Sailing Corner - Design
By Tom Schnackenberg
Source: teamnewzealand.com

Each design team has much to think about before even embarking on the design programme, and it is worthwhile thinking about the rule from first principles, including the history of how we got where we are.

The America's Cup is a Challenge Cup and from its very inception serves to demonstrate the supremacy of a nation's marine design industry. You can imagine what a shock and embarrassment it was to the British when the upstart Americans, with whom they had been at war only 40 years previously, showed up at Cowes and cleaned up the fleet. The famous remark "There is no second, Ma'am", expresses the horror of the situation perfectly. It has moved a little away from that lately, with a lot of emphasis on sailing skills, but for many observers, it is still a design game and a chance for the marine industry to show its capabilities.

After the "Big Boat" challenge in 1988, there came the opportunity to develope a new class, and so a group of interested designers gathered informally over several months and the current America's Cup Class was borne.

The America's Cup Class Rule is intended:

(a) to produce wholesome day sailing monohulls of similar performance, while fostering design developments that will flow through to the mainstream of yachting; and

(b) for yachts that are raced "around the buoys" with tenders present, as opposed to off-shore in high wind and rough sea conditions with or without tenders.

This rule was established over a period of time during 1989 and 1990, with an informal group of designers sharing ideas, systemising the key parameters, and establishing formulae. Boat design rules are notorious for "type-casting" boats, and key features of the America's Cup Class Rule were designed to make the Class aesthetically pleasing, and to avoid the many distortions which may arise when a limited number of hull measurements are made.

As well, there were and are several rules designed to control the cost of ACC yachts and to avoid a "space race" in ancilliary technologies. For example, the number of moveable appendages is limited to two, and surface finishes are limited by the words "No coating or substance (.....) may be applied to the outside of the hull or appendage surface except for polyurethane, epoxy paint or commonly available paint"

A Velocity Prediction Program (VPP) was used to predict the likely effects of length, sail area, and displacement, and the result was a formula which was expected to rate a range of boat dimensions fairly. The formula was designed to allow boats weighing from 16 to 25 tonnes to race competitively, and is summed up in the following equation:

L + 1.25 x (S)^(1/2) - 9.8 x (DSP)^(1/3) ------------------------------------------------ <= 24.000 metres 0.679

L is the rated length in metres. S is the sail area, the sum of the foretriangle area and the mainsail area. DSP is the displacement in cubic metres.

The formula works to create a trade-off among the speed-gain factors of length and sail area, and the speed-loss effect of displacement. The rule-makers spent a great deal of time on the constants in this equation, hoping that the rule would accommodate a variety of dimensions and thus allow quite different boats to race under roughly equal terms.

They were aware that time might prove that these factors were incorrect, and so arranged for "boundaries" so that boats would not become extreme. There are penalties at each end of a designed range in sail area and length, and the weight is effectively limited to a range from 16 to 25 tonnes. The draft has a maximum of 4m, while the beam has a maximum of 5.5m. The boats these days are all well under the maximum beam. As you make the boat narrower, it loses both friction drag and wave drag. Less friction and less drag are good features, but they are offset by the fact that a narrow boat loses stability and heels more, which means you have a little less driving force available.

One feature which is still "under development" is the bow shape. Under the rule, you can have remarkably different bow shapes with the same rating, and quite similar performance. Because there are still plenty of variations possible, it would be a brave person who said the problem was already solved. Most designs in the current fleet seem to be at the upper end of the weight range. This comes about because when you increase the weight, the rule knows that the boat is becoming slower, and allows you to compensate with more length and sail area. However, with additional weight you are also gaining stability, because almost all the extra weight goes into the keel bulb, nearly 4m below the surface.

After a lot of testing, people have found that the heavier boats turn out to be faster under the rule. The rule seems to have overdone the compensation for increased weight! In a situation where there are few, if any, easy gains, everybody will grab any advantage that is available, hence the tendency for the boats to be similar in their key dimensions.

From a designer's perspective, it would be interesting to explore the possibility of changing the basic formula for future contests to bring more variety back into the competition. Whether this would be an improvement to the America's Cup is a moot point ­there is plenty of expensive uncertainty already. Perhaps, after all, there is a good argument that having the boats settled into a corner of the Rule is just fine for the future strength of the event.

Enough about the rule and the key parameters. How do we design the fastest boat? The overall challenge is to define and then explore the design options as thoroughly as time and budget allow, and then to decide on which design best fulfils the ideals of good performance and good adaptability. As much as possible, one tries to break down the problem, or the questions, into bite-sized chunks, and also consider the methods in a similar isolated fashion. The questions are myriad and can very quickly become too complex for simple analysis, especially as many of them are inter-related. Answering one question in a certain way has implications that ripple through all the other questions.

