Technology
Unravelling the mystique
A-sail design principles are oftenmisunderstood. Top sailmakers explain… A running sail could have a 105%
To explain the thinking that goes into designing asymmetrical spinnakers, we interviewed three sail designers from UK Sailmakers: lead designer Pat Considine, UK Sailmakers Chicago; Geoff Bishop, UK Sailmakers Fremantle, and Stuart Dahlgren, UK Sailmakers Northwest - Sidney, BC. Their thoughts on spinnaker design fell into two buckets: 1. The boat itself Type of boat - high-performance, displacement, one-design Exact measurements - pin-to-pin
halyard length, sheeting position, etc. 2. The type of sailing to be done – IRC, ORC, offshore, coastal, one- design, professional vs. Corinthian.
Key data points When designing an asymmetrical spinnaker, a designer will focus on three primary data points: 1. The point-to-point distance between where the spinnaker will be tacked (on a sprit or a lowered pole) and the max height for the halyard, 2. The mid-girth (SMG) luff to leech measurement, expressed as a percentage of the foot length, and 3. The trim position for the clew in terms of height off the deck and where the sheet will lead. Pat Considine explains: ‘A
symmetrical spinnaker is probably the easiest sail to design. It has no twist and both sides are the same. An asymmetrical spinnaker on the other hand is challenging to design
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because the luff is free flying, there’s twist in the leech and there are many other things to consider.’ Our sailmaker panel agreed that
once the boat characteristics and racing plans are known, additional pre-design considerations include: The performance characteristics
of the boat such as polars, reaching/ running angles and so on The other spinnakers in the
inventory: can this new sail have a narrow range or should it be all-purpose? Rating system to be sailed, optimisation for IRC, ORC, etc Budget: Yes, there is less
expensive nylon and more expensive nylon. With spinnakers you get what you pay for The skill of the owner: experienced
sailors (helm and trimmers) will not need as forgiving a spinnaker as less experienced sailors. Geoff Bishop adds: ‘Once you get
a good sail design and you know the boat’s measurements, you pretty much stick with it. That said, the critical thing for me is getting the luff length and the mid-girth ratio right. ‘Start with the straight-line measurement from the tack to the max hoist position (straight line), the tack point to the sheeting position, plus the sheeting position to the max hoist position. It’s critical to get those measurements right because every percent of difference in mid-girth in relation to the foot and luff length changes performance and ratings.’
Above: there are a few key differences between Asymmetric spinnakers designed for offshore racing and those that are optimised for windward- leeward races inshore. One of these is the mid-girth measurement which tends to be shorter for offshore sails because they are often used at tighter wind angles
mid-girth (SMG) of the foot length (SF) and a luff length longer than the straight-line measurement. Conversely, a reaching sail will have a smaller SMG in relation to the foot length as well as a shorter luff.
Luff length The luff length helps to determine the projection of the sail. For windward/leeward racing, mostly in displacement boats, the goal is to soak for VMG. For that you want a luff that is longer than the pin-to-pin measurement; this creates positive luff curve allowing the sail to project to windward further so you can sail deeper on those shorter round-the- buoys races. If a spinnaker luff keeps collapsing when properly trimmed, the entry is probably too flat. One of your tools is the tackline: if it has been eased to allowmore projection, pull it back down to help round the entry, making it easier to trim. To design a good A-sail, the entry
must be deep enough to support the luff round and allow the luff to curl without collapsing. Consider a heavy J1 jib. To sail that J1 upwind in a groove, you need curvature in the luff so the telltales work. Reachers (A1, A3, A5) will be designed flatter and straighter in the aft section to avoid the sail getting overpowered too quickly. Runners (A2, A4) tend to have deeper, rounder aft sections. For running asyms, the luff length can range from 105-112% of the
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