Pete Melvin drops a little knowledge…
There have been many questions about some of the unique features of the NACRA 17 that my company, Morrelli & Melvin, designed in association with NACRA, and builders CMI. I thought I would shed some light on why our team decided to design and build this new class of catamaran, and some of the thinking that went into the concept.
The NACRA 17 was designed from scratch to meet the specific criteria set out by ISAF for the 2016 Olympic multihull. NACRA also produce a Formula 16 and a Formula 18 catamaran and we had initially considered entering these products, but with some changes in order to more closely match the ISAF requirements. Since ISAF’s specification did not require the design to conform to any existing class rule, we were considering adding performance-enhancing features like a carbon mast and curved daggerboards. The F16 class and F18 class rules do not allow either of these features.
After doing some further research into the ISAF requirements and what was possible within this design space, we determined that neither the F16 nor F18 platforms would be ideal.
The F16 is on the small end of the scale for the crew weight range specified (120 kg to 140 kg) and, in our opinion, would be more exciting and challenging to sail for Olympic-caliber sailors if it had a more powerful sailplan. The F18 typical crew weights exceed the range specified by ISAF. The F18 is also quite a heavy boat for its length and could be made lighter, but the hull volume and surface area would be needlessly large for a lighter Olympic spec F18 platform.
Since no existing design or class fit the ISAF specs, we decided to create an all-new design that is about 17 feet long, called the NACRA 17. Compared to an F16 class catamaran, it is 250mm longer, 100mm wider, has a taller mast and more sail area, and curved daggerboards.
Curved daggerboards are a relatively new technology that is quickly gaining acceptance in catamarans. They have been used mostly on offshore trimarans in the past, and used primarily as a way to increase longitudinal stability and shift underwater center of effort forward to prevent leeward helm downwind. These trimarans all have center-hull straight daggerboards for upwind sailing.
In the last few years curved foils have appeared on A Class catamarans, the new NACRA 20 and SL33 catamarans, and some custom and limited production catamarans. All of these designs successfully incorporate constant-curvature daggerboards, commonly referred to as “c foils”. The innovation with the curved foils on catamarans is that they are used to enhance performance for both upwind and downwind sailing.
Performance improvements with the c-foils have been well documented in the A class catamarans and c-foil equipped boats have won the World Championships for the last several years. Other foil geometries have been tested on the A Class cats but so far the c-foils have proven to provide the best all-around performance and are by far the most user-friendly and practical solution.
Lifting foils on catamarans are an area of intense development at the moment, especially with the next AC being contested in catamarans that allow lifting foils. I do not think anyone knows exactly what developments will prove to be best in the future. Each class of boat will evolve toward a foil geometry that is best suited to their specific rules and requirements.
For the NACRA 17 we chose c-foils for the following reasons:
- C foils have proven to make catamarans easier to sail, especially downwind.
- They provide lift which helps keep the bows up and reduces the tendency to pitchpole.
- The extra lift generated from the foil helps reduce hull wetted area and markedly reduces hull drag.
- There are other foil geometries that could provide efficient lift but all the ones we evaluated for the NACRA 17 had issues that we determined were not suitable for this type of boat. For instance, L foils are not able to be retracted from the top and would make launching and beaching problematic. We are also not convinced that L foils would be faster on this type of boat. They would certainly create more drag in light air, due to having more wetted area. The windward L board would also need to be lifted to reduce drag and unwanted vertical lift on the windward side, thereby making the boat much harder to tack. C foils can be left down on the windward side since they automatically reduce their vertical lift when on the windward side.
- C foils make the boat more exciting to sail. The ISAF Olympic Multihull Evaluation Committee report and individual sailor reports confirms this. There is an added dimension of control and speed that can be attained after gaining some experience sailing with them. We believe that this is an area that should be exploited with a modern racing catamaran and is why you see them on the NACRA 17, NACRA 20, and SL33 catamarans.
The hull shape of the NACRA 17 was designed specifically to work in harmony with the curved foils. You can add lifting foils to conventional catamaran hull shapes, but you will not realize the full benefit of the foils. This fact has been proven in the A Class catamarans and is apparent to those who are designing the next generation of Americas Cup catamarans.
The NACRA 17’s new-generation hulls have about 40% greater longitudinal stability than recent conventional shaped catamarans. This increased stability comes without the drag normally associated with creating high longitudinal stability in conventional shapes, and allows the sailors to push the boat much harder downwind.
The NACRA 17 was designed in late 2011 and two prototypes were built and tested prior to the ISAF Equipment Evaluation Trials in Santander, Spain March 17-25.
The Evaluation Panel Report went on to recommend the NACRA 17 as their top choice for the 2016 Olympic Multihull and stated in their opening paragraph “The clear preference of the MNA Sailors and the Evaluation Panel was the innovative new NACRA 17. Designed specifically for the Mixed Multihull criteria the Evaluation Panel concluded the NACRA 17 is seen as the best option. Featuring curved dagger boards providing vertical lift, the NACRA 17 will carry a wider –range of crew weight better than the 16 footers and is considerably lighter than a Formula 18. The modern NACRA 17 also offers the sailors in the Mixed Multihull Event the exciting challenge of mastering the potential lift of the curved daggerboards.”
Of the 18 sailors evaluating the multihulls at the trials, 14 chose the NACRA 17 as their top choice. The next runner-up, receiving four votes was the Viper F16. Nacra video right here.
Morrelli & Melvin Design & Engineering, Inc.