I do not understand why the position of the foil in the US BOX matters so much

Hello foilers,

I do not understand why the position of the foil in the US BOX matters so much (edit: once up on the board).

Intuitively, I would think that moving the foil 3m backwards could be perfectly balanced by moving your feet 3cm backwards and that it would not make any difference.

As a first hand experience, I know that a few cm forward/backwards in the US BOX can make a foil setup feel unbalanced and unhealthy, and that moving your feet around does not entirely compensate it.

I do not understand why.

The weight of the board itself (hanging forward or backwards of the foil) can be neglected compared to the weight of the rider.

So why?

Pop-up - you can’t dial in your pop-up like that and end up “just 3 cm forward”. Your body mechanics with the board are more of a fixed quantity, you end up where you end up and its much easier to move the foil than move where your feet end up.

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Good point, I should have mentioned “once up on the board”.
I will edit the original post.

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I don’t think it matters much at all. Maybe if you were riding strapped?
My current board has fixed M8 inserts. Moving my feet around all the time while riding and even while taking off depending on how steep/late drop is. So I see no need for moving my mast around.
Have to add that I’m riding the same brand for years now. Maybe if you are swopping back and fourth between differend brands that have big differences in front foil to mast (differet center of lift), then tracks could be helpful.

The way I think about this is that your body and foot stance will always feel most natural in a certain position on any specific board, and you need to match the foil’s lift distribution to that position. You cannot just move the foil forward or backwards and then scoot your feet to compensate because you may not like where that puts your weight in relation to a certain part of the rails (for turning) or how much nose/tail is in front/behind you (for turning and especially pumping). Probably a good amount is just mental, your brain probably just thinks you should be in a certain area on some board and there is probably some room to change your brain to an extent to have different expectations, but not so much that you can make any box position and foil work together. It’s why some people get boards and sell them because they think the boxes are just in the wrong position for them.

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I don’t think it matters once up on foil. There isn’t a logical explanation.

it feels like I CAN move my feet sufficiently to compensate

shimming is different, but mast position I think is fine.

I do think there is ergonomics issues with having the mast in the wrong place, ie touchdowns will be worse if mast is too far back, and popups are more awkward, and if a board has a narrow tail then you lose space for back foot leverage.

But purely from a forward backward trim, without straps, I haven’t personally found a reason to agree with you

Looking at foil box position (or foil position) in the absence of pre-takeoff balance of the board is a purely abstract exercise. As others have noted, the position of the box is largely related to how the board balances pre takeoff. Too far forward and you are going to dive the nose on takeoff. Too far back and you are going to induce tail drag inefficiency on takeoff. Putting aside touchdowns, once you are up it only matters in relation to your stance (and weight), hence why a minimal tray board works for something like towing.

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Mast positional also matters a lot for swing weight IMO, which affects both turning and pumping.

Greg from Omen Foils put out a good video explaining this exact topic.

This is a tough one for me because my shifting feet around gracefully is still a WIP. My up and riding position prefers for my wing to be more forward than my paddling in position. If I move my wing forward to match my riding preference, my takeoffs buck unnecessarily and need to be actively managed.

Center of gravity vs center of lift. Drag the arrows around and watch them get bigger. That’s the extent of effort I’m willing to put in to learn about it, but there is the research topic you are looking for.

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I’ve actually been meaning to shoot a follow up for a while. In the video I stated it was due to turning dynamics but I’ve come to believe now that it’s purely based on weighting/unweighting the foil (such as in a turn, pumping, or when dropping down a wave face) rather than the change in forces from the turn itself.

@mediterranean_foiler you are completely right that in steady flight, if you move the foil 3cm further and then move your stance approximately 3cm further forward than the system will still be balanced. However, as you mentioned, we are in a dynamic system which is where this falls apart.

Let’s assume your original position was perfectly balanced, meaning that if you jumped off the board in flight the foil would continue to fly straight. This means that as long as you stand directly centred over the lift of the foil when you unweight the foil it continues to fly straight.

By moving the foil further forward, the