Comments for Experimenting with Rudders at http://www.surfski.info , comment 0 to 12 out of 12 comments http://www.surfski.info Fri, 04 Jul 2008 22:37:17 +0100 FeedCreator 1.7.2 http://www.surfski.info/content/view/682/147/#pc_2274 Hi Dale, For completeness how about testing another version where the tip of the rudder extends in front of the rudder post as in this aircraft rudder? http://www.diracdelta.co.uk/science/source/r/u/rudder/image002.jpg Thanks for the excellent article, I find this sort of experimenting and research fascinating. - Alain Jaques Wed, 23 Apr 2008 16:00:54 +0100 http://www.surfski.info/content/view/682/147/#pc_2273 Hi Jonathan Comparison to skeg hung rudders is unavoidable. My analysis (admittedly without any detailed knowledge) is that “keel rudders” died along with the low aspect keels that were the standard in yacht design in years gone by. There are however many reasons to conclude that the factors that drove the evolution of yacht keels do not apply to ski rudders. Keels and rudders effectively work against each other so basic mechanics suggests that they should be separated - a yacht turns to port by the interaction of the lift of the rudder to starboard and the lift of the keel to port. The only question is therefore how far apart to put the rudder and the keel and this just a trade off between manoeuvrability and directional stability. Beyond this high aspect keels are way more efficient in terms of lift to drag even ignoring the considerable benefit of displacing the keel weight lower. None of the above applies to ski rudders. The only criterion are to produce the most tuning “lift” with the lowest drag through the range of operation, achieve the broadest range of operation without stalling and minimise unhelpful rudder characteristics such as rudder induced roll and rudder “bite” My findings to date are that an articulated rudder is best with all but the first criterion. The trouble is that the first is the most important one so some compromise is necessary. I see 2 solutions: 1. The V1 rudder together with a ventral fin under the seat. This will replicate the keel/rudder effect as in modern yachts. I plan to do this over the weekend but expect partial success and don’t really like such a complex solution anyway. 2.V3 which I have always believed is the best but which was saved for last because it is inherently a compromise. As regards scientific proof Anton Erasmus at Red 7 has forwarded details of my experiments to Intelligent Fluid Solutions in the UK (they do computer simulations for Red 7 ski designs) and I will post any feedback that they provide. As I mentioned in my article the nose mounted hydrofoil still needs to lot of development. The thinking behind the concept was that the volume required to lift the nose of a ski downwind could be replaced by a hydrofoil. The expected benefits lie in the fact that hydrofoils are way more efficient (by a factor of many times) than displacement hulls in terms of lift to drag. The hope was that the volume of the nose of the ski could be dramatically reduced and the huge braking effect of a buried nose could be eliminated. I envisaged a low volume nose similar to a K1 sprint canoe with a sharply angled deck designed to shed water. The hydrofoil would be mounted on top of the deck and only come into operation once the nose started to bury. The trouble was that I did not have a ski that I could afford to cut up and modify so I simply mounted the hydrofoil on top of the nose of my V10. As expected even a small hydrofoil easily produced enough lift to stop the nose burying but there was a problem. I had set the hydrofoil at a shallow positive angle so that it would not produce too much lift and drag when paddling into a chop. This was fine in typical downwind conditions but was a liability when coming down the face of a steep shore break when the hydrofoil actually delayed the release of the nose (hydrofoils are as good at holding things underwater as lifting them out of it!). The obvious solution is to have a spring loaded pivoting hydrofoil This is a lot less difficult than it sounds but requires more modification to the ski than I am prepared to do hence my comment about being on the lookout for wrecked ski to use as a test bed. Until then this experiment is on hold. Regards - Dale Lippstreu Wed, 23 Apr 2008 15:37:02 +0100 http://www.surfski.info/content/view/682/147/#pc_2263 I love to see people try new ideas. We all have ideas, but most never leave our heads and become reality. Kudos to you for taking your ideas beyond the napkin stage. Unless I am missing something, this concept is simply a skeg hung rudder. Skeg hung rudders were quite common on older sailboat designs as an intermediate step as they evolved from full keels to modern fin keels with spade rudders. The purpose of the skeg was to provide protection to the rudder in case of impact with a submerged object. You still can see skeg hung rudders on modern boats aimed at the more traditional market who put more of a value on safety over performance. Unless I am missing something about this concept, it will be difficult to convince me that a skeg hung rudder on a surfski is more efficient than a standard fully articulating rudder. If there was something to it, you would already see it done on racing sailboats. Serious racing sailboats have used fully articulating spade rudders for decades. If there are examples