Dynamic Stability System - a new superyacht solution
by Marilyn Mower
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.
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.