Secrets behind Wild Oats XI's DSS foil

Revolutionary foil helped Bob Oatley's maxi to seventh Hobart race line honours victory

Part of the success of Bob Oatley’s Wild Oats XI in securing a record-levelling seventh line honours victory in this year's Rolex Sydney Hobart can be attributed to the Dynamic Stability Systems foil fitted to the Australia 100ft maxi this season.

We have reported on the DSS foil in the past (see here, here and here). To recap, the foil runs laterally across the boat and when required can be deployed to leeward beneath the waterline. The vertical lift this provides, which increases with the speed of the yacht, dramatically improves righting moment, an effect similar to having many more crew on the weather rail. This in turn translates into the boat having more power, less drag and therefore improved performance.

On Wild Oats XI, the DSS foil protrudes from the leeward side of the hull by just over 2m and the foil provides eight to ten tonnes of vertical lift. The added righting moment the foil provided in the Rolex Sydney Hobart was also vital in enabling Wild Oats XI, with her narrow beam of 5.2m, to compete with the newer and more powerful Perpetual Loyal, which has a beam of 7.4m, in bigger conditions and points of sail when righting moment was a key factor.

In the build-up to and aftermath of this year's race, the DSS foil received many endorsements: “The trial results were quite remarkable. There was an impressive increase in speed,” said Wild Oats XI skipper Mark Richards prior to the Boxing Day start. Following the race he enthused how they hit 35 knots boat speed at one stage. “The new foils were phenomenal,' he said. “They gave us a lot more control in the extreme conditions, and the hard running we did today.”

Chris Links, one of Wild Oats XI’s helmsmen, said of the DSS foil before the race: “It gives us some more stability in those tighter angles and in the heavy downwind it lifts the bow 300mm. In those steep little seas it helps get us on to the waves and surf for longer. We have seen it really work a couple of times, which has really blown us away when it clicks in.”

Arriving in Hobart, Wild Oats XI's wizened tactician Iain Murray attributed a 5% gain in Wild Oats XI’s speed to the DSS foil, which had been put to good work as the wind built to more than 30 knots coming down the east coast of Tasmania and continued to build as they hardened up to cross the aptly named Storm Bay.

In fact according to the DSS’s creator Hugh Welbourn, Wild Oats XI’s retro-fitted foil can be considered a ‘DSS-lite’, and therefore only achieves a relatively small percentage increase in performance compared to a design when the DSS foil is integral from the outset. The DSS foil on Wild Oats XI has a smaller area and is shorter than would normally be fitted to a 100ft maxi, such as the Welbourn-penned Infiniti 100R, being marketed by Monaco-based broker Yachtzoo. The Oats foil is also straight, whereas DSS foils normally have some curvature to them that raises the tip of the foil when it is deployed, creating yet more lift.

Rather than gaining righting moment from traditional features such as keel bulb weight or draft or waterline beam, the Welbourn-designed Infiniti Yacht range are typically light and slender for their length, deriving a greater portion of their righting moment from the DSS foil. The Infiniti 100R for example weighs just over 20 tonnes making her the world’s lightest 100ft offshore racing monohull, capable of covering more than 700 miles/day.

“It is great to see Wild Oats XI perform so well and this gives us even greater confidence that the Infiniti 100R, which is lighter and has much more optimised DSS foils, will perform as we expect and will deliver our clients something truly spectacular and unique,” said Infiniti Yachts’ Gordon Kay.

Chris Richardson from Yachtzoo added: “We at Yachtzoo are delighted to see DSS leading the field in a top quality event and are proud to be the exclusive sales team for Infiniti Yachts.”

DSS creator Hugh Welbourn said: “This race has been a great demonstration of the effectiveness of DSS even when not allied with a purpose-designed hull and rig. However even when the configuration is less than ideal, it’s been good to see the dramatic benefits being well repaid in this victory for Wild Oats XI against a quality fleet, and reinforcing the belief we have in our technology and its application to varying design configurations.”