Experimenting with Rudders
I enjoy the science and design of surfskis nearly as much as paddling them and have experimented with a number of things including:
- A low aspect ventral fin under the seat of the paddler
- A winged rudder with an endplate
- A hydrofoil located on the bow of the ski
![Articulated Surfski rudder Articulated Surfski rudder](http://www.surfski.info/images/stories/2008/04/rudders/rudder.jpg)
These experiments have produced interesting and positive results but not sufficiently so to justify to the complexity and fabrication hassles involved in incorporating them in productions skis. The hydrofoil needs a lot more refinement but I abandoned this because it also requires substantial modifications to the bow of the ski. Maybe I will pick a damaged ski sometime which can be used as a "test bed" to take the concept further.
Experimenting with Rudders
My latest experiment is an articulated rudder comprising a fixed leading edge and moving aft section.
The intended benefits are:
- Reduced tendency to stall due to the fixed leading edge
- Smooth progressive actuation with positive feedback due to the fact that the rudder is fully trailing
- No rudder "bite" and associated drag
- Reduced rudder induced roll
- Less drag around the rudder /hull interface
- A "built in" weed deflector
The obvious downside is that the introduction of a fixed portion of the rudder and a reduced movable area is likely to result in reduced rudder authority. My hope was that the "flap" effect of the moving trailing edge would produce a sufficient increase in lift to compensate for this but I had no way of finding out without building a test rudder.
3 Versions
I envisaged 3 basic implementations of the concept (shown as versions 1 - 3) which varying percentages of fixed versus moving areas. V1 is the most radical implementation with a 50:50 ratio and I decided to build it first - if it worked the concept had potential. The resulting rudder is shown in the accompanying pictures
Having tested the V1 rudder in flat conditions and on 2 Millers Runs (30knots / choppy seas) my findings are:
- Tested on flat water the rudder does produce less turning authority resulting in an increased turning radius. It does however produce a smoothly progressive effect with no "bite" and hopefully less drag.
- Downwind the rudder comes into its own. The ski tracks and generally steers better. Both Millers Runs were done in conditions that I would normally expect to spin out several times but this did not happen. I felt that I could manoeuvre the ski and place it where I wanted better than usual. I suspect that this is because the rudder with its fixed leading edge is way less prone to stalling.
{gallery}2008/04/rudders/litebox:150:120{/gallery}
Where to go from here? The increase in turning radius is not at all bad but my home series is Hout Bay which invariably involves racing around buoys.
I therefore have to improve turning and plan to test V2 this weekend to find out. My guess is that it will improve on both 1 & 2 and I am pretty confident that I will find it generally preferable over a standard rudder in pretty much any conditions other than racing around cans.
If as I suspect turning is still slightly compromised I will make v3.2. My guess is that it will perform at least as well as a good standard rudder (I take the Epic surf rudder as the benchmark) but will hopefully retain some of the downwind benefits of the articulated rudder. I am pretty convinced that this will be better than the standard alternatives but whether the benefits are sufficiently great to justify the fabrication hassles remains to be seen.