|
 [Editor: Dale Lippstreu is a paddler and craftsman of note. In this article he describes how he's improved the V10 rudder by adding winglets to improve performance, particularly in downwind conditions]
Downwind paddling in big conditions is for me what might be politely called a "development area". This is not an unimportant skill in Cape Town which is renowned for its gale force South Easters and paddles such the Millers Run and Downwind Dash. My problem is that I find myself broaching off runs on a regular basis and, while some skis are clearly better than others, I have yet to find one that eliminates the problem entirely.
I am not sure of the mechanics which cause skis to broach but the effectiveness of the rudder clearly decreases as it becomes partially exposed on the crest of the swell. I had heard that the addition of an endplate on the tip of the rudder produces a marked improvement particularly in the case of swept rudders. The theory is that water tends to be shed off the tip of the rudder rather than flowing around it once part of the rudder becomes exposed. An endplate eliminates this problem and thus allows a rudder to remain effective even when partially exposed. The theory is entirely plausible and is in fact consistent with practice in aircraft design - witness the winglets or endplates on most modern jet aircraft wings.
Without seeing a winged rudder I set about building one of my own design to test the theory. Rather than start from scratch I decided to modify a spare rudder that I have for my Epic V10. For those who may wish to do an experiment of their own the following represents a brief description of the method used:
- I decided to add my winglet at the very tip of the rudder which required that it be ground flat. To do this I marked out 2 parallel lines on a board - one at the top of the rudder and the other about 1cm from the tip. I then placed the top of the rudder exactly on the top line and ran a strip of masking tape across the bottom so that it connected across the partially covered bottom line. This gave me a marked off line parallel to the top of the rudder. I used a bench disc sander to grind off the rudder tip to the masked line.
- My trial endplate was made up 3 layers of fibre glass chop strand laid up on a polished piece of glass. While this cured I made a plywood template which I cut and sanded until I had the right shape and proportions. This outline was then traced onto the GRP before rough cutting with a jig saw. The endplate was then shaped to an airfoil with a blunt entry and fine trailing edge and sanded to finish.
- The endplate was bonded to the rudder tip with epoxy and the joint filleted with ultra light automotive body filler to ensure the cleanest flow of water possible. I finished the job with gloss black spray paint to make it look good.
Well after all this trouble does the rudder work? Just about everything I have tested in skis involves a degree of subjectivity but this is one instance where the answer is clear. In theory the rudder should improve steering under all conditions but on flat water it makes no discernable difference as regards either steering or drag. I suspect this is because the increase in turning performance is very small. Going downwind however the rudder comes into its own and the difference is marked. Steering remains positive under a far broader range of conditions and I find myself able to position the ski where I want it on the swell. The rudder greatly reduces the possibility of broaches and used in combination with a ventral fin (maybe the subject of another article) it pretty much eliminates the problem entirely.
 Photo 1: First version of winglet rudder
 Photo 2: First version of winglet rudder
Photos 1 & 2 show my first prototype with a tip mounted endplate and photos 3 and 4 my 2nd version which differs slightly in that the end plate it is mounted above the tip rather than on it.
 3. Version two of the winglet rudder
 4. Bad photo of version two (blame accepted by the photographer)
The sole motivation for moving the endplate upwards in v2 was that I found that it was necessary to grind through to foam core of the rudder to achieve a flat base for the end plate. This left me with little bearing material to bond to. I have avoided this problem in v2 by bonding the fin to the carbon skin of the rudder. I don't believe that there is any difference in terms of effectiveness but I believe v2 might be slightly better in shedding kelp and other debris. Keeping the end plate off the tip also affords it slightly better protection from damage.
Conclusion
The problem of "water shedding" increases as rudders are more swept. Epic have gotten round this problem with their new elliptical rudder which is both deeper and more vertical but this has its own issues. Longer more vertical rudders are less progressive in that they tend to "bite" and sudden rudder deflections = sudden drag. Also deeper rudders tend to rock the ski and unsettle it. My conclusion is that a winged swept rudder represents the optimum solution in that it retains a smooth progressive response while at the same time remaining effective when partially exposed. My conclusion is that winged rudders produce dramatic increases in both directional stability and steering downwind and will be offered as options on production skis in the not too distant future.
 |
Subscribe to this comment's feed