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RS Sailing 2021 - LEADERBOARD

Anhedral-angle rudder foils

by Finbar Anderson 14 Mar 2014 08:54 PDT 14 March 2014
International Moth anhedral-angle rudder foils rendering © Mach2

'Evolution, not revolution.' This has been the mantra adopted by the International Moth class ever since the complete sea-change that came about with the introduction of hydrofoils over a decade ago. Since then, evolution has been gradual, with refinements in foil and sail technology.

Because of the Moth's necessary emphasis on vertical lift, some of the developments within the class have been inspired by sailing's faster, often flashier cousin: aviation. The associations are by no means new: those who grew up watching the VHS of 'Awesome Aussie Skiffs' will remember the commentary team describing Nokia's downwind performance on a fresh-to-frightening Auckland harbour as 'Top Gun, jet fighter stuff; ready for take-off'.

Now that dinghies have quite literally taken off, in the Moth fleet we are starting to see 'fighter jet' technology visibly trickle down onto production foilers. A particularly intriguing example is the recent partial adoption of anhedral-angle rudder foils (the foil horizontals slope downwards). For years, the majority of fighter jets have been designed with the wings mounted at an anhedral angle (in other words, the wings slope downwards from the fuselage). This serves a number of functions, one of them being to give the jet more maneuverability. If you imagine a jet with anhedral wings pitching to the right, for example, the port side wing will gain increased lift as it comes closer to horizontal, while the starboard side wing will lose lift, helping the jet to effectively fall into the turn.

The application of this effect on the International Moth has nothing to do with maneuverability, but in fact helps the Moth go faster in a straight line. Many will be familiar with the sight of the Moth heeled to windward when going upwind. The effect of the anhedral rudder horizontal is that, with the boat heeled to windward, the leeward part of the foil comes closer to horizontal and gains lift in the vertical plane, while the windward side drops away from horizontal, thus losing vertical lift. This makes the boat want to 'fall' to windward, giving the sailor more righting moment.

This comes as a response to a problem that has been facing Moth designers - indeed, boat designers in any class - of how to make their boat go faster: they must either increase the power going into the boat or reduce its drag. With only 8 square metres of sail allowed under class rules, increasing power is difficult. The average sailor weight has remained at a reasonably modest 75-80 kilograms; much more and Moth sailors often struggle in lighter conditions. As such, the anhedral rudder foil is an elegant solution to the problem of how to increase effective righting moment without increasing overall weight.

The anhedral rudder foil was made available to sailors of the Mach 2 - currently the most popular International Moth design, and brainchild of Australian Andrew Mcdougall - just before the 2012 Moth World Championships on Lake Garda. It was used that year by four of the top ten finishers; by the following year over half of the top 40 were using anhedral rudders.

The obvious popularity and success the anhedral rudder has enjoyed in the Mach 2 begs the question as to why other Moth designers have not yet employed it. Simon Payne, former Moth World, European, and National champion, and Mach 2 Sales Manager, fully expects them to: "There is a lot of inertia sometimes, and when someone else has introduced it first, it is natural not to want to follow unless you can see why there is an advantage or it is totally proven to be faster."

He cites the case of the Mach 2's predecessor, the Bladerider. In 2006, Mcdougall added a torpedo-shaped bulb to the join between the foil verticals and horizontals, and saw a marked reduction in drag and corresponding increase in performance. 'It took four years for other Moth designers to take it up and has taken eight years for it to be accepted as an advantage outside the Moth, with Oracle Team USA introducing it to great effect in the middle of the last America's Cup event'.

We might therefore expect to see anhedral rudder horizontals on other Moth designs in the future, and can wonder where else this technology might be used. It seems obvious that the next step for the Moth is to extend this clearly successful technology to the main foil horizontal. This is not currently possible, Payne tells me, for one reason. While the rudder horizontal is an individual unit, the main horizontal must incorporate the flap that, attached to the wand at the bow of the boat, controls the Moth's ride height. Making this foil anhedral would involve splitting the flap, and it's not something they feel they can currently realise in a reliable and cost-effective way.

And further afield - will this particular technology also filter down to other classes? There are a number of monohull classes that use winglets on their rudders, such as the Cherub, International 14 or National 12. Cherubs or 14s are designed to be sailed as flat as possible, and so the introduction of an anhedral angle on their winglets would not be of any advantage. The National 12s have been experimenting with considerable success for a number of years with dihedral rudder winglets, which help to counteract the moment of the boat heeling to leeward in breeze.

What's certain is that the International Moth, with its alternative approach not just to boat design, but to sailing itself, will continue to be an exciting act to follow. This year's World Championships in Hayling Island, already with well over a hundred boats entered, will surely be one of the highlights of the coming season.

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