That’s a great question and there is indeed a torque being induced about the centre of gravity from the off axis downforce generated by the stab.
In a well balanced foil system though, that moment generated by the stab is exactly equal and opposite to the pitching moment from the front wing + the total drag of the system about the centre of mass. So with these moments balance out in level flight the change in the effective COL over the speed range of the foil is very subtle.
Of course, the complication is that for an imperfectly tuned foil system the rate of change of these moments changes at different rates relative to speed. So for most foil setups that are not well tuned the rider will have to shift their weight around to compensate for these different moments.
To get really nuanced there is also a small pitching moment being generated by the stab but fortunately that works in the opposite direction of the front wing’s pitching moment - so it ends up helping stabilize the system along with the stab’s downforce. To really consider foil dynamics I think one needs to shift their thinking from analyzing a static problem to a “set” of problems with variables changing with respect to speed. It’s rather simple as far as calculus goes but the tricky part is not so much the math but actually visualizing and working on these problems over a continuous speed range.
All this to say is that yes, you are correct you when analyzing a foil the torque/moments need to be considered. BUT, for a perfectly tuned system these moments all sum to zero so do not need to be taken into account when thinking about mast position. If your foil is not tuned as such, my goal would be to adjust the tail shim or change your tail to get as close to this point as possible. I think of this as a separate tuning operation from track position, but as you pointed out everything is connected so if your stab tuning is way off there is no “perfect position” for your mast in the tracks since the effective centre of lift wanders aggressively with speed changes.
I disagree. I mean… kinda.
You should position your fore-aft mast position based on balancing the system.
but you would change mast rake angle based on speed, with a general rule of more rake for more speed (like a wing board or tow board) and less for slower speed (like a pump or prone board).
Indeed, as you change the rake though, it might slightly change the balance and therefore track position, but that’s secondary in response to the rake change, not in response to speed.
Moving a mast in response to speed may sometimes feel better, but that probably just means something else is out of balance, and it’s a compensation. It would work better to properly fix whatever is the original issue.
Upon closer inspection, something seems amiss here.
If unweight the foil still keep same speed (let’s say at least in early stage), the lift will too much for the less weight (just for the foil and board itself) that means the whole system will lift but not fly straight (as we want). Since the body’s weight is far greater than that of the board plus foils, therefore we need set the COG of foil & board must in front of the COL of whole system, to prevent the board tip from rising too quickly when unweight.
I find it funny how many people think there’s one right answer or how many things are right vs wrong. Foiling is the most multi dimensional board sport I’m aware of and for the most part to date we are not foiling native, some come from water boardsports, some wind sports, some land board sports. So we all bring a bias.
Some may want to move the mast the mast forward winging in light wind or some will rake the mast negatively, some may even shim the stabilizer. None of those are right or wrong and all three will help light wind wing for example, or help pumping.
Same goes for high wind some may move the mast back, some may rake the mast positively. Again no right or wrong just preference.
Just because someone prefers to move the mast doesn’t mean something is off in their set up. Some don’t have mast shims and don’t want them. They may also only have one foil and one hand wing. All these tweaks can maximize equipment without needing to buy more.
Not to mention you will likely move it around as your skill set evolves, I used to pump terribly so I had my mast a little forward to compensate, now I can have it back a little which to me feels like it helps my carve in addition to a bit more high speed stability.
all that to say experiment with the gear you have (slam the mast forward, slam it back, set it in the middle, see how it all feels and adjust), have fun, and foil.
yes position in the foil box has a massive effect for which I don’t have the answer but I feel it
