Hi everyone. Firstly thank you to everyone for all the huge amount of information on this forum, it’s really helped my foil journey (still very much on the low end!).
I was wondering what foil people are using for what energy on any given day when prone - or even if people take note of energy (KJ) at all? I know there are a lot of other factors at play eg tide height, bottom profile in terms of how the wave itself forms - also rider ability in managing a foil but in terms of pure energy is there a range best suited to the 125/140/170/200? For example is 500KJ always going to be too much for a 170 and 50KJ always too little for a 125?
While it could be useful to integrate wave heigh and wave period to have a good reference for wave size (or power) on the same spot, there a few problem with using this metric to choose your foil size.
Wavepower is typicaly used for deep water wave (depth more than half wavelength)
It would be tidious at best to calculate wave power for each spot because you have to factor in wave transformation from deepwater to shallow water (refraction from the seabed, wave shoaling, and a bunch of other factor).
Wavepower on your foiling wave would be hard to calculate but easy to understand : think about your different spot on the same swell, some break can be so much bigger and powerfull than others (classic case is Black beach in California and Nazare in Spain). In that case, your (or others) experiences foiling on the same wave as you would be a far easier (and probably better) proxy than actual wavepower to choose your foil size.
If you really want to use wave power, you could calculate it for a deepwater point near your spot (or if you use Surfline, I think they use it in there forecast display). Than you could note if you where underfoiled, correct or overfoiled. You could than to that exercise for every spot that you ride…
Sorry about the nerdy stuff. I’m a coastal geomorphologist and analyse wave data in my work. And take my answer with a grain of salt, I have never foiled elsewhere than my locals that consist of bay runs and semi-enclosed sea. Plus, I’m not and expert foiler. Maybe others use wave power as a proxy for foil size…
For my weight 85kg and the foil im using best wave kj are around 250 , range is 150kj to 500kj max. 150 kj is more for my 7s 1400cm2 or my enduro 1100 11 ratio , 500kj more a 900cm2 sk8 950 for exemple. Wave range from 40cm to 1m max
As said here, the energy calculated is just an approximation of the swell’s energy offshore. Depending on bathymetry, one spot or another could varie wildly, and if you take into account swell direction, even more so.
What I would recommend is the same as @LaPaloude try to anotate your sessions for different swells at the same spot to see what works best for you.
I’ve got a 1350cm^2 mid aspect and feel i can get overfoiled at around chest high with it. On another 800cm^2 High aspect, i can still feel good with head high waves, haven’t tried more than that, periods between 10 and 12 mostly, i would say.
I take every data i can, website are supposed to compile all the data , sometimes it’s good sometimes not.
no model is perfect, they have some issue with sheltered spot most of the time.
I cross check on other website too (no kj on those)
I have good Result with period between 8sec to 10 sec i noticed too, best tide coef around 40-60 a good session is all the above and little bit of luck.
A pattern for a good session would be 40 to 60 tide factor (not sure how you said in english) 45 to 85 cm, 8-10 sec period, 150 to 500 kj , from side on to side off, wind below 10 knots , everybody got it’s secret sauce dont juge
of course those apply on some of my spot but could be totally different for your.
Ps: they are outliers too, it’s not an exact science.
Not judging. I’ve done the same for kitesurf wind predictions over the years. I prefer to stick to one forecast/source and adjust my prediction based on experience over time.
RE the “40cm” wave height - my first engineering boss was an old German guy who used to say “only engineers will calculate to the 4th decimal and then multiply by a safety factor of 3”
Thanks for the comments, especially the science detail - keen on this! I think the “know your spot” philosophy is probably the best one. Just done mine in 1ft 10 sec with predicted energy of 12KJ and had a great session on the p170 with glassy conditions. That was off Surfline. The other day it was barely rideable in a 3ft 6 sec onshore windswell and 70KJ.
So my only contribution was kinda pointless, but hopefully it’ll satisfy another person at some point!
Energy and wave size aren’t the biggest drivers of my foil selection. I ride a north SF 930 and a 680. In longer period , bigger “energy” conditions the wave is usually on the nose, closing out, and there’s not beneficial wind so I end up on the 930 for that extra pump - to dig out of a double up I’m forced to take off on because the shore break is sketch, to pump out the back to a line that is farther away, etc. Also bigger longer period energy moves faster so there’s more pumping for every second on wave and it always feels to me like the faces have more energy and the backs somehow have more negative energy to pump through. 2.5 at 11 secs I want my bigger foil.
I ride the 680 anytime pump is not a priority or assisted by the wind. The smaller foil eats up small waves. Glide, fits in the pocket better, tighter turns on smaller faces. 2.5 at 6 seconds with the wind at my back - give me the small foil all day. Also that small period is WIND SWELL so usually there’s wind with it so yeah wind assist and for us it is usually running down the beach better so fewer closeouts. For those that say this slow swell is too slow for the foil you just need to do more turns so you cover more distance than the wave! If I can’t get the 680 up then yeah I fall back to the 930 but if there’s wind at my back that threshold is low.
I’m no pro by any means, far from it, I look at both wave size/period (windfinde pro) and kJ (windy.app pro) forecast , on top of knowing local conditions.
Depends also if you want do straighter gliding runs or wave riding in the more critical sections and what kind of swell section your riding, glide between sections, wave speed etc.
Rider weight and skills too obviously.
Another topic where there’s no “general” perfect foil
I personally have been using windguru for 15 years for surfing, and the amount of information you can gather is better than any other app or website. It is definitely tricky to read and to know what is what, but I usually only look at the primary swell and the wind forecast from the model AROME 1.3km, which is the most precise for my location. They updated recently and now it also shows swell energy!
There has to be some crazy refraction at your spot to be able to surf with just 12 KJ! I don’t know what would be the minimum for my spots, but I think at least 50 to be able to catch something.
Another spot a little bit more sheltered start to be foileable at high tide with 150 or even 200KJ if the direction is right.
Do you think it has to do with the coefficient or is it just that mid tide works best at your spot? I’ve never felt the tide coefficient be relevant for the same given height on the tide, just that the session is shorter on a higher coefficient because the tide comes and goes faster
I also use https://www.surf-forecast.com/.
Their “energy” indicator (a mix of swell, and period) is reliable indicator of swell size, it’s stable tough time.
But it doesn’t tell you much about the wave size, which is dependent on underwater features (reef, sand bank, tide), so no, the same energy can lead to dramatically different forces of waves.
Wind also plays a huge role, flattening waves, or curving them.
you are right, this a disclaimer that surf-forecast put if you check the ? near energy
So you have to check what going on at your spot and take note when a session work, that what i did for my spot for like 1 year, trying to figuring out what make it work and noticed that each spot were different.
Wave energy (in kilo Joules, kJ) on the swell forecast tables and maps indicates the power of the predicted waves and can be the most useful guide to how powerful the surf is likely to be at your local beach / reef. It is a function of both wave size and period (the larger the waves and longer the period the greater the power). Small ocean waves with a long period can create the same size surf as larger waves with a short period but they can have the same wave energy, making it a potentially better guide to likely surf size. We would suggest making a note of the forecast wave energy on a small but surfable day and on a day that is the biggest you are happy to surf. This should give you the energy range that is in your comfort zone. As a rough guide, wave energy of 100kJ can be just about surfable at many breaks, 200-1000kJ should produce increasingly punchy waves while 1000-5,000+ can start to get really heavy and even dangerous at some breaks.
