Ground Effect 2.0

A story for the advanced speedsurfer / recordhunter or interested windsurfers. This story involves a lot of techtalk and recordhunting stuff (just remember speedsurfing is NOT only about records).

Since almost 1 year after witness some amazing stuff but also rumors I believe there is something special in the (dutch) GPS speedsurfing scene, maybe caused by the origine of the Netherlands (living on a delta of rivers with a lot of soft ground (soil) like clay, peat/veen). There is a new ground effect and some speedsurfers used this principle for a longer period of time (without knowing?). The "ground effect 2.0". You go faster, its easier and can be done with current equipment. Really?? Anyway I decided to share the story and hopefully get some responses.

The white stripe behind this
"boatplane" looks like the windsurfer 
But first what did we mean with the "original Ground Effect"?
It is about compressing a liquid/gas. At the bottom of a foil/board there is always a high pressure. Ground effect does show when the distance between foil/board and ground is extreme low (with or without water inbetween). Example: when using waterski and behind a speedboat entering a spot with 5cm deep water instead the normal 100cm, there will be more vertical lift and therefor drag surface will decrease. In other words at one point it become much better.

Can we use the original ground effect for a windsurfer?
I can not imagen a situation where the water is shallow enough for the ground effect on water. Also with any chop it will not be possible for ground effect in combination with water. Compressing water is no option. But the boardshape in the AIR is VERY interesting, because we can use the board and benefit from some ground effect. We use a little bit ground effect with the current boardshapes, just think about the white area behind the board. If the water is choppy enough it is possible to make a little airflow underneath the board for less drag (the original ground effect).

Ground Effect 2.0* ==> what ??
Ground Effect 2.0 this is speedsurfing and running a part of the fin IN the ground. Example: a speedsurfer sailing in 20cm water with 23cm fin. What do we need for a positive effect, a soft kind of soil like clay, it could be 50/50 mixed with some fine sand (It should be soft enough to just sink in a few cm when standing there). Also the ground (bottom) NEEDS to be reasonable flat. How can running the fintip in the ground be a REAL benefit?

Effects (theory):
  • Maximum Efficiency on the finpart still in the water. When using a 30cm or smaller fin in less than 30cm deep water, you can imagen VERY little chop can build. Soo maximum efficiency for the complete fin surface, not average xx % out of the water flying between chop.
  • Fin does generate extra drag, when the fin gets deeper in the ground the drag increases fast. A lot of speedsurfers will know what I mean, at one point you feel it is going totally wrong. In medium/ rough sand it would happen in 2-3cm or soo.  You can imagen any small underwater sand dunes can lead to HUGE crashes.
  • The fin/board angle of attack stabilises and will be less on average. Normally on chop/ open water the angle of attack is anything BUT consistent. This little bit less angle of attack will generate less drag, but  NOT near enough to compensate this drag the fintip generates. The stabilisation is important, I remember a extreme safe feeling, possible to generate MUCH more forward forces with the sail. This is big benefit NR.1, good enough to compensate the extra fintip drag.
  • Last effect is the vertical lift becomes a bit more at the back of the board without losing grip (horizontal lift). More vertical lift without losing grip, if there is anything THIS is the key to records. For your information: very well trained speedsurfers can pull the back of the board in the air with the backhand/ harnass. Extremely dangerous stuff (remember this Albeau crash during his recordday)
Experiment description (I simply had to have some real world experience):
Tested on a tidal spot with very soft soil, using a reasonable proto thin fin, that would break when hitting something massive (for safety reasons). THe ground was tested to be soft enough when standing there my legs dissapeared at least 30cm! In this very soft soil there were shells, when running speedruns I definitly heard hard tapping against the fin. From the moment the tapping begon the equipment started to accelerate significant, it is easy to feel the fastest spot. In this case my fintip was about xx cm in the soil for maximum performance. NOTE: my fin 30cm had scratches up to 10cm from the base without any crash.... Speeds were double the windspeed 38kn boardspeed in 19kn GUSTS or soo.. And in deep water I lost at least 5% speed.

Running the fintip in the ground... Its legal according to the GPS-Speedsurfing records, definitly NOT legal for official World Speed Record. No worries whatever you do (but keep your own safety/health as priority, since nobody does make a living from speedsurfing these days)

General facts that (could) back-up the theory:
  • Strand Horst records lately with extreme shallow water
  • Smashed up fintips that often do records
  • EXTREME close to shore pictures from top riders running at high speed. (where there is clearly no waterstate difference the first 8-10m from shore)
*Ground Effect 2.0 is no proven theory, I believe to see some experiences/facts pointing towards this theory. The size of this possible advantage is also not clear. Maybe it is only good for accelerating up to 35-40-45kn...

Erik Loots

Erik is windsurfer for 10+ years. In his daily life he is professional in construction dewatering, advisor, troubleshooter. Erik likes adventures, explore and to challenge himself. During his life he is trying to get the best out of it and have respect for the earth, nature and future generations. The modern world is about sharing, in this blog Erik shares his experiences, selfreflection and lessons learned.

6 comments:

  1. I have often wondered about the comments made about doing a better speed after smashing a fin tip. I was thinking it could be something to do with turbulent flow at the tip creating less drag - like the foils they put on the tips of aircraft wings which save a lot of fuel..........
    I do not want to experiment with this on my new caspar elite 27 & 31cm fins though ;-)

    ReplyDelete
  2. the effect you are talking about is not actually called "ground effect" as your foil is vertical to the ground.... it's an endplate effect.
    the only way you can have a ground effect on a windsurfer is between the bottom of the board and the seabed you are sailing over.

    Boogie
    www.C3-fins.com

    ReplyDelete
  3. Lea, you are right your Caspar Elite fins are way to expensive for that kind of stuff! As I told, it was a pretty much old and forgotten proto which I used. But it did at the end far over 40kn average which is fast for a 30cm foil...

    Boogie, thanks for giving it the right name. Soo with ground effect 2.0 I mean the endplate effect. Never read a lot about this effect though.

    ReplyDelete
  4. Studies have been done on keels for sailboats, the effect is the same as "closing the gap", it increases the aspect ratio of the fin/sail which increases the Lift/Drag ratio. I believe this starts to be significant on a "gap" size of 2%, so for a 30 cm fin it is theoratically noticeable when you have a gap of 0,6 cm. However when surfing over a soft soil the effect diminishes again since a soft soil behaves like water..
    Better go for an asymetric foil for maximum efficiency!

    ReplyDelete
  5. So this is interesting. How does it apply to kiteboarding then? I heard that the WSSRC claimed kiteboarders had this advantage although they have never presented the papers to back it up.

    ReplyDelete
  6. the result you're referring to isn't really known as "ground effect" because your own foil is actually up and down towards the floor.... it is a good endplate impact.





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