Much of the FD's well known power and upwind speed in particular is rightfully attributed to its generous sail area in relation to the boat's weight. Fairly unique in the world of high performance dinghies is the FD's use of an overlapping genoa. This feature provides special challenges for the sail designer in trying to achieve optimal shapes. The effective range of sail designs is of course broadened by the adjustability of the rig, but let's take a look at some special features.
In the latter stages of the Olympic status of the class, some fairly standardized shapes were developed. The Diamond DMU-9 main was developed as a powerful, relatively deep sail. This was in the days when crews could wear water weight on their shoulders during racing. Fortunately for the long term health of competitors, wearing weight has been outlawed. As a consequence, designers have taken other looks and generally determined that a somewhat flatter overall shape was faster for today's lighter crews. At the end of the day all sails attempt to achieve the best lift to drag ratio possible. Carbon spars have been legal for some time now, which takes weight out of the rig and allows even more variables to manipulate to achieve a better bend and tip dynamic.
Compared to other boats, and not just other dinghies, the FD has a fairly wide sheeting angle for the genoa. Before fully adjustable sheet leads were fully developed, FD sail designers would induce more curvature into the leech area (often called leech return) in an effort to add power and "fool" the boat / sail plan that the sheeting angle was narrower than it really was. Because of this feature of the boat, most modern FD's now rely on fully adjustable genoa leads. These are moveable in both a vertical and horizontal axis with controls easily manipulated by the helms man.
Even with these controls, there have been several designs that hope to maximize performance in a certain wind range since no one design can be optimal in every condition. The trick is to provide enough overlap in these designs so that you do not find yourself too far out of range on the race course. Many choose to opt out of that choice by going with an all purpose design. At the loft I work in, we have three different genoa designs, all intended to maximize performance in a certain wind range.
There is a light to medium air sail that has significant leech return, overall depth and power. This sail is excellent in breeze 4 to 11 or so knots, and is especially effective in that range with any chop. The next step up for many is an all purpose design that has the widest effective range of the sail options. This approach has many advantages, in that the AP design is never far off in performance terms from the more specialized sails, and the sail choice decision before racing is far easier! This sail features a bit less depth and leech return low and is noticeably flatter up top. The idea is that when it is time to rake and move the sheet attachment to one of the higher leech grommets, there is a natural flattening of the sail shape.
We also build a heavy air design that is flatter than the two previous designs mentioned, with straighter leech exits.
One way to look at the differences between these designs is to imagine what happens to airplanes when they alter the amount of flaps on their wings. When the pilots of planes want the maximum amount of lifting force, you'll see those flaps down, and when there is enough airflow across the control surfaces to provide the level of lift they want, they are retracted to reduce drag. So in an incredibly general way, equate flaps with the amount of leech return and depth in a sail. Both planes and boats are trying to maximize the lift to drag ratio of their lifting surfaces.
The FD started out life with a very small by today's standards spinnaker. In the mid 1990's the class voted to enlarge the sail, which was a significant improvement in horsepower. Modern sail cloth engineering has naturally progressed over the years, and Nylon cloth is no exception. Spinnakers can be built out of lighter cloth and hold their shape to a much higher degree than in years past. That is good because it is a maxim that all other things being equal, lighter spinnakers are faster.
One of the real world factors in the life of our spinnakers is they be tough enough to survive the trauma of being drug in and out of the launcher tube. Sailors often coat their sails with friction reducing compounds like Holmenkol or McLube to make that task easier on the sail and helmsman.
Interestingly in the case of the FD, building to the absolute size limits of the class rules does not result in a very fast or efficient shape. The best sails have maximum length leeches, but smooth vertical distributions that allow easy attachment of air across the sail when reaching. There is art and science in all the sail development.
In future articles, we'll explore how the various rig controls effect the shaping of sails, and how best to manipulate them in the FD.
Discussion of the overall sail plan and design idiosyncrasies. Then individual discussion of each sail, different manufacturer approaches, pro/con, rating method, wind range, etc. construction options.