Most syndicates have a limited budget and so one of the very first questions is whom shall we get to help answer these questions? You need a MIX of designers, ideas people, scientists, and engineers, and you also need to consider how the design team relates to the sailing team. Different syndicates establish different hierarchies. In Team New Zealand, for instance, the model we pursue is to put the sailing team at the top of the pyramid, as much as possible. There are many other situations where the sailing team might defer to the design team, so some syndicates reverse the order and put the designer on top. We tend to answer the design questions at the very last minute before the decisions need to be made. The hulls are built first and are among the hardest things to change, so people start there and spend a disproportionate amount of effort on hull design. Rigs, keels and rudders come next, and sails come last. That being said, everything interacts and has a bearing on performance, so we definitely think about sails when we are deciding on hull dimensions and rig position for instance.

The rule allows you to trade-off upwind and downwind speed, and also light and heavy air performance. This brings in the subject of weather. As you can imagine, we spend quite a lot of time studying the environment, so that we can target the correct wind range and build a suitable boat for the contest. In San Diego, settled predictable weather conditions made this part of the task relatively straightforward. There was a high degree of certainty that the boats would be racing in a narrow wind band between 7-11 knots. Auckland typically has much more varied weather conditions, posing a real headache for design teams, as the boats may have to race in anything from 7 knots to 25 knots.

Once we have settled on the hull shape and size, we consider the construction techniques. The challenge is to produce hull structures that are extremely light, but extremely strong and reliable! Great effort goes into stripping weight out of the hull structure, because every gram of weight that is saved in this area can be added to the bulb, increasing the stability of the yacht and hence the upwind performance. In similar fashion a lot of attention is paid to the deck layout and selection of racing fittings. Here again there is a trade-off between "race-ability" and weight saving. The former helps us to shift gears and turn corners, while weight saving translates into stability and power.

As soon as the hull and deck structures are decided, the focus shifts to fins, bulbs, rudders and wings, collectively referred to as appendages. Things are decided in concert; structural arguments and drag calculations go hand in hand. For the bulbs, we start with the imaginary shape with the lowest drag, and then consider distorting the shape a little to lower the centre of gravity, without increasing the drag too much. Hours are spent calculating the changes in drag that come from different shapes and trading this off against the gains made by increasing the stability.

The fin and rudder designs are equally contentious. Often we make trade-offs between pure drag minimisation arguments and others which are more practical, such as handling qualities, and general robustness of design.

The rigs are there to hold up the sails, and no rig meeting is complete without someone to speak for the sail designers and trimmers. As well as desiring a rig which is down to the minimum allowed weight with the minimum aerodynamic drag and good bend considerations, we are looking to support the sails in the most effective fashion.

There are a few structural rules that come into rigs and rigging, and we also have to comply with minimum weight and minimum centre of gravity (CG) requirements. Because of this, every mast and rigging discussion considers the effects of design changes on weight and CG.

Rig and appendage design are both much more associated with engineering considerations than hulls, for instance, and at the meetings, a lot of attention is paid to the scientists in the group, who come armed with calculations that show the effect of various changes on the overall behaviour of the rig.

Seemingly all the teams are employing developments of the millennium rig which we introduced during the last America's Cup.

In this, the diagonals run from the ends of the spreader up and down into the middle of the adjacent mast panels rather than to the base of the next higher spreader, which was conventional arrangement.

When we consider sails, and sail design, we have a large interest group available to offer opinions and sometimes expertise. There are the sail designers and sailmakers, who report on choices, design techniques, new materials, and so on. And there are the trimmers, who feed back the results of testing on the water, past experience, and so on. As well as this, the helmsmen often have opinions and expertise of their own.

The decisions as to which sails to make and how they are to be designed are made in committee and this is one of the areas where the committee approach is most effective. Sail decisions are being made continually, and you might say that the final decisions are actually being made by the trimmers on the water.

Once again, the task is to create foil shapes that will promote the most efficient flow of air across the sail surfaces, which, in turn, translates into forward drive. Sail design and construction remains an area where significant speed advantages can be achieved ­ and it is an area where research and development continues right through the campaign.

At the end of the day, America's Cup design is a complex exercise, conducted across a wide range of fronts, all dedicated to a simple premise: Make the boat go faster. If only the solution was as straightforward as the proposition.

Copyright 2002, teamnewzealand.com

[Note: This article is reprinted with permission from Team New Zealand and teamnewzealand.com]