I wouldn't call those "extreme" AoA's, because we encounter them quite often in foiling. For aviation, you want to avoid being close to stall speed, but in foiling I'd say we get close to (and exceed) stall speeds quite often (for example when pumping prone, DW sup/prone, or when winging when the wind drops on you..)

And close to stall speed the rear wing is definitely creating upward force. Even for asymmetrical profiles, the Cl is zero at alpha's somewhere in the range of -1 or -2 deg.

So if we assume a foil setup where the front wing is set at 2 or 3 deg (wrt the fuse), and the rear wing is set at -1 (wrt the fuse) and has an asymmetric profile (lets say it has Cl=0 at -2deg), you can see that if the foil set up going through the water at 3 deg AoA or more, the rear wing will be generating positive lift. At 3deg AoA for the foil setup, the front wing will actually be at 5 or 6deg AoA, which is way below stall speed, and I wouldn't call that "extreme". Stall angles are around 12-15 deg for most profiles used in foil designs.

Knowing that at stall speeds the rear wing is definitely providing positive lift, this means that obviously a bigger rear wing means you'll have more upward force at the stall AoA. So therefore your stall speed will be lower than if you had a smaller rear wing.

HOWEVER, if we take prone foiling or DW'ing, we don't have an external power source. If we assume a body of flat water and we're up to speed and then we let go of a tow rope and we just try to glide for as long as possible, our speed will start decreasing because of the drag the foil system experiences by moving through the water. As our speed decreases, we shift our weight backwards to increase the AoA, up to the point where we stall. So even if our eventual stall speed might be lower by using a bigger rear wing, the added drag means we get to that stall point much quicker, especially given that V is squared in the lift equation.

It would be pretty cool to model such an unpropelled "glide" of a foil system going through a body of water. Starting at a certain speed, with the requirement to maintain a certain altitude (either level, or drop by max mast length). It would have to be a dynamic model where the only control input is the x position of the cg of the rider, which would then influence the AoA of the system, which would then change the L and D of the front wing and L and D of the back wing, up until the point where stall is reached... (one day if I'm bored )

Oh, there will a few who will question my contention that 1400HA should have similar lift to 1550 LA foils.
I windfoil more often than wing. Got maybe 100 days on 1220 Naish. Got close to 50 days on 600 Naish HA. With the same stab, lift is within 3mph, either board speed or wind.
Half the sq cm, 600 sq cm less, but very similar lift.

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greg87foil said..
I wouldn't call those "extreme" AoA's, because we encounter them quite often in foiling. For aviation, you want to avoid being close to stall speed, but in foiling I'd say we get close to (and exceed) stall speeds quite often (for example when pumping prone, DW sup/prone, or when winging when the wind drops on you..)

And close to stall speed the rear wing is definitely creating upward force. Even for asymmetrical profiles, the Cl is zero at alpha's somewhere in the range of -1 or -2 deg.

So if we assume a foil setup where the front wing is set at 2 or 3 deg (wrt the fuse), and the rear wing is set at -1 (wrt the fuse) and has an asymmetric profile (lets say it has Cl=0 at -2deg), you can see that if the foil set up going through the water at 3 deg AoA or more, the rear wing will be generating positive lift. At 3deg AoA for the foil setup, the front wing will actually be at 5 or 6deg AoA, which is way below stall speed, and I wouldn't call that "extreme". Stall angles are around 12-15 deg for most profiles used in foil designs.

Knowing that at stall speeds the rear wing is definitely providing positive lift, this means that obviously a bigger rear wing means you'll have more upward force at the stall AoA. So therefore your stall speed will be lower than if you had a smaller rear wing.

HOWEVER, if we take prone foiling or DW'ing, we don't have an external power source. If we assume a body of flat water and we're up to speed and then we let go of a tow rope and we just try to glide for as long as possible, our speed will start decreasing because of the drag the foil system experiences by moving through the water. As our speed decreases, we shift our weight backwards to increase the AoA, up to the point where we stall. So even if our eventual stall speed might be lower by using a bigger rear wing, the added drag means we get to that stall point much quicker, especially given that V is squared in the lift equation.

