Open 60 support veteran reports on design trends

To the editor: For the past few years I have been working on various Open 60 race campaigns, such as the Vendée Globe, and I thought voyaging sailors would find some of the aspects of these powerful race boats interesting with possible future applications to voyaging boats. Three intriguing elements of Open 60s are their canting keels, hull forms and cassette rudders.

Canting keels are a remarkably simple idea. You cant the keel to windward, and the downward force of the bulb adds massive righting moment to the boat. With a canting keel, it’s no longer necessary to have excessively beamy boats that carry the additional weight of water ballast — in some cases as much as 3 tons. Instead, more moderate hull shapes can be designed while still retaining the same powerful righting moment. The reduced beam means the boat will sail better upwind while retaining a flat planing surface for downwind sailing.

Canting keels do give the boats stability, but when the keel is canted to windward by as much as 45°, there is no keel fin on the centerline to produce lift and resist the side forces associated with leeway. This makes it very difficult for the boats to sail efficiently to windward. The day of the appendage has arrived.

Until canting keels came along, daggerboards and other appendages were an afterthought, just small protuberances that often as not slowed the boat down rather than increasing speed. With the first canting keels, daggerboards on centerline were added, but like the single rudders of old, they are not very efficient. Then twin asymmetric daggerboards were installed. The daggerboards, like twin rudders, are toed outboard so when the boat is heeled, the leeward daggerboard is close to vertical and therefore a very efficient foil shape for going to windward — even more than a fixed keel, since a fixed fin would ordinarily be at a 15° angle of attack when sailing to windward.

Slowly, almost incrementally, the size of the boards increased. The daggerboards on the latest generation of Open boats are nearly 13 feet deep. Board positions have moved aft, and in some cases — the new boats from designer Marc Lombard being good examples — they are now heavily raked aft. Raking the boards aft allows the center of lateral resistance of the hull to be moved fore and aft, according to how far down the board is.

A new Open 60 from the design office of Bruce Farr has the daggerboards well aft, in line with the mast and keel. They are straight up and down (when the boat’s at rest) with no toe out angle. By having the boards vertical, the designer is trying to maximize lifting force to give the boat a bow-up attitude when sailing upwind.

The refinements may be small, but the improvement in performance and handling is immeasurable. Twin asymmetric boards also have a safety advantage. Although a daunting task, if one board is sheared off in a collision, the other board can be inverted and put in its place, as Ellen MacArthur was able to do in the 2000-01 Vendée Globe.

With hull shapes stabilized around an average beam of 18 feet and emphasis placed squarely on the underwater appendages, designers turned their attention to safety in the form of cabin-top design, and weight savings by trimming unnecessary structure from the hull. By rule, all Open 60s have to be able to be righted by the skipper while inside the upturned hull. This is usually done by canting the keel to one side, but in some cases, especially where the wide decks are flat or even concave, the surface tension keeps the boats upside down. Large, rounded cabins add sufficient buoyancy to an upturned hull to destabilize it and allow the boat to come upright.

Designers also began to take a closer look at the area where the hull and deck meet, realizing it was wasted space. By rounding the deck edge, they were able to trim weight from the hull. Angling the edges of the cabin and deck structures, like on the new Lombard designs, not only saved weight, but also added some structural rigidity to the hull.

The most recent striking changes in rudder and hull design once again originate from the French and are found on the newest Lombard boats. A distinct but shallow chine runs from the aft quarter at the transom, forward, fading into the hull at about the same place where the daggerboards exit. The idea behind this chine is to reduce drag and optimize wetted surface area while improving lateral resistance on the hull. The chines should result in less wetted area, but the same hull-form power when the boat is heeled. Also, the improved lateral resistance relieves load on the autopilot, a very important feature on Open boats.

Another fundamental difference between the new and old generation boats is the rudders. In the interest of safety, several of the new boats have transom-hung kick-up rudders. These rudders fit into a carbon cassette that is hung on the transom via a simple pintle-and-gudgeon system. The base of the cassette, where the blade exits, is fitted with a fuse. This is a stainless-steel pin led into the cassette housing. The pin will eject from its seat in the cassette when a certain load is applied. If the rudder strikes an object and the impact force exceeds the load limit of the fuse pin, the rudder swings up. After the collision, if the rudder is not damaged, it can be pivoted back down into position and the fuse reinserted.

Some designers feel transom-hung rudders are not as efficient as a traditional spade rudder that has its top positioned under the hull to provide a plate effect and prevent ventilation. Other designers think ventilation of the top of the rudder can be advantageous under some conditions, such as power reaching. Surely the advantage of a kick-up rudder system on a long race, where the windward rudder can be lifted clear out of the water to prevent any drag and reduce the chance of collision, outweighs any slight effects to performance.

– Brian Harris was the owner and operator of his own boatyard. For the past seven years he has been living in Europe involved in the project management and campaigning of a wide variety of offshore and around-the-world racing yachts.

By Ocean Navigator