Hi all,
I'm currently riding a 1250 frontwing MA, with alu mast. I'm enjoying this setup, it's fast en nimble enough for my level. What does happen to me is that when I push fore more speed I can crash. It feels like I hit something underwater, like a fish ore some weed. It happens on longer straighter lines when I push for speed. My brother has a different setup up and he has this much less he says. Doe anybody recognise this behaviore of the foil?
Is this technique, can I prevent this?
Any tips are welcome thanks 
!
Ventilation does not feel like hitting something. If the foil ventilates, you hear a distinctive sound (maybe similar to frothing milk or so) and if you not correct immediately you crash very suddenly. You can find videos of how ventilation "works" on YouTube.
Sounds to me like you would actually hit a fish or something else.
Buy a action camera, mount it to the wing and then check what is the cause for your crash.
Sounds like mast ventilation, feels like a sudden loss of lift as the air travels down the mast and then ventilates the stabilizer causing a nosedive. Compared to a foil breach/ventilation it's more gentle but still usually not recoverable. I used to get it on demand with an Axis AL mast, 18 knots leaned over to windward would do it every time.
First, give the foil a good clean with window cleaner just before you hit the water, don't touch the foil with your fingers after cleaning.
Push some speed during that session, if you notice an improvement it is definitely mast ventilation. Google foil race sanding to get better results. Some foils are more prone to ventilation than others.
Thansk so much for your replys. I googled mast ventilation and found a lot of info. On another forum i found a large thread about mast ventilation, it looks like this is what I'm experiencing. I will try to clean the mast before entering te water. In the other thread someone also says it can happen when you push upwind with a lot of 'yaw', so this maybe also a factor. Mast profile also can contribute I read. But its nice to know that it is a common thing
the link to the thread:
Getting ejected out the front door... (standupzone.com)
Mast ventilation is also a big issue when windfoiling, since you exert a lot more sideways pressure. I noticed a huge difference between alu and carbon masts with a sail. Shorter masts also make things a lot worse; perhaps trying to fly high to get better speed is similar to using a shorter mast?
Does you problem mostly happen when you push speed at beam reach or upwind angles, or also when you are going downwind? If you judge speed by feeling, then you'll probably prefer upwind angles, since they feel faster due to increased apparent wind. If you're measuring speed with a GPS, you'll see that downwind angles tend to be faster, even if they feel slower due to lower apparent wind.
It mostly hapens when I build up speed. So when I'm just cruising its no problem. I have it on upwind/straight but also downwind runs. But i do think on downwind runs it could also be just the forntwing popping up for air because it gets harder to maintain a good foil hight because every move gets exaggerated. For me going fast is anything between 32-40 km max 2 seconds (about 17-20 knots). I use a garmin watch so accuracy is a guess. I use a 85cm mast.
Select to expand quoteFishbone80 said..
... on downwind runs it could also be just the frontwing popping up for air because it gets harder to maintain a good foil hight because every move gets exaggerated
Interesting point. I just switch to Axis, and was a bit surprised to read that the 19 mm Axis alu masts seem to be prone to ventilation in the link you gave. I did have more "sudden falls" on the new gear, which theoretically could be ventilation. Seems unlikely in my case, though, since I'm going slow (in general, and even more so with a new foil). Also, it mostly happens in jibes, where I am used to adding back foot pressure from being on a draggier foil. With the more efficient higher aspect foil, that just leads to overfoiling.
one of the videos in the standup zone discussion, the nose of the board is going downwind when the mast ventilates - presumably due to yaw of the board from front foot pressure. Interestingly, this is the opposite of what happens when windfoiling, where the tail of the board slides downwind, because the "heavy" foot is the back foot. That's similar to spinouts in windsurfing. The technique adjustment is to control the pressure on the "heavy" foot. Freestyle windsurfers are quite good at this, since they use very small fins.