It would be pretty cool to model such an unpropelled "glide" of a foil system going through a body of water. Starting at a certain speed, with the requirement to maintain a certain altitude (either level, or drop by max mast length). It would have to be a dynamic model where the only control input is the x position of the cg of the rider, which would then influence the AoA of the system, which would then change the L and D of the front wing and L and D of the back wing, up until the point where stall is reached... (one day if I'm bored )

Good post.

I don't know what the exact lift/AoA curve for a typical foil stabiliser looks like. For a Cessna wing you get Cl=0 at -5degree AoA, and that's the only aerofoil I know that figure for. My Sabfoil fuselage has a stab angle of -2.3degrees, I've no idea what the front wing is. But using those figures we're only getting to +ive lift on the stab at AoA at around 7-8 degrees ( assuming a flat front wing).

Where I would disagree is that I don't think that when foiling we do foil at high AoA often, other than inadvertent stalls ( and not at takeoff). At speeds below L/Dmax (corresponding to an AoA greater than ~6 degrees), we are in the speed unstable region, where drag increases with reducing speed ( we call it being on the wrong side of the drag curve when flying).

I have an idea for a shim that deforms at higher speeds, altering the AoA on the stab when going fast so you don't get such extreme front foot pressure.

AoA, AoI, CoG and zero pumping, all other mechanics aside(or equal), if I understand correctly.

A larger 330cm stab, could have 61Lbs of down force, when the 1350cm front wing 250Lbs of Up force in order to lift a 189Lbs man.
At the same speed
A smaller 220cm stab would only have 42 Lbs of down force, the 1350cm front wing would still have 250 lbs of up force which could lift a 208lbs man. But if we only wanted to fly a 189 lbs man, the set up with the smaller stab would be able to do it at a slower speed.

The larger stab also has more drag.... Maybe it's the pumping of the larger stab that adds more forward propulsion, and that's what more than compensates for earlier take off.

Sounds great.
Hasn't worked that way for me in real world....yet.

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greg87foil said..
I wouldn't call those "extreme" AoA's, because we encounter them quite often in foiling. For aviation, you want to avoid being close to stall speed, but in foiling I'd say we get close to (and exceed) stall speeds quite often (for example when pumping prone, DW sup/prone, or when winging when the wind drops on you..)

And close to stall speed the rear wing is definitely creating upward force. Even for asymmetrical profiles, the Cl is zero at alpha's somewhere in the range of -1 or -2 deg.

So if we assume a foil setup where the front wing is set at 2 or 3 deg (wrt the fuse), and the rear wing is set at -1 (wrt the fuse) and has an asymmetric profile (lets say it has Cl=0 at -2deg), you can see that if the foil set up going through the water at 3 deg AoA or more, the rear wing will be generating positive lift. At 3deg AoA for the foil setup, the front wing will actually be at 5 or 6deg AoA, which is way below stall speed, and I wouldn't call that "extreme". Stall angles are around 12-15 deg for most profiles used in foil designs.

Knowing that at stall speeds the rear wing is definitely providing positive lift, this means that obviously a bigger rear wing means you'll have more upward force at the stall AoA. So therefore your stall speed will be lower than if you had a smaller rear wing.

HOWEVER, if we take prone foiling or DW'ing, we don't have an external power source. If we assume a body of flat water and we're up to speed and then we let go of a tow rope and we just try to glide for as long as possible, our speed will start decreasing because of the drag the foil system experiences by moving through the water. As our speed decreases, we shift our weight backwards to increase the AoA, up to the point where we stall. So even if our eventual stall speed might be lower by using a bigger rear wing, the added drag means we get to that stall point much quicker, especially given that V is squared in the lift equation.

It would be pretty cool to model such an unpropelled "glide" of a foil system going through a body of water. Starting at a certain speed, with the requirement to maintain a certain altitude (either level, or drop by max mast length). It would have to be a dynamic model where the only control input is the x position of the cg of the rider, which would then influence the AoA of the system, which would then change the L and D of the front wing and L and D of the back wing, up until the point where stall is reached... (one day if I'm bored )

Wake thief has lots of vids doing something like that, and he measures the distance to see which of the reviewed foils has better glide.

