That's a subject that has been recently discussed extensively on internet forums even if the answer to that question has been given by the most prominent NAs on the design of most recent cruising boats: Almost all feature a twin rudder set up. To the more distracted, let me point out that a two rudder set up has nothing to do with a twin wheel set up. A twin wheel set up can drive a single rudder or a twin, as a single wheel can drive a twin rudder system. That's true that a twin wheel setup is today the norm but some drive a single rudder and the reason for its almost universal use on cruising and also most racing boats has nothing to do with a single or twin rudder setup.
So, let's focus on the rudders: It seems obvious that the more protected rudder is the one that comes on the continuation of a full keel. But there is an efficiency problem: the rudder to work well has to be in the way of the water flux (not to mention the efficiency of a full keel), the further away from the keel the better and in what regards that, it cannot be a worse situation that a rudder immediately aft a full keel.
The next step, following the search of a more efficient and better sailboat, in what regards rudder, it was to separate the rudder from the keel, using modified fin or fin keels. Almost all the first rudders of that era used a skeg, most of the times an integral one that protected all the length of the rudder. That improved the rudder efficiency but also revealed some problems regarding the use of skegs: They found out that a skeg could not be strong, otherwise the skeg would resist to a strong impact or grounding, but the force transmitted to the hull, multiplied by the long arm due to the depth of the rudder, would risk to break the hull at the insertion point.
After finding out that problem, the skegs become thinner and as the rudders, sacrificial, face to the hull integrity.One more step brought boat design to modern spade rudders, rudders designed for max efficiency, away from the keel and designed, as the skegs, with a sacrificial function: They should be strong enough to sustain all the abuse strong sea conditions could have on them, resist small shocks but face big shocks, they should break or bend in in a way that protect hull integrity. Not an easy compromise designing a rudder that responds to these two apparently opposed constraints.
Better keels, better hulls (with the beam more brought aft) and lighter sailboats give cruising boats a better sailing performance. The rudders have to be designed not just to respond to a slower and more sluggish sailing, but to boats that have an increasingly better performance and that, in what regards rudder design translated in more efficient rudders, auto aligned, balanced and more deep.
Deep rudders, some almost the depth of the keel, are more vulnerable, not only due to the bigger efforts that a deeper rudder generates, but are also more exposed to breakage due to groundings or eventual shocks with submerged objects. Besides it was discovered that in beamier designs with the beam brought aft, due to the strongly asymmetrically wet surface when the boat is heeled, the best position to a rudder is not at the center of the hull, but at the center of the wet surface (while the boat is heeled) and that means on the side and therefore a rudder on a twin rudder setup, that would respond to this demand, could be much smaller, much less deep and have less efforts than a single rudder positioned at the center of the hull.
Rob Humphreys describes here the benefits observed through a tank testing that was performed on a Oyster 885 model and that convinced Oyster management regarding the benefits of using twin rudders on their line of yachts:
In our tank testing we were focusing our attention on a fully-pressed up set of sailing conditions, with the boat heeled over to twenty degrees and sailing at nine knots, with a variety of leeway angles and load conditions. .. We tested a lot of other things as well but the rudder testing part was most interesting and was totally supportive of all that we had learnt to be true in the field. ... For example, with the twin rudders set to just two degrees to the flow the spade rudder needed to be at over six for an equivalent moment, and the skeg-hung rudder at eight – all for the same yaw moment.Put another way, the leeward twin rudder provided 4 times as much force than a skeg rudder!..
From our perspective twin rudders represent a huge benefit and an Oyster owner will really appreciate it too as soon as he has the wheel at his finger-tips. But what’s also interesting is that the system fares a lot better in terms of potential reliability, especially against the spade rudder. The blades are significantly smaller and more lightly loaded, and the span is considerably shorter, making it almost impossible to damage the steering gear when reversing into a quay. And of course, with two rudders rather than one there is an obvious increase in the level of redundancy. Unlike some twin-rudder installations, the arrangement we have for Oyster means that even assuming the worst-case loss of one rudder it would be possible to still sail the boat on the compromised tack, albeit with reduced canvas."
