Revolution in stability?

The story of sailboat stability has many chapters — from deep keels to skimming dishes to water ballast to canting keels. It appears a new chapter is abaout to be written. Hugh Welbourn, a British naval architect, and a company called Dynamic Stability Systems (DSS) have developed a stability approach that uses a retractable underwater foil amidships to provide stability. According to Welbourn, in addition to stability the foil also adds other benefits. Some big-name backers, Harken, Doyle Sailmakers and Pantaenius Insurance, signed on as partners.

What makes this approach different is that the DSS foils run athwartships, at 90° to the fore-and-aft run of the hull. In other words, the foil has the same relationship to a vessel’s hull as an airplane wing does to a plane’s fuselage. And like an airplane wing, the foil has a profile to create lift.

It is this lift that plays the main role in greater stability. In practice, the DSS foil is deployed to the leeward side of the boat. Although the foil exits the side of boat just above the waterline, the combination of heel and an upward curve to the foil keeps it just underwater. As the foil moves through the water, it provides lift just like the foils on a hydrofoil-equipped vessel. The purpose of a standard hydrofoil is to lift a displacement hull out of water. The DSS foil, however, is designed to produce just enough lift on the leeward side to counter the heel of the vessel and keep the boat upright. According to Welbourn, the foil also acts to dampen roll and even diminish the tendency of the vessel to pitch. In addition to the added stability, Welbourn says lift from the foil prevents the boat from heeling over, which reduces wetted surface area and thus reduces drag. “So for the drag cost of one item [the foil],” Welbourn wrote in an email, “you are getting benefits in two areas — more stability, reduced hull drag. Effectively, you get one of these for free in drag terms.”

For racers, the foil means added stability and more speed by reducing drag, pitch and yaw through the waves. For shorthanded ocean racers, the added stability of the DSS foil can mean less course corrections are needed from an autopilot, reducing the vessel’s electrical needs. And offshore voyagers also benefit from reduced autopilot electrical consumption. Probably the biggest benefit for the voyager, however, is simple comfort: The boat doesn’t heel over as far, doesn’t roll as much and even pitching is reduced.

Welbourn doesn’t see the addition of a DSS foil changing a voyaging boat’s ballast package. The vessel would carry the same keel arrangement. Thus, if the DSS foil were to strike an obstruction, it would break off and the boat would be left with its normal ballast stability intact.

Welbourn got the idea for the DSS foil roughly six years ago when designing a 100-foot ocean-racing maxi. Welbourn wasn’t able to incorporate his leeward foil into that design, but he continued working on the concept. This work involved an incremental development plan. First, Welbourn undertook virtual simulations of various foil shapes and dimensions. Then he tried out the concept in the real world using radio-controlled boats on an English pond. This was followed by two rounds of tank testing before once again taking the concept to the water and retrofitting a foil to one side of a 23-foot sport boat. The foil system was then built into a 27-foot sailboat and extensively, and secretly, tested on Italy’s Lake Garda. Additional tests and more data logging were carried out on a 40-foot cruising boat in Australia.

How would DSS foils be fitted to a voyaging boat? The 27-foot DSS test boat has an internal tunnel that runs from one side of the hull to the other. On a voyaging sailboat, the options would presumably be greater, depending on the size of the vessel and its configuration. Rather than a tunnel running across the beam of the boat, it would be possible to build foils into individual tunnels on either side of the boat. Welbourn comments via email: “The choice between individual P&S foils or the straight-through foil system can depend on several aspects of the overall design of the boat in question. On many typical medium displacement boats then the transverse foil system will disappear under the floorboards and so not impinge on any internal arrangements. This is the simplest and lightest solution, but sometimes there is a case to be made for separate foils — in this case typically they fit outboard of the bunk fronts so have no impact on the saloon area.”

As for moving a single DSS foil from one side of an internal tunnel to the other, Welbourn explains how it can be done using a simple line with blocks: “In either case, then we can use simple rope haulers to move the foils — on the 40-foot yacht in Australia we had 2:1 purchase on the board to move it from one side to the other and even under full load at pace then you only just needed to take the line onto a winch. This was on plain bearings too — we are developing suitable bearing systems with Harken for the boats where the frictional loads get a bit higher. On the 27 footer — even at 18-plus knots and fully loaded up, the 2:1 line is more than adequate to hand pull the foil.”

If the benefits of the DSS foil are realized, it could have a major impact on recreational sailing in the years to come.      

By Ocean Navigator