With constant height the model should not be too complicated (brave words :) ) because the lift has to be equal to rider+board weight up to the stall.Decreasing speed,increasing AOA.
I would bet the glidiest setup would be stabless,by the time the back wing contributes lift you are slowing down fast in a very draggy attitude.Add energy or sink :)

Larger stab has a lot more force once away from neutral AOA but I don't think all that much when running neutral provided similar thickness.

Bit of an edge case but on my windfoil setup I'm significantly faster upwind with a bigger HA stab vs a lower aspect one. Performs better in all speed regimes actually. I do not think it would be the same winging however since there is not anywhere near as much need for downward stabilizer force compared to windfoil.

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Fishdude said..A larger 330cm stab, could have 61Lbs of down force, when the 1350cm front wing 250Lbs of Up force in order to lift a 189Lbs man.

Not sure that is true, because the downforce at speed pushes the front up. We have all felt the effect of the faster you go the harder it is to keep the front down. The rear flying down lifts the front and changes AOA then you breach and fly over the front at speed.

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hilly said..

Not sure that is true, because the downforce at speed pushes the front up. We have all felt the effect of the faster you go the harder it is to keep the front down. The rear flying down lifts the front and changes AOA then you breach and fly over the front at speed.

At higher speeds, the AoA on the front wing is lower, but because the stabiliser is upside down, it has a higher AoA, therefore more downward lift, which pushes the balance point (where you need to have your bodyweight) forward. When flying we adjust the stabiliser AoA so that it doesn't produce more downward lift at higher speeds, but you can't do that with a fixed stabiliser like you have in foiling.


Going back to large AoA's, it's worth pointing out that pitch attitude is not the same as AoA. On takeoff for example as you come out of the water, the board has say a 10 degree pitch angle to the water, but is travelling upward at 5 degrees, with a 5 degree AoA. The same will be true with pumping where the board is performing a sinusoidal path through the water.

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BritWindfoiler said..

hilly said..

Not sure that is true, because the downforce at speed pushes the front up. We have all felt the effect of the faster you go the harder it is to keep the front down. The rear flying down lifts the front and changes AOA then you breach and fly over the front at speed.

At higher speeds, the AoA on the front wing is lower, but because the stabiliser is upside down, it has a higher AoA, therefore more downward lift, which pushes the balance point (where you need to have your bodyweight) forward. When flying we adjust the stabiliser AoA so that it doesn't produce more downward lift at higher speeds, but you can't do that with a fixed stabiliser like you have in foiling.


Going back to large AoA's, it's worth pointing out that pitch attitude is not the same as AoA. On takeoff for example as you come out of the water, the board has say a 10 degree pitch angle to the water, but is travelling upward at 5 degrees, with a 5 degree AoA. The same will be true with pumping where the board is performing a sinusoidal path through the water.

Indeed, pitch angle is not AoA, but I would still argue there are many times we are near or exceed stall AoA's. Especially with HA wings, which have a steeper Cl-Alpha curve and lower stall angles (compared to low aspect wings).

For example: if you're not careful using a HA wing and push too hard on the back foot when pumping, you can stall the wing. If you're winging and you go through a lull and the wind drops on you and you desperately try to stay on foil, you will get close to or exceed stall speed and drop off foil. When you're pumping up on foil (winging) but you don't have enough forward speed (yet) and you increase pitch too much, you will stall and drop back in the water. There are many more situations...

To me, the fact that as foilers we care and talk about stall speeds, means we find ourselves in situations where we are close (under, at or just above) to stall speeds more often than not. Otherwise we wouldn't be obsessing about "what stall speed is this new HA wing?" And when we are close to stall speeds, we are close to stall angles.

And I would argue that unless the rear wing is set at a substantial angle wrt the front wing (which almost always it won't because it will feel too draggy), the rear wing is producing an upward force, at stall AoA's or stall speeds.

But then, will a bigger rear wing make getting up on foil easier?

If we assume a stable experiment where a foiler is dragged through the water (from V=0 steadily increasing), where the pitch angle is set at the stall angle (for max Cl) and stays constant until liftoff, at some point when we reach a certain speed the foil will generate enough lift to pop the rider out of the water (once lift force exceeds weight of rider+board+foil). Since I argue that the rear wing at this pitch angle is also producing an upward force, the speed at which takeoff occurs will therefore be lower than if a smaller rear wing was used (which would provide a smaller upward force).

