Rudders – all you wanted to know but were afraid to ask!
Thursday, 01 July 2010
"Nel" posted a comment to my review of the EOS to the affect that it would be good if we did an article of rudder design. The topic being an area of interest to me and having experimented with rudders and fins over many years I needed no second invitation.
What is one looking for in a Surfski rudder?
Essentially one is looking for the most “lift” (i.e. turning force) with the least drag with good all round characteristics. The latter includes the broadest possible range of operation without stalling, soft and progressive actuation (i.e. no sudden bite) and the least amount of rolling effect on the ski. For me the easiest frame of reference for considering rudders is to think of them as wings (as in aircraft). It scientifically sound, readily accessible and the issues are pretty much the same. Essentially it leaves one with 4 parameters to play with i.e:
- Aspect ratio
- Airfoil section
I will review each of these and then make some conclusions based on both theory and my experience.
Planform is the essentially the shape of the rudder and for present purposes it includes the angle of sweep (i.e. the angle of the leading edge of the rudder to the hull of the ski). Clearly this is the most noticeable characteristic of rudders and everyone will have noticed the difference between shark fin type rudders on Fenns and elliptical rudders such as the Epic downwind rudder. The shape of a rudder affects its lift to drag efficiency (LD ratio) quite dramatically so it is a very important parameter. The biggest component of inefficiency in an aircraft wing (and for that matter a rudder) is induced drag. This is caused by the flow around the tip from the high pressure underside to the low pressure topside. As far as planforms go elliptical wings are by far and away the most efficient. The Supermarine Spitfire design was based around this and it goes a long way to explain its success as fighter plane. The angle of sweep also affects drag because spanwise flow increases with sweep and this is the source of induced drag – this serves to explain the small vertical fins on modern airliners (which require swept wings for other reasons) that are there to minimize flows around the wingtips.
Radical planform! (Spotted in San Francisco in 2009)
The bottom line as far as rudders go is that elliptical rudders are the most efficient and swept shark fin rudders the least. The benefit of sweep is of course that swept rudders shed debris whereas more vertical ones tend to collect it. There is also a secondary benefit in that a less efficient rudder by definition does not produce big effects and is therefore more forgiving. Anyone who has switched from an Epic standard rudder to their elliptical downwind rudder would have noticed just how more “whippy” the ski becomes.
The conclusion is therefore unless you’re an inexperienced paddler or paddle in water with weed and debris an elliptical rudder is the way to go. If sweep is required is a tilted back elliptical rudder is a good compromise.
By and large my experiments bear out what they say in woman’s magazines i.e. that within acceptable parameters size does not make a big difference. It seems to me that manufacturers would often do better make smaller more efficient rudders that larger inefficient ones.
Simply put this is the ratio of length to area. A high aspect ratio rudder is long and thin while a low aspect ratio one is short and fat. In principle the higher the aspect ratio the greater the LD efficiency (hence the long wings of sailplanes) but there are practical considerations. Firstly and most obviously high aspect rudders tend to roll the ski when deflected and can make an otherwise docile skis skittish. Following on what I said above the trickiest rudder of all is a high aspect elliptical one.
Extremely high aspect ratio rudder (compared with the conventional swept back Epic rudder)
The second problem with high aspect ratio rudders is that they stall sooner and more severely. Given that it is a fundamental requirement that rudders remain effective through the broadest possible range of deflection before stalling (and loss of control) this is a big reason for keeping the aspect ratio down. I have seen some absurdly long rudders on skis and I can only assume that their users keep them fitted only because their real effectiveness has not been tested in proper downwind conditions.
This is most probably the most misunderstood area of all. I have frequently heard paddlers equate the thinness of a rudder to its efficiency. This would be at least partially true if rudders were fixed and static but the practical reality is that they have to remain effective through a broad range of deflection. Perhaps the best way to explain the problem is to consider a knife edge. Encountered straight on the water has to deflect very little to move around the blade. If, however, the blade is deflected say 20 degrees the water has to take a 20 degree bend in a fraction of millimeter. This is a very hard ask and if it does not the flow will separate and a stall will occur. Contrast this with a thicker and rounded entry and will become clear that the water is given longer to change direction and so remain flowing across the section. So in essence it all boils down to the fact that thin is not necessarily good and that the profile needs to be optimized for the range of use. I suspect that the sharp entries of many rudders have more to do with the manufacturers moulding techniques than design considerations.
Some other considerations
The positive castor rudder
Johann van Blerck recently published an article on his new PC rudder and I am waiting for my new EOS 660 to be delivered with one. This requires quite a radical modification to the rudder mounting but I am happy to try it out despite not having paddled with a PC rudder because it makes logical sense. The logic is supported by the fact that Johann has demonstrated that he can now sit on the side slip of trawlers coming in and out the bay – previous attempts at this invariably resulted in uncontrollable broaches. He thinks that PC rudders will be standard fitment in the near future – I will soon know if he is right.
Some time ago I fitted a 2 piece rudder to my Red7 Surf70 pro in an attempt to get it to turn better (see attached photo). The improvement was marginal but it did bring about a big improvement downwind.
The rudder proved to be practically unstallable with the result that I retained directional control over a far broader range of conditions. I am entirely convinced that it is an improvement over standard rudders but it does involve a degree of fabrication difficulty and creates issues around packaging for delivery (even with the rudder removed you have a substantial protrusion from the hull).
Rudder tip winglets
A few years ago I experimented with a few rudders with endplates or winglets which were aimed at retaining rudder effectiveness downwind. Although many people saw them as small hydrofoils intended to create lift they were in fact was based on aeronautical practice as described above. I think that they did help but the improvement was not big enough to warrant the fabrication effort and practical disadvantages such increased potential to snag weeds etc.
When I first started paddling downwind it was largely case of proceeding from one broach to the next. When I got onto runs I steered as straight as possible if only to delay the onset of the next broach. I recall being amazed at the ineptitude of ski designers in that they had not found ways to solve such a basic steering problem!
Now many years later with my technique greatly improved broaches are fairly rare events. The challenge has now shifted more towards positioning myself on the run and retaining positive steering so that I can link runs rather than just stay on them. The fact most input in ski design comes from good paddlers together with the fact that that good paddlers can through a combination of acceleration and technique position and control their skis on runs most probably serves to explain why rudders have not received the attention they deserve. This does not however mean that all paddlers would not benefit from better rudders.
Maybe the PCR will demonstrate the benefits and give rise to whole new area of development. I hope so because it can only improve my downwind performance!