Basically, there are two ways to handle the increased lift a foil generates as your speed increases. One is to increase the pressure on the front foot, which reduces lift by lowering the angle of attack. That's a very intuitive way: the nose comes up, we push it down. The second way is to change the angle if the foil by rolling the board (dipping the windward edge). That's what foil racers do with sails, wings, and kites; the kite racers have quite extreme angles when going upwind, with the mast being almost horizontal - and they are the fastest. Controlling lift by increasing roll is intuitive to everyone who thinks in force vectors. I thinks it's safe to assume that his is a very small minority
.
Looking at the two options to control increased lift from higher speeds, the intuitive "more front foot pressure" approach has some clear limitations. The obvious ones are that your front leg will get tired, and that there's a clear limit: once all your weight is on the front foot, that's it. Another one that you describe for downwind runs is that it can be quite hard to adjust to exactly the right amount. Finally, the "hidden" limitation is that more front foot pressure pushes the nose of the board downwind, increasing yaw and angle of attack on the mast. With a higher angle of attack, and a mediocre profile that's typical for alu masts, ventilation will happen.
So the tip in the standupzone discussion to use roll rather than front foot pressure is spot-on.
It could be lots of things.
Check your foils carefully for defects, and fill+sand anything suspicious. Clean your alum mast with windex or acetone, and beware of oily finger prints on it. Check all you fuselage joins for any mechanical freeplay.
It's unlikely to be anything you are doing, but when you go upwind with lean, see if you can train yourself to not yaw the nose of the board up-away, because that increases the lift generated by the mast (fyi: wingers+ kite foilers experience the opposite direction mast side loading as windfoilers).
You could try 800 grit sanding your alum mast along the foil (perp to the water), but beware that once you cut away the anodising you might have to continue periodic sanding to remove corrosion.
Lastly, it could just be the design of your foils, or construction, or mast section shape. Some foils tolerate/recover from tip breaching and flat breaching far better than others. Some foil combos are far more yaw stable than others, which helps reduce sudden mast ventilation. Some masts are very flexy and you just crash for no apparent reason if trying to go fast.
I had a similar sounding problem with a SAB Leviathan 1350 frontwing (big lightwind pumpwing) on a Slinshot Phantasm foil with the small 325 rear stab. The stab was working flawless with the original and smaller and faster Slingshot PTM 926 frontwing.
But with the Leviathan I had the problem that when hitting a gust and gaining speed the nose was suddenly sucked in the water, where I first thought it felt like ventilation or breaching but I was riding horizontally and not too high so that couldn't be the case.
Also I felt like I had to put more pressure on the back foot when gaining speed, normally it's the other way round.
Then I watched many videos about tail shimming and finally found one where a guy described the effects of different rear shim angles by using a model glider plane. And especially the described speedstall and nose sucked down effect when you have too less longitudinal V between front and rear wing. Bingo.
Unfortunately I couldn't find the video anymore on YT to put in the link here, maybe somebody is better in searching YT for it.
Your problem sounds like it's the same cause.
So then I decided to play with shiming the rear stab and finally ended at 2 degrees (more V) that solved this issue. 2 degrees, that's a lot I thought, though.
Afterwards I found out that Slingshot had reduced the rake angle on this particular 325 stab by 1,5 degrees (if i remember it correctly) since the 325 ist their smallest, most "radical" stab, designed for speed and turns and high wind and waves, and not for a combination wit a big pump wing like the Leviathan (my aim was to improve pumping with the small stab and gain some manouverability).
I had terrible ventilation problems with my Axis Alu mast. In the end I mostly fixed it by etch priming and sanding the mast. However it still ventilated in choppy water occasionally.
I'm now using an AFS and Unifoil before that. No such problems with them. I have also had the AFS at over 25 knots.
My conclusion was that Axis Alu masts are not really designed to be pushed.
The ventilation felt as though someone had just chopped off the stab. Resulting in a crash. It mostly happened when loading the mast from one side. So very often (but not always) on a close reach.
The faster I went (20+ knots) the worse it was.
Some good tips in this topic, it's too cold to ride for me at the moment. I'm gonna windex and pay ayttention to my technique when the temperatures get better. And if that doens't work, I'll blame my gear
I finally found the video where it gets explained what happens if you have a too small stabilizer ankle respectively too less downforce produced by the stabilizer.
The nose pitches down without any ventilation or breaching effects.