HOWEVER, let's not forget that popping up on foil is not a static movement, and we don't have unlimited power on tap (such as if you're dragged behind a boat). In order to generate enough forward speed to lift off, the drag of the whole foil system needs to be overcome. And in this case, a bigger rear wing is not working in our advantage since it has more drag than a smaller rear wing.

So going back to the OP question "Is it going to make getting on the foil easier?" my answer would be: it could be, but not necessarily. It could be that the size of the rear wing reduces the stall speed (for the reasons explained above), but the added drag could also mean that it's more difficult to reach that stall speed. Obviously there are other factors at play here that imo play a more significant role on reaching the required lift-off speed (board design, size of hand wing).

But for sure a bigger rear wing will make everything more stable, so imo a no-brainer for someone who is learning. Even with HA wings, it just adds stability, even though it adds drag. Drag is not a bad thing when learning.

Greg- everything you write is correct, but I don't think we're ever near those AoAs, and certainly don't want to be. We do stall on occasion, especially when learning. But I don't think that we ever intend to be near the stall, and aren't near there at takeoff if done correctly. It's very draggy at these speeds, and speed rapidly decays away.

This is what drag vs speed looks like, once up on the foil:


There are two regions:
1. Speed stable, where a slight drop in speed reduced drag, and allows a new slightly lower stable speed to be maintained
2. Speed unstable, where a slight drop in speed increases drag, meaning a positive feedback loop resulting in a rapid loss of speed and subsequent stall. The only way to get out of this is a big burst of power.


In a successful takeoff we allow speed to build up, then we push up to get the board out of the water, at which point that graph becomes active. A beginner issue can be doing this at too slow a speed (in the speed UNSTABLE region), where they pop up and are well into the SPEED UNSTABLE area, instantly get loads of drag and speed rapidly decays to the stall. Those more experienced allow more speed to build up so that they pop up at Min Drag (also L/D max), which is around 6 degrees AoA. At that AoA is a bigger stabiliser producing any lift, and if so is it worth it for the extra drag? I think you'd need to test it as I'm not sure, but my suspicion is no.

In winging we can temporarily reduce the foil wing loading, which greatly changes stall characteristics. I think if anything winging is too dynamic to model using an understanding of the physics based on aircraft.

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BritWindfoiler said..
Greg- everything you write is correct, but I don't think we're ever near those AoAs, and certainly don't want to be. We do stall on occasion, especially when learning. But I don't think that we ever intend to be near the stall, and aren't near there at takeoff if done correctly. It's very draggy at these speeds, and speed rapidly decays away.

This is what drag vs speed looks like, once up on the foil:


There are two regions:
1. Speed stable, where a slight drop in speed reduced drag, and allows a new slightly lower stable speed to be maintained
2. Speed unstable, where a slight drop in speed increases drag, meaning a positive feedback loop resulting in a rapid loss of speed and subsequent stall. The only way to get out of this is a big burst of power.


In a successful takeoff we allow speed to build up, then we push up to get the board out of the water, at which point that graph becomes active. A beginner issue can be doing this at too slow a speed (in the speed UNSTABLE region), where they pop up and are well into the SPEED UNSTABLE area, instantly get loads of drag and speed rapidly decays to the stall. Those more experienced allow more speed to build up so that they pop up at Min Drag (also L/D max), which is around 6 degrees AoA. At that AoA is a bigger stabiliser producing any lift, and if so is it worth it for the extra drag? I think you'd need to test it as I'm not sure, but my suspicion is no.

We might be saying the same thing in different ways.

Rather than a beginner trying to pop up in the speed unstable region (basically increasing pitch too much too early, before enough lift is generated to counteract the weight) I've seen this happen more with people switching to HA wings. HA wings definitely require more of a gradual speed build up, and not big pitch inputs to pop up on foil. So when people pitch up too much at a speed that's too low, they stall out and drop back in the water.

So perhaps, if you were to micro-analyze, it could be that people use various pop-up techniques: (1) where mostly first speed is built up, and pitch remains relatively small, and (2) where speed remains relatively low, but pitch is increased to get to an AoA where Cl=max.

We probably all do a mix of these techniques depending on conditions and preference, when there's enough wind we probably do something closer to method 1, and in light winds we're probably doing more something like method 2.

