From memory, I'm pretty sure Robby Naish set the world record on a single fin directional though - is that not the case? Maybe because it was long and thin, it was acting like current speed boards and getting its directonal stability from edging.
ronnie wrote:From memory, I'm pretty sure Robby Naish set the world record on a single fin directional though - is that not the case? Maybe because it was long and thin, it was acting like current speed boards and getting its directonal stability from edging.
I think we are talking about two completely different things here...
Speed boards are going on a reach or broad reach if windy, and only need so much fin area so they can keep good track, but apart from that as small as possible for control and less drag
With a race board you want fins as big as possible (almost), that you can still control reasonably both upwind and going faster downwind, and the increased fin drag is outweighed by less "board surface" drag and a higher angle to the wind. Of course there is a limit where the angle dont increase really, but fin drag does - so here we have met the "roof" for a specific board/fin type. But still - a huge amount of fin area is desired
So dont make sense to use more than one fin for speedkiting
i am onto this topic for one and a half years now. i tried a lot of different configurations since i could not find too many reports on different configurations with detailed explanations. i started with a 09 twoag hellracer with single fin.
these are in short the result of my tests for single fin: - as mentioned before: the direction stability is quite bad. especially going upwind in choppy water. - if you go upwind and ride the board flat the fin is too far in the back. you have to bring all your weight to the back foot. bad for back leg and not good for low wind because board sinks in too much. if you try to compensate this by riding you board angled you loose too much speed. of course i tried a single fin with the fin further in front, but the board gets very unstable when going downwind. it sometimes just jumps into one direction. very unfunny. - if you angle your board for a sharp turn the fin is not in the water all the way so it looses the grip. + downwindspeed ist just great. no turbulances in the water what so ever. the board is just gliding. the differenece compared to a quadfin is amazing especially on flat water.
i am about to finish a new (for me at least) concept at the moment. i will let you know how it works. personally i hate the quad- and trifin concept although they seem to be the best compromize at the moment. in my opinion more energy has to be put in other concepts. i want to go upwind like hell and also want to have fun downwind.
Last edited by holden on Fri Sep 17, 2010 10:06 am, edited 1 time in total.
@norm: this was my next approach. i tried many combinations of different finsizes and shapes.
these are in short the results of my tests for 2 fins on center line: + direction stability is solved + point of combined force of fins (please someone translate lateralschwerpunkt!) can be controlled by finpositions and finsizes - if you angle the board for a sharp turn no fin fully in water = no girp + downwindspeed great - the biggest downside of this setup is, that, as mentioned earier in this thread, the size of the front fin is limited. upwind is no problem. take a 30 cm fin and you are still fine. but downwind is just impossible. the maximum size of the front fin with which i was able to go downwind rather ok was 20 cm. in combination with a 28 cm back fin i still had too much load on my back leg. if you take smaller fins you do not have enough grip for hardcore upwind.
@ronnie: i don't think this shape will cure the problem. never tried it but i just don't think it can replace a 30+cm racefin.
@tautologies: this was discussed in some other forum recently. someones argument was that those little dents, like on golfballs, help to reduce drag on objects that create big turbulences due to their shape (sphere). the little dents kind of split the big single turbulence into many little ones which seem to be better. in the 80is some motorbike helmets also had those little dents. and they worked great too. since the other bikers laughed too much at the design it disappeared again.
by the way, i also tried weed single fin. better, but does not solve problems.
Nice work holden, no substitute for getting out there and testing prototype ideas!
My old man(40yrs as a hydrodynamicist/engineer) explained to me at some length that single fin[sailing] configurations are actually directionally stable when sailing off the wind and can theoretically be trimmed for a given course without constant correctional control input, but that upwind they become directionally unstable and require constant correctional input[rider work] in order to maintain a desired course. this fine constant correctional input is probably easier on a windsurfer with the longer moments resulting from the spread of mastfoot and rider feet. This wasn't based on any practice or anything, but was core to the physics of the thing, which he was able to explain to me with a lot of trig/equations and vector diagrams. I wouldn't be able to replicate the explanation now i'm afraid...
i always wondered why two big in-line fins hadn't proved successful on the racing circuit though, but thanks to your work holden i think i get why it is proving difficult. However, i do reckon though that with more developement and competition a balanced(upwind+downwind) in-line twin concept will be developed over time. There is a reason why almost all other sailing vessels use two-inline fins and planes have a wing and a horizontal stabilizer. it is the ultimate compromise of stability and reduced number of surfaces(and their efficient use).
there is ofcourse the elephant in the room here, that does solve your problems of compromising downwind speed and upwind lift in a two fin configuration, which has proven successful for many years for many people:
whether that is a direction we don't want to go in in this sport remains to be seen. if there is a competitive advantage you can be sure people will try it, but if that makes the top end competition less fun and even more inaccesible due to big heavy complex gear then maybe we'd all benefit from some class rules or something.