Have twin rudders been given a fair chance?Jan 1, 2003
With reference to the article by Dave Gerr on broaching ("Spun sideways," Issue No. 61), I must take exception with Gerr's blunt dismissal of the benefits that twin rudders might afford a sailboat. It is opinions like this that repress the development of many design innovations that might otherwise bring significant benefits to voyaging sailors.
Personally, I don't think twin rudder installations have been given a fair trial. The reason is as simple as it is stupid. Race management organizations, authors of such gems of type forming as the International Offshore Rule, typically class as "illegal" any technique or design feature that they can't figure out how to rate. Consequently, these things don't get developed. The fully-battened mainsail has only recently fought its way to respectability after decades of being restricted to iceboats, multihulls, BOC boats, and other "outsiders." How many other invaluable features have been consigned to oblivion by the self-serving shortsightedness of the racing fraternity?
Which brings us back to twin rudders. Firstly, I believe Mr. Gerr is dead wrong in asserting that "each of the twin rudders must be nearly as large as a standard single centerline rudder would be." He only needs to look at the profile of the boat used to illustrate the article, Groupe Sceta, which Chistophe Auguin steered to victory in the 1990/91 BOC Challenge, and compare it to that of Generali Concorde, the boat's elder sibling which Alan Gautier steered to second place. Generali Concorde, with her single rudder, experienced steering difficulties in the inaugural Globe Challenge.
Groupe Sceta was the next Open Class 60 off the boards of Groupe Finot, and was fitted with twin rudders which look to me as though they are less than half the size of the single rudder they replaced.
I worked with Rodger Martin on the design of Mike Plant's Duracell, which he sailed in that first Globe Challenge, 1989/90, as well as the 1990/91 BOC. Rodger had been paying close attention to what the French designers were doing and opted for twin rudders. Each was about half the size of the single rudder which would have been specified otherwise. Admittedly, when we first saw them installed they looked mighty small compared with the barn doors on all the other boats in the yard, but they worked just fine for Mike.
Duracell had moderate beam for a BOC boat. At optimum heel, the weather rudder was clear of the water and creating no drag. Sailing flat, the combined wetted surface was no more than the single rudder would have had, but the frontal area was significantly less. This was because the smaller area and stock length of the individual twin rudders results in the bending load in the stock being reduced by about two thirds, with a consequent major reduction in stock diameter and rudder thickness. So, instead of increased drag, as Mr. Gerr postulates, a properly designed twin system has both less wetted surface and less frontal area, which sounds to me like less drag.
The reduction in rudder loading is extended to the rudder ports, bearings, and internal structure of the hull, not to mention the smaller hole left in the hull should a rudder fall out (note the experience of Brooksfield in the recent Whitbread Race).
Even if, with all factors taken into account, there is a minor performance penalty incurred by twin rudders (I have yet to see any empirical data to demonstrate it one way or the other), what would the voyaging sailor care about a small loss in ultimate speed when twin rudders give him increased control and confidence in the conditions he seeks most: reaching and running, preferably in the Trade Winds.
As for denying that twins give greater control, I can only suggest that Mr. Gerr has not had the good fortune to sail on a boat with a well-designed twin rudder system. I have never steered any boat that had the instant, positive response to the helm that Duracell exhibited. In a stiff summer sea breeze and a building quarterly sea, a thumb and forefinger on the wheel was all she needed, while helmsmen all around with "conventional" steering were getting a thorough aerobic workout, at half the boat speed.
Another factor helping the immersed twin is that it is operating in clear water and not in the slipstream of the keel, or propeller if there is one. Turbulence from a propeller, even a feathered one, can seriously reduce the effective area of a rudder. The keel has a significant effect also.
Hull shape, too, can contribute to the effectiveness of twin rudders. Mr. Gerr makes the assumption that all boats that are beamy aft will dip their bows when heeled. This is true of IOR types and their derivatives, which unfortunately describes altogether too high a proportion of the current inventory of sailing boats. Because the waterline is distorted so that the boat measures low, or worse, fits an aesthetic detail that some designers admire, this type of boat looks as though it is sucking a lemon right where the rudder is attached at the aft end of the DWL. The heeled waterplane is fuller than that of the DWL and the center of flotation much further aft, so the hull naturally trims down at the bow. The yacht in the illustration accompanying Mr. Gerr's article does not exhibit this vice. The hull is fair, the DWL stretches all the way to the transom. The heeled waterplane is a narrow lozenge with the same longitudinal center of area as the level waterplane, and not dissimilar in shape. It has a rudder right in the middle of its aft end and mounted perpendicular to it. This yacht steers like a race car and holds the bow up when heeled.
This arrangement makes an awful lot of sense to me, and suggests that, rather than hull design dictating twin rudders, the choice of twin rudders would dictate how the hull should be designed.
As for being susceptible to damage from passing waves, I suspect Mr. Gerr is waffling. We should have seen some of that in the several BOC, Globe, and transatlantic races that have found many twin rudder boats in very rough conditions, but I am not aware of a single incident of damage.
Twin rudders will not work for everyone on every boat, but they should not be dismissed out of hand.
I would like to see more research done, on this topic as well as many others, but this will only happen when the racing authorities decide to allow experimentation within their conservative ranks. Racing is, after all is said and done, the best test-bed for ideas because heavily campaigned boats are pushed harder and cover more ground than voyaging boats, so that flaws in design, and likewise, benefits, come to light quite quickly. And the high visibility of racing boats impresses their style and attributes on the voyaging as well as racing sailor.
The discussion of stability, of which David Gerr's article was a valuable contribution, is welcome and important. Its impact will be wasted if we fail to follow through and investigate ways to best achieve stability and control, two factors which are critical to safety and comfort in sailing boats. I would be dismayed if readers were to take Mr. Gerr's dismissal of twin rudders to heart, without hearing from the other side. Who, in their right mind, would forgo the ability to steer out of a broach, even at a small price in performance?
Jeremy McGeary is a naval architect living in Newport, R.I.