I recently noticed all the GOAT prone foils have a wingspan of about 82cm give or take 1-2cm. All with different areas and aspect ratios. What gives?
Lift 90 ha- 81cm
Sk8 850- 82.5cm
Silk 850- 82.4cm
Eagle 690- 82cm
Army 680 HA- 81.8cm
Axis art v2 819- 81.9cm
Army MA2 790- 81.7cm
Unifoil p100- 82.4cm
Code 720s- 83cm
Code 810x- 81.4cm
Miss any?
Adrian always said span was more important than area… maybe he’s on to something.
I’d suggest that 82 is not “best” but more like a rough maximum span to get acceptably responsive roll and feel.
We are trying to always minimize span for turning while we still want accessible pump.
I agree with Chris in that it represents the limitations of reducing span and still providing pump.
The exception is the new flite 707 as it reportedly pumps better than Army 680 and eagle 690 and would have a much reduced span compared to the rest of the list.
It can only turn a lot better.
707 span is 77cm 
. First protos of the 606 pump good too 
Not bad. Feels like how 6’0 surfboard is a general central point for surfboards.
Bet it turns better but what magic physics did the 707 unlock to pump better?
That tracks. I’d say the 750 SK8 (77cm) and smaller are actually the SK8 sizes that feel good, and the 750 retains plenty of pump (actually, even the 650 does). 850 feels too slow, regardless of span.
I’m sure some of the newer foil sections are refining the right balance of speed, responsiveness in turns, and reliable pump. At the end of the day, I rather wrap turns to the beach on one wave and paddle back smiling vs. connecting a million waves without a single satisfying turn.
I ride fone mostly currently.
The 690 eagle pumps a lot better than the 650 SK8 yet the eagle is not what you would call reliable pump in messy water. The 650 SK8 is a real challenge to pump unless you get an ideal exit and have clean water yet its the right size for me to surf in most conditions unless its really small. The 707 seems to be in another league compared to these and the army 680 for reliable pump. It would be great if there was a metric around this as some foilers would be surprised by the differences.
Unlike in the early era of prone foiling where everyone just wanted to pump around the lineup, I think most prone foilers are willing to sacrifice as much span as possible to where there is only enough pump to get a 2 or 3 for one, but it has to work in messy water.
KT nomad
700 = 78cm
830 = 86cm
Given your weight that the foil needs to lift, wingspan is what determines induced drag. Induced drag makes up about 2/3rd of total drag at low speed, and about half at mid speed. So while profile, planform, anhedral and AR will impact takeoff performance, span will determine most of the drag you experience.
Of course wingspan also impacts roll resistance: how quickly you can turn. A designer can design a very tapered foil shape and play with anhedral to gain 1 or 2 cm span without increasing roll resistance, but that is difficult to get right and has drawbacks too.
I think that is why you are right that foils with similar span will have very similar performance at low speeds (meaning just above takeoff), which is most relevant for prone foiling. A foil designer can only impact 1/3rd to 1/2 of the drag by playing with profiles and foil shape, and most designs heavily optimize early takeoff because that is where gains can be made.
At high speed, it’s very different: viscous drag becomes bigger than induced drag and dominates total drag. At the top end, it’s all about having a small area and low drag profile and span doesn’t matter much anymore.
Maybe 80-ish cm is about optimal for an average-weight rider.
But then it shouldn’t be optimal for lightweights or heavyweights…
I notice this first hand when I’m wing foiling vs kite foiling. The same foil feels slower to turn when kiting, I think the kite takes off some weight.
The second moment of area (not sure if thats the right term) would be a useful stat for determining how wing rolls. Essentially a metric for measuring the distribution of area against how far from the fuselage - thin tips = easy roll
Wing tip breach at our typical mast lenghts and lean angles is IMHO the biggest factor.Modern foils can tip breach and keep going but they all work better fully submerged efficiency wise.
A less-wide foil will turn better - generally speaking. Adrian is right, that is the most important number for TURNING. Anything else and it’s not. If you think 80cm is the magic number wait until you try 70cm! Since most people are not rippers and are mostly concerned with stall speed, why axis would use that as the main number to describe their foils is… interesting…
There is a simple tradeoff on narrower wings and size and pump ability. If you can’t pump back out, then prone foiling is a drag. If your foil gets large in area but still small then it is a slow foil based on the AR and the chord length, so the sweet spot for people is IMO, 90cm or less.
I ride a 9AR 700cm2 foil which is about 79cm wide. It rips. My 90cm wide 10AR 800cm foil also rips and pumps out better. I can’t really pump the 700 all the way back to the top 300 yards into the wind, but I can on the 800 barely.
Prone foiling is pretty rough and demanding, so I can ride a 540 or smaller but I could only pump back one wave, not back to the top. Then I’ll have to wait my turn again lameeeeee, otherwise I’d be on that all the time.
If you took a great surfing foil with an 82cm wingspan, why couldn’t you use the same plan shape but adjust the foil section to be thicker for slower/heavier rides and thinner for faster/lighter rides? I know there’s a lot of give and take with design parameters but I wonder how much the glide is affected by the aspect ratio vs the thickness of the foil sections.
I think “Bending Moment” is most commonly used to express this. It is used to calculate the bending load at the root of a wing, but the same thing also opposes roll. Ludwig Prandtl proved in 1932 that a wing having Bell shaped lift distribution (not elliptical) results in the minimum induced drag against bending moment. In practice that comes down to a more tapered shape.
This is my quiver for La Ventana this January. I reckon it will cover every condition and style of riding I want to do. 75kg winging.
I won’t ride foils over 1m span. Every time I have tried it feels terrible. My largest foil is the AV2-999 and it feels like a dog. I only ride it when the wind is very light. As soon as there is any wind I get on a sub 900 span foil.
How’s the surge?
I haven’t ridden it yet!
