Dynamic Stability System - a new superyacht solution

21 January 2015 • Written by Marilyn Mower
The horizontal foil dampens motion and reduces leeway and provides lift to allows more sail to be carried further up the range

‘Revolution’ is perhaps an overused word. Not everything is equivalent to the overthrow of an existing regime. But the Dynamic Stability System (DSS) developed by naval architect Hugh Welbourn, business partner Gordon Kay, and engineer Will Brooks probably is.

These fellows are such good sailors that if they didn’t have degrees and day jobs, they could be making a living out of it. Welbourn is a foil and ratings specialist and has ‘optimised’ and raced some of the most interesting boats in the world, from Olympic class dinghies to maxis and Swans. Brooks, a structural engineer to five America’s Cup campaigns, is the half of the technical team that has to make it all work.

Welbourn Design was an early adopter of velocity prediction programs (VPP) and computational fluid dynamics (CFD) to study the interactions of foils with water and the effects of foil types on motion. Sailors of fast boats know all about motion and the fatigue it puts on both structures and crew. Existing keel designs have had a tremendous impact on speed, says Welbourn, but do nothing for pitching, yawing and vertical acceleration. Looking for ways to reduce motion without reducing speed, in 1997 he began running computer analysis of the lift-to-drag ratios of horizontal retracting foils.

The DSS system extends a foil to the side of the yacht to help it stabilise.

While other engineers were putting effort into canting keels or extremely deep bulb keels, Brooks was not a fan of either. In 2002 he began working with Welbourn to test ideas on radio controlled models. Next, for design development, they used tank, VPP, and real-time sailing data from the 28m supermaxi Bols (now Med Spirit) as a baseline, then tank tested a 1/10 scale model of the next generation 30m raceboat. This showed them just how critical parameters such as fore and aft location of the horizontal foil were to performance. Testing was performed with the Wolfson unit at the Southampton Institute, and higher speed runs at the GKN tank in Cowes – the expensive way to gain hard data.

The team then put a fixed foil on half of a seven metre boat in Australia. ‘What it did was give us instant comparative data tack to tack,’ says Welbourn. When the side with the foil consistently out-performed the foil-free side in all conditions by up to 35 per cent, they knew they were on to something that was more than a scientific exercise. Welbourn patented the design as the Dynamic Stability System (DSS).

The curved foil slides through a casing in the bilge area, and slides out along roller bearings during a tack

By 2007, they had built a scaled-down working model of a 30m yacht. This 8m test bed gave them data from both fresh and saltwater sailing in varied conditions. Next, they plugged in CFD to calculate the hypothetical performance enhancement of a Wally 94 retrofitted with DSS. The results were staggering. Now they are utilising test tanks to refine designs for a 16.5m daysailer and two 30.5m fast cruisers.

So how does it work? A curved foil slides through a watertight casing in the bilge area. Its span, chord length, sections and curve radius are determined by each boat’s mission profile and general configuration.

When the boat tacks, the foil is slid to the new leeward side along Harken roller bearings, where it generates lift, forcing the boat to sail more upright, thus allowing it to be trimmed for more speed. Secondly, the foil’s position underwater dampens pitch and roll, improving rig efficiency while reducing motion and strain on the helmsman.

The foil extends outboard less than the boom or the mast when the boat is heeled even at only 5 degrees, so it is unlikely the foil would come into contact with another yacht or structure. It is, however, designed to break away on impact.

With no keel box or mechanism, the DSS doesn’t intrude on saloon space

Without the DSS foil, the yacht would have plenty of stability from its fixed keel, and while the sails might need to be adjusted, the yacht would remain completely safe. Any certified yacht is already required to have a minimum stability such as STIX, as used in IRC racing, AVS (angle of vanishing stability) in IMS/ORC, or another classification body’s regulations.

However, to extract good performance from most modern boats with their big sail plans, most have significantly more weight in their keels than would be strictly required for the safety aspects. It creates a vicious cycle: more weight means more sail, more sail means greater rig height, which means a deeper keel, which means more effort to move the boat in light air, not to mention more cost.

The DSS system, however, promises an inwardly reducing spiral: less keel weight is required, as the foil provides a significant part of the stability profile, so less sail is required, so less draught and less overall displacement.

According to Welbourn, DSS can provide benefits for almost all types and sizes of monohull; he has scaled it up to a 46m and it can be retrofitted onto existing boats. The technology licence can be purchased, but Welbourn and Brooks, working with Gordon Kay, have drawn a line of yachts optimised for the horizontal foil under the marque ‘Infiniti’.

Conservatively, says Kay, the Infiniti Yacht is 5 to 10 per cent faster upwind and significantly more comfortable, with 5 to 7 degrees less heel and much less pitching in a seaway. Because the boat increases stability without additional lead or water ballast, the Infiniti Yacht is measurably faster in light air and more easily driven. In windier conditions, especially reaching, 30 to 40 per cent more speed is a conservative comparison for some of the more performance orientated designs.

The motion dampening effect of the DSS foil acts as an anti-seasickness device

The racer will see a cleaner interior compared to a canting keel yacht. There are no vertical dagger-boards or canting keel mechanisms impinging on interior volume and sail handling and packing areas. There are no ballast tanks, so volume is maximised throughout.

The performance cruiser will see an interior that equals the finest interiors of fixed keel yachts with a comparable design brief. Infiniti has been working closely with Design Unlimited, which has designed interiors for such signature vessels as Visione, Ran, Pink Gin and Singularity. The DSS unit is below the cabin sole, and the drive units for the foil itself are easily accessed and serviced, even while sailing.

‘Everything gets lighter. Racing maxis come in at 28 to 30 tonnes – we can do this at 20 tonnes.’

Hugh Welbourn, Welbourn Design

‘The Infiniti 100 S was developed in response to a client brief for an ultra-fast cruising yacht,’ says Kay. ‘He wanted a yacht that was focused on Mediterranean sailing in both light and heavy winds. The yacht should be stylish, provide plenty of outside living with a focus on relaxing, swimming and enjoying the experience of sailing, although he also plans to race. The interior is developed around six guests in three cabins and four crew in two cabins.’

‘The brief for the 100 R was to be capable of breaking all records but without using diesel power to run winches or cant keels,’ says Welbourn. ‘Because it weighs less than 50 per cent of a yacht such as Leopard 3 at the same length, we are able to have a sail plan that can be managed manually – therefore it is eligible for some of the key records such as the transatlantic.’

Indeed, at the 2011 Fort Lauderdale show specialist builder Danish Yachts announced a partnership with Dynamic Stability Systems and Infiniti to build the 100 S under the name FLITE 100, along with others in the Infiniti range, with Yachtzoo appointed as worldwide sales agents.

Employing Danish Yachts’ specialist knowledge of building in carbon, the yacht will be fully pre-preg. It’s all part of that inward spiral of construction.

‘Everything gets lighter,’ Welbourn says. ‘Racing maxis come in at 28 to 30 tonnes – we can do this at 20 tonnes.’

A scaled-down demonstrator has already been constructed at the Danish Yachts yard, and is proving her worth in the Mediterranean.

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