But I agree that it remains to be seen if a bigger stabilizer actually helps or doesn't help to pop up on foil. If the rider is inclined to have a pop up technique more like 1 then a smaller stabilizer might help him/her to lower take-off speed (because it would have less drag), but if the rider is used to popping up using a technique more like 2 then a bigger stabilizer might help.

I think all this is very very marginal though, because the majority of drag at take-off comes from the board, and not from the foil. And the decision wrt using a bigger rear wing should be driven by how much stability the rider desires.

It's fun to do thought experiments like this though

Can someone come up with mini pitot tubes and mini AoA sensors already? so we can gather this data while we foil

Instead of theory, why not apply real world practice?
In practice, bigger stab provide more stability when low to medium powered, and earlier sustained lift when low to medium powered.
Negatives when well to overpowered.
OP thread is for beginner foiler...not well to overpowered.

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greg87foil said..
perhaps, if you were to micro-analyze, it could be that people use various pop-up techniques: (1) where mostly first speed is built up, and pitch remains relatively small, and (2) where speed remains relatively low, but pitch is increased to get to an AoA where Cl=max.

We probably all do a mix of these techniques depending on conditions and preference, when there's enough wind we probably do something closer to method 1, and in light winds we're probably doing more something like method 2.


And the decision wrt using a bigger rear wing should be driven by how much stability the rider desires.

Yeah, if you have loads of power, enough to overcome the higher drag, you can pop up at a lower speed in the unstable region. (I think with the HA wings and latest boards with very flat undersides, method 1 is how people are getting going)

For a beginner lots of power is good, because they can keep the board fairly flat and start to foil, having only to move their weight slightly forward as the board leaves the water and accelerates. The less having to move weight around for a beginner, the better.

I am currently teaching my partner. Here is a stall on takeoff due to a lack of speed:


And one where she keeps the board flatter with more speed and starts to come out of the water with no stall:




Yep, stabilisers are there to stabilise!

For David John.....Naish makes 270 and 280 stabs, both too close to 310 to notice. They don't offer a 290.
They should make a 245 stab.

For Qtwind....
Lots of conflicting theory....
Let use an example. We know i76, i84, and i99 are great STABLE foils with huge range.
Consider, they are 3.8-4.5 aspect ratios...very low.
Now, add a big stabilizer and what do you get?
Lower overall aspect...more stability, more low end!
That's what helps people with less experience.

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LeeD said..
For Qtwind....
Lots of conflicting theory....
Let use an example. We know i76, i84, and i99 are great STABLE foils with huge range.
Consider, they are 3.8-4.5 aspect ratios...very low.
Now, add a big stabilizer and what do you get?
Lower overall aspect...more stability, more low end!
That's what helps people with less experience.

Thanks leeD. I had planned on getting the bigger stabiliser I saw but screwed up and missed the auction end time (am not pm lol). So I will keep my eyes peeled for something else.

Yeah, I miss stuff on sale all the time, like complete 1650, 440, 64, and 75 mast for $550 in A condition.
Has wife tried SUP with wing? After 3 sessions on that, she can ride weight plus 30 liters and almost any size foil. Slogging at first, of course.
I'm a poor example, I still wingsurf on 140 liters and 1220 foil. I cannot stand up consistently with the 122 Hover. Bad legs and 73.

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LeeD said..
Yeah, I miss stuff on sale all the time, like complete 1650, 440, 64, and 75 mast for $550 in A condition.
Has wife tried SUP with wing? After 3 sessions on that, she can ride weight plus 30 liters and almost any size foil. Slogging at first, of course.
I'm a poor example, I still wingsurf on 140 liters and 1220 foil. I cannot stand up consistently with the 122 Hover. Bad legs and 73.

Yeh she has covered that step. I had a naish hover 120l (7'6) cross over foil board that was good for curising around, but it was a lot of board to handle on the foil for a smaller person and since sold it to fund my 90l.

I am considering getting a jet 320 stabiliser, not as extreme in size as the 450 and may double up to help give me some stability in my gybes while I try and smooth them up. I have only just realised that my 280 has been chopped down, not sure if it's not as wide or had vertical tips chopped off. Sure is fast and smooth tho.

