On Saturday 4th of October the world speed sailing record (on water) was increased to 50.56 knots (93.6 Km/h) by kite surfer Alexandre Caizergues (FRA) at Luderitz in Namibia. This phenomenal achievement eclipsed the previous official record of Robert Douglas (also a kitesurfer) at 49.84 knots. Before Robert, the record was held by windsurfer Antoine ALBEAU (FRA) at 49.09 knots.
So why is this phenomenal? Well, it is phenomenal because it is an incredible feat of skill, strength and endurance. The record was set in winds averaging around 40 knots obviously with higher gusts. To stand up in this is difficult, but to control a kite and board over 500m (the record distance) is amazing. Furthermore, to record an average of 50, you need peak speeds well above this, and according to Alex’s GPS, he hit 58 knots peak speed with a 54.5 knot average over 200m. This is skill indeed and we at Greenbird salute you all.
So what’s with the 50 knot barrier? For years speed sailing records were set in boats, then later sailboards. Both these require a foil in the water to generate the side forces necessary to counteract the force from the sail. As the boats and boards got faster and faster, the rate of increase per record decreased and it seemed like we were approaching a theoretical limit due to the amount of force a water foil can hold before it cavitates (or more likely ventilates). To avoid a very lengthy technical discussion on the theory of cavitation and ventilation, it quite simply means, that at some point (at very high loading) the flow detaches from the low pressure side of the foil and almost all lift is lost, while extra drag is created. This transition normally results in catastrophic wipe-out, so lots of work was put into trying to delay this transition. We all also knew there are such thing as super cavitating (or fully ventilated) foils that work efficiently at high speeds way beyond the transition speed for conventional foils, but these are very poor at low speeds so switching from one type of foil to the other was always going to be tricky. It became pretty clear that this transition speed from attached to ventilated flow was around 50 knots and hence this was initially proposed as a physical barrier for speed sailors. Strangely enough, it turned out that ventilation was successfully delayed on windsurfer fins and the fastest boat, Macquarie Innovation and the limiting factor became drag. Had they been able to reduce the drag further and increase power, it would have come back to ye olde foil ventilation.
So why have Kite surfers eclipsed it so seemingly effortlessly? Well, a couple of reasons. Firstly, the kites are higher in the air than a windsurf sail meaning they are effectively sailing in higher wind speeds than their windsurf counterparts. Secondly, the boards already work exactly like a super cavitating (or fully ventilated) foil which uses mass (or momentum) transfer to achieve a reaction and hence side force. Using this principle, there is no hydrodynamic limit other than drag (air and water) factors to limit top speed. Therefore, I predict a new barrier will soon be established, determined by the sum of the aerodynamic drag from the kite, line and person, plus the hydrodynamic drag from the board.
I am not going to propose what that new speed limit might be, but there will be a limit ,and once the technical evolution is fully evolved (as it pretty much is in windsurfers) the top speed will be directly proportional to the wind speed. So, we may see different record categories for different wind speeds, which would give more credit to technical efficiency rather than just skill, body weight and brute force.
So will that be the final barrier? No far from it. The problem with kite surfers is that the maximum kite force will always be limited by the weight or strength of the person. To get real ‘pull’ you need proper ‘grip’ in the water and this can only be done with some kind of submerged foil. Once you get this to work, you can have a more powerful, efficient, high aspect wing in the air. Then there is a whole new ceiling. Maybe we wil be talking about the 100 knot barrier in a few years?
So how is it done, exactly? Don’t think I am going to tell you in a single blog, but one thing for sure, once the land and ice records are done, I am very much back on the case!
Here are a few links of interest, showing some of the different principles currently being tried by teams around the world:
Sailrocket – http://www.sailrocket.com
Macquarie – http://www.macquarie.com.au/speedsailing.htm
MoS – http://www.mastersofspeed.com
Luderitz – http://www.luderitz-speed.com