Few aspects of yacht design have received such free rein as the design of yacht rudders – sometimes they have taken directions that have been far removed from all hydrodynamic concerns.
I have found it immensely amusing to sit in a yacht club bar listening to the self proclaimed “experts” passing judgement on the twin rudder set ups on my designs. It would appear most consider design has not advanced since Nat. Herreshoff used separate spade rudders on “Dilemma” in 1891 or Van de Stadt transom yachts such as “Zeevalk” and Zeeslag” in the 1950’s. For those on the surfing persuasion, the comparison is the move from single fin mal’s (long boards) to the smaller, lighter, three fin “thrusters” is applicable – similarly the twin rudder set up cannot be evaluated independently of the entire yacht design. When the fashion (rating rules) moved away from the 60’s and 70’s narrow sterns to the more modern broad sterned “dinghy – type” hull shapes rudder design did not keep pace. Most keel boat sailors will be familiar with the bow down – stern up aspect depicted in the accompanying sketch (Figure 1).
It doesn’t take a brain surgeon to see less than half the length of this rudder is actually in the water and working. Not only that but it is attempting to operate in the most disturbed water flow. Because this generation of yachts frequently trimmed by the head and heeling to a certain degree at the same time the upper part of the blade often emerges from the water, so the blade must therefore be long.
The rating of the period (50 – 90) penalised fuller bow sections in fact Bruce Farr, Paul Whiting and others actually had designs with hollow water lines and this tended to exasperate the problems.
The modern “open” designs are the complete opposite. The hull buoyancy is more evenly distributed along the hull’s length and in fact when sailing upwind these boats exhibit a bow – up attitude.
All this leads logically to a rudder or rudders located at the position of maximum waterline length and horizontal to the optimum angle of heel.
Now this can only be accomplished with either a single canting rudder (which has been done) or twin rudders. The arguments I have heard against twin rudders don’t stand up to scrutiny.
1. The people “bagging” them have never sailed a (properly designed) twin rudder boat.
2. There is not more wanted surface with twin rudders, in fact there is less! – For example on a modern 8.5m yacht a single rudder
would be between 4.5 and 1.7 metres below the waterline when static the twin blades are only 700 – 800mm deep.
3. There is less drag as at optimum heel angle (15 – 20 degrees) only one of the blades is in the water ie 50% less drag.
4. Safety is enhanced as you can have a “spare” blade.
5. When comparing a single transom hung rudder with twin rudders the access from the water (or a dinghy) is hugely superior with the twin set up.
In summary unless you have sailed yachts with twin rudders you are in no position to comment – I’ve certainly sailed yachts with one rudder. Sailing a twin rudder yacht in 35 knots of wind both upwind and downwind is a two finger experience.
Footnote: To reduce ventilation and improve the effective aspect ratio of twin rudders I recommend fitting a fence or cavitation plate positioned to form a prolongation of the hull. As shown in figure 2.