Might just come down to her weight.
Someone around 130 lbs could use a 250 stab as a light wind setup, while a 160 lb person needs a 320.

Buy her the right main foil. She is your wife after all ;). Its an investment....

I ride the smallest tail with the least angle of attack that I can control and steer effectively. increased stab angle seemed to do nothing for early take off, because of the extra drag. But it does seem to help to carve better. Less angle gives a smoother takeoff. Too little, at negative angle and I'm standing right back on the tail. A bigger or lower aspect stab helps me when the conditions are a bit out of control, when glide is perhaps not so important as control. I'm not sure that any change in fuse length, stab size or angle is going to be anywhere near as good as a good beginner foil. They're holding their value that well, you can move it on when she's stoked on the HA in a month or two. But perhaps I should've started marital advice thread

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mojoflex said..
Buy her the right main foil. She is your wife after all ;). Its an investment....

I'll 2nd this.. Nearly sold my 880 last month but my wife objected wholeheartedly stating she had her eyes set on it.. Just had a week of pumping surf on said 880 and I'm very glad its still 'ours' ..

I've tried shims and many different stabs - the difference vs changing the front wing is practically insignificant (IMHO) at an early stage..

A true marriage councillor you are Mojoflex..

Unfortunately it seems to have "stalled" the thread Smee......loving the tech talk you guys keep it coming

I've just got my hands on a 320 tail, and now thinking of just selling the 1400 ha front wing and getting one of the bigger jet surfs.

The 1400 is too close to my 1240 and I've suddenly found I don't have much use for it . So will hopefully sell to fund something better suited suited learning.

Alternatively eyeing up one of the complete gong packages as a learner setup.

Appreciate the marital advice, catch 22 that the wife doesn't want to spend money lol.

Here's another perspective on economy....I have two foils, axis 1150 and art 999. I bought the 1150 to learn how to sup downwind and dock start, but I've found it great as a light wind wing foil. I can get it going in about 10 knot gusts with a 4m cabrinha mantis ( I'm 76kg). The 4m covers me to 20kn plus, then I have a 2.5 that covers me to 30+. The 1150 has allowed me to have at least one less hand wing, and is also a great learner foil....and it'll last a lot longer than a hand wing. Bit off topic, but I find the hand wings are the biggest ongoing cost.....

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mojoflex said..
Here's another perspective on economy....I have two foils, axis 1150 and art 999. I bought the 1150 to learn how to sup downwind and dock start, but I've found it great as a light wind wing foil. I can get it going in about 10 knot gusts with a 4m cabrinha mantis ( I'm 76kg). The 4m covers me to 20kn plus, then I have a 2.5 that covers me to 30+. The 1150 has allowed me to have at least one less hand wing, and is also a great learner foil....and it'll last a lot longer than a hand wing. Bit off topic, but I find the hand wings are the biggest ongoing cost.....

I'm not familiar with axis gear, the 1150 is high aspect that correct? Interesting that you say its a good learner foil, I generally thought ha wings would be harder to learn on, hence why I started this topic.

I can get a 1800 naish ha and wonder if that would be better suited to giving me more of a quiver and still double up for learning on. I see a lot of people claim their small wings still work in light winds, but you must loose atleast some light wind lift.

I'm determined to stick to one hand wing, I still windsurf once it's over 20 knots.

New 320 rear wing arrived today and as luck would have it, it was windy for a test ride

It was much more stable, helping me immensely with my gybes. It did seem to limit my top speed, but I felt more comfortable crusing at lower speeds, partly because I feel a little too loose with the other wing.

So in summary it's going to help some of my own progression and confident it's going to aid my wife's learning. I'll revert back to my smaller rear wing at some point, but will enjoy my "stabiliser" for a little while.

If you want to push some lower limits, the Naish 2140HA w the 280HA stabilizer w my 6M FOne is a Summer Saver. I also have the 1800HA but have found I spend most of my time on the 1240HA now until it's silly light. Then I break out the 2140 Glider. I've used 7M and up but at the end of the day, maybe I should be mowing the yard when it's that light. ;) so I'm staying w a 6M. The 2140 actually turns pretty well and for such a large foil is pretty fast. Can't use it in any more than thigh high waves though. 1800 is hood up to stomach or rib, 1240 chest to shoulder, 1040 for head.