of racing boats with the occasional skeg hung rudder then it was likley for protection against impact, not for any hydrodynamic benefit. Despite my skepticism, I would love to be proven wrong with a scientific study. If you could find some magic bullet that would reduce drag and provide protection of a skeg, then you would have the attention of the whole marine design community. I would like to hear more about your testing of a foil in the middle of the boat. I believe there could be some benefits to combining a fixed foil near the middle of the boat and a normal rudder aft. The benefits would be in a cross breeze, however it would likely be detrimental when on downwind runs, so perhaps not practical for surfskis. Unless it was retractable like a small daggerboard. Keep experimenting. Its cool to see. - Jonathan Smith Tue, 22 Apr 2008 20:54:49 +0100 http://www.surfski.info/content/view/682/147/#pc_2262 Hi Kirby The idea for my ventral fin came from a friend who related how he had very many years ago fitted a "dagger board" type fin to an old Hammerhead ski in the hope that he could make it more stable (he was a complete novice at the time). He was surprised to find that the fin did not affect stability at all but the ski "turned on a tickey". My purpose was different in that I was rather trying to reduce broaching when running downwind and I was looking more to improve directional rather than rolling stability and the fin was successful in this. Based on my friends and my own experience I do not believe that a ventral fin will do much to improve the rolling stability of the ski and so make it easier for your friends to get going. regards Dale - Dale Lippstreu Tue, 22 Apr 2008 15:15:49 +0100 http://www.surfski.info/content/view/682/147/#pc_2259 Hi Dale, Any news on how your ventral fin worked? I have an older ski I want to fit a skeg / fin to as a trainer boat to try anf get some friends introduced to the sport. The boat is about 14 years old and has a very shallow seat, and rounded hull profile so primary and secondary stability is poor and she is very tippy. Would this help for stability and how much would turning performance suffer? Thanks Kirby - kirby mackintosh Mon, 21 Apr 2008 17:41:27 +0100 http://www.surfski.info/content/view/682/147/#pc_2254 I tried V2 this weekend and was very surprised that despite increasing the area of the moving portion of the rudder by 40% there was no discernable difference in performamce. Both paddles were in downwind conditions and I still want to test the turning radius before changing to V3. At this point my conclusions remain unchanged i.e. the articulated rudder produces smoother progressive actuation and all round better control downwind but at the cost of reduced manouvreability. This reduced turning radius is explained the fact that the rudder cannot achieve the same effective angle of attach as a standard one peice rudder because a portion is not deflected. The improved performance is explained by the fact that rudder does not stall when deflected rapidly. - Dale Lippstreu Mon, 21 Apr 2008 09:54:10 +0100 http://www.surfski.info/content/view/682/147/#pc_2253 Paul, move the post back ... further than you 'think' you should ... some of the 'leading' or blade in front of the post will will act as a 'powersteering' counter force and aid to balance. Take a look @ my rudder blades again ... can be same area as stock Epic, swept back to 45 degrees AND still be 3mm deeper. 20mm deeper @ 50 degrees and 33mm deeper @ 55 degrees. - onnopaddle Sat, 19 Apr 2008 14:32:02 +0100 http://www.surfski.info/content/view/682/147/#pc_2248 Where I paddle in Sydney we have a lot of problem with weed, especially an introduced South Africa weed, so to counter this we add weed deflectors that end up looking just like your designs. - Tom Balaam Fri, 18 Apr 2008 03:30:19 +0100 http://www.surfski.info/content/view/682/147/#pc_2246 I also like playing with rudders and I've thought about building a rudder just like that. Your results sound encouraging so I might pursue them myself after all. Here in So Cal our biggest problem is eelgrass so I'm trying to find a rudder that sheds grass but is still efficient and balanced in downwind conditions. The problem with most swept rudders is that the feel becomes heavy due to most of the area of the rudder being so far behind the post and the rudder area is further towards the stern which causes the rudder to come out of the water in steep conditions. Your rudder might be able to help those problems. - Paul Fri, 18 Apr 2008 00:02:10 +0100 http://www.surfski.info/content/view/682/147/#pc_2245 That reminds me of the (skeg hung) rudders used on racing yachts from the 60's. - Ritchie Cunningham Thu, 17 Apr 2008 23:54:49 +0100 http://www.surfski.info/content/view/682/147/#pc_2244 Has the surface area between the original rudder vs. this new rudder been held constant? If not, the improved turning might simply be due to the fact that the rudder is bigger. - Thomas Yonley Thu, 17 Apr 2008 18:37:25 +0100 http://www.surfski.info/content/view/682/147/#pc_2243 Great article Dale... Your low aspect ventral fin that I fitted to my Mako6 has proved invaluable to me. I'm sure that you will find the best rudder variation to suit over the next few weeks. One wonders how long it will take for the major manufacturers to catch up to your scientific design ideas. Well done! - David Cade Thu, 17 Apr 2008 18:18:04 +0100