Tuesday, November 11, 2014


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:

"David’s naval architecture back -ground (David Tydeman is the CEO of Oyster Group) quickly helped me persuade him that twin rudders were the way to go! It was clear that this was going to be a bit of a sea-change for Oyster and I was pleased that David was keen to push this onwards and also to support this breakthrough with a decent budget for tank testing. We both felt it would be helpful to have fairly tangible reference information for those owners trying to understand the shift from a skeg-rudder to the twin rudder form for this exciting new model.

In fact our testing session set out to do more than just this because we also used the opportunity to let the spade rudder have its say, just for some form of completeness. We have often been asked why Oyster has tended to steer clear of spade rudders and the answer has more to do with potential vulnerability than any disrespect for its potential qualities. As any Oyster owner knows a blue-water cruising yacht has to be accomplished in a number of different ways, and one of the lower profile requirements has to be an ability to slide backwards against a Mediterranean harbour wall without necessarily endangering the steering gear. ..

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."

This pretty much says it almost all regarding why a twin rudder is more adequate to a cruising boat and does not tell the full story. There are certain things you can't measure on a tank test and one of them is the feel you have on the boat and the easiness to control it. In what regards that ,sailors that experienced twin rudder configurations, reported in almost in all cases a superior easiness to control the boat, specially downwind, and that makes sense if we take into account the superior directional stability that the two rudders on the sides will provide compared with a single one at the middle.

So, the single spade rudder is as dead as the skeg rudder? Not quite. I continue to see designs of racing boats and of top cruiser racers, designed with CFD and VPP extensive use, that continue to use a deep very narrow spade rudder. Rating formulas can have to do with that but I am more inclined to think that for absolute maximum performance, less drag and maximum rudder sensibility, those rudders offer advantages but in all the cases we are talking about boats designed to offer maximum performance, with a crew and that demand a very exigent and controlled sailing. Solo racers have for many years adopted twin rudders as the better solution and that has probably not to do with absolute performance but with the requirement such boats have to be easily controllable, since they are sailed solo and many times on autopilot.

No comments:

Post a Comment