers is themid-girthmeasurement (SMG). Offshore, where you typically sail at higher wind angles, your asym’s SMG is likely to be narrower depending on whether it is an A1, A3 or A5. Coastal or one-design spinnakers, usedmore for VMG running (A2 and A4) will have a wider SMG, as you’re looking to soak deeper. This wider SMG design will allow the sail to rotate out in front and to windward further in the VMG soakmode.
Sheeting position/leech length The leech in an offshore asymmetric isn’tmuch different than a coastal or one-design sail, yet it’s still critical to get the clew height and sheeting angle right. The amount of fullness in the back end comes down to the type of spinnaker it is. Reachers are flatter in their aft sections so that you don’t get overpowered as quickly. Runners’ leech sections tend to be rounder, generatingmore power. Pat Considine continues: ‘With
our computer design software it’s very easy for us to change the depth of the sail but it’s important that you understand the corner-to-corner loads. Someonemay be looking to save time andmoney by designing an asymwith fewer panels but that’s no good because of the bias loads going across the panels. Designing a sail to havemore panels that are narrower will result in a structurally stronger
sail.We orient panels so the warp and fill yarns reflect the load paths of the sail.We also specify heavier fabric with narrower panels in luff, wheremost of the load is.’
Tweaking the leech shape As the true wind angles being sailed change, you will want to control the leech shape and sheeting angle of the sail. Attaching a tweaker to the sheet allows you to raise or lower the sheeting angle and optimize the twist and exit angle of the sail. Geoff Bishop notes: ‘With our
running sails, if you’re in eight knots with a 150-degree target angle, we’d put on a little tweaker. But if we had to head up to 140 degrees to gain speed, we’d release the tweaker. We constantly play the tweaker up and down to optimise leech shape and control twist.’ Overtightening the tweaker will
close the leech and prevent it from exhausting properly. To sail deeper angles, ease the sheet so the sail rotates forward and tension the tweaker to control twist.
Summary Taking the time to understand the boat and themanner of sailing it will do is critical to designing and building an asymmetrical spinnaker that will help you win races. After that, it’s up to you to properly adjust and set the spinnaker so it willmatch the design’s optimal flying shape. If you are looking for a new
asymmetrical spinnaker for your boat, don’t just take what you’re given. Ask questions about the key measurement points of the sail, understand the performance differences of various options and then practice to sail it effectively. If you do this, you’ll be able to sail with confidence.
www.uksailmakers.com
❑ SEAHORSE 67
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