Twin keel, what's the deal?

Monday’s launch of USA-87 has raised a great deal of speculation on what’s going on below the waterline at BMW Oracle Racing. To the novice, the yacht looks extremely similar to her predecessors. However amongst the aware, heads are being scratched as clear signs are apparent, this yacht is quite different.

Underneath an AC yacht, what’s going on?
Below the waterline of a classical ACC yacht, there is one keel, and one rudder. Centrally attached to the hull, the 3 metre keel fin supports a 20 tonne torpedo shaped bulb. The rudder is another 2-3 metre fin about 10 metres further toward the stern (back) which turns to steer the yacht. In harmony, these two large fins give the yacht stability and the ability to sail in a straight line. More importantly, it allows the yacht to sail close angles toward the wind by providing ‘grip’ in the water. Until now, over 90 percent of ACC yachts have been built to this specification.

What is a tandem keel?
A normal keel sees the keel bulb supported by a single fixed fin in its centre that attaches to one point centrally in the hull. A ‘tandem’ keel uses two fins, one at each end of a long slim bulb that is connected to the hull at two points. Each keel blade is able to move independently on its own axis, allowing its attacking angle to the water to be adjusted. With this arrangement the yacht sails with no rudder, instead totally relying on the movement of the keel fins to steer.

Sails 20, 29, 31, 59, 78, 87?
The idea of a tandem keel is not new. In fact, in every AC event since 1992 tandem keel designs have actually made it into the water but not all competed in regattas. Oracle’s designer, Farr Yacht Design tasted some success with 1992 tandem keel design NZL20. The New Zealand Challenger made the LV Cup final, only to lose in a cliff hanger series against Italy’s Il Moro Di Venezia. The yacht was fast but difficult to sail, and only performed well in the hands of those who had time to truly understand her. Since that time other yachts trialled these concepts without success and most later changed back to classical arrangements. The current +39 challenger, (SUI)ITA59 started her life as a tandem keel yacht, only to be later purchased and converted by Alinghi to her current format. To this day, a tandem keel yacht has not competed in the America’s Cup Match.

General Benefits
A tandem keel can make a boat faster in a straight line, especially upwind. In essence, being able to adjust the angle of both fins independently reduces drag and increases efficiency. Both ‘lift’ and ‘wave’ drag are reduced meaning twin keels can do the job better than a more traditional single keel design. Other benefits include moving the yachts centre of gravity forward, which increases down force on the bow. Wave drag is reduced with the boat actually creating less wake (disturbance) in the water. It pays to note, the super long bulbs amongst the most successful of recent conventional yachts, are not that different to those used on tandem keel designs of the past. Past Tandem bulb trials may have already influenced modern bulb designs.

General Weaknesses
Historically, the main downfall tandem keel designs have been manoeuvrability issues. This is due to the removal of the rudder, and rather relying on two central fins with limited movement to direct the hull through the water. Once engaged in tactical duelling situations, these designs can be slow to regain their momentum after several manoeuvres in a row. This doesn’t mix well with AC Match racing, as an opponent could attack this as a weakness.

Tandem keels also have a larger wetted surface, meaning more resistance pushing through the water. Also eroding efficiency gain is ‘junction’ drag, which is where the actual fins attach to the hull and keel bulb. These moving parts create areas of inefficiency in their design. Overall, the design gains in a tandem keel have always proved small, and the challenges faced both in engineering and actual sailing have made it a risky choice.

USA87 – What's raising questions?
On first impressions, there are distinct design differences between USA76 and USA87.

Mast Position - One of the first things we notice, that mast is a long way forward! Some have estimated it at between 2 and 3 metres further forward in this yacht, in comparison to other designs.

Forestay Position - The forestay is the high strength cable that supports the mast from the bow (front) of the yacht, also supporting the Genoa sail. The distance from the bow that this connects into the hull, is far shorter on USA87, than her predecessors.

What is this saying?
When eliminating a rudder from the design, a great deal of ‘grip’ on the water is lost at the back of the yacht. The rudder cancels the ‘spin-out’ effect upwind, created by the mainsail putting pressure on the back end of the boat. A tandem keel requires the force in the sails to balance more centrally in the boat, so the designers must place the sail plan’s centre of effort further toward the bow. They do this by moving the mast, and complete sail plan forward on the yacht. This may go some way to explaining why the mast and forestay on USA87 are closer to the bow.

Use of Bowsprit?
By moving the yachts mast forward, a bowsprit may be needed to provide a better angle in controlling the yachts spinnaker pole. The tip of the spinnaker pole will also extend 2-3 metres further past the bow than a classic design. This bowsprit is legal, and compensates for the extra ‘overhang’ of the spinnaker pole forward. This ‘extension’ provides a strong point further forward than the yachts actual bow, in which to provide downward force on the spinnaker pole ‘tip’. This is required to correctly trim the sail, & safely manoeuvre the pole when turning or ‘jibing’ downwind. We shouldn’t be expecting to see any sails being flown directly from this point.

What’s likely to be beneath the skirts??
It’s hard to say considering other advances in AC technology. Rigs and sails continue to progress in their development, and this affects the design of the hull. USA76 was Bruce Farr’s interpretation of V4 of the AC rule. USA87 may simply show some new Version 5 thinking in how the mast, sails, keel and bulb are positioned on the hull. What we have seen doesn’t necessarily confirm a Tandem keel.

It’s obvious that Farr has been given a license to think by the BMW Oracle team. The frightening upwind advantage seen from Alinghi’s SUI75 last season may have heightened the sense of urgency to push design parameters further. This is a natural trial and error process always seen in the America’s Cup, and is what produces champion yachts. BMW Oracle will get a good initial feel for the performance of USA87 over the next few weeks, and use that experience to make a decision on her debut. This will not only depend on her initial performance data, but also on the launch of other competitor’s yachts. Already we’ve seen Luna Rossa's willingness to show their hand in the next Act, but the chance of BMW Oracle and others following suit is still to see. The question remains, if USA87 is a ‘rocket’, will Chris Dickson, CEO and skipper of BMW Oracle Racing, keep his cards close to his chest?

Patrick Carpenter. Valencia, 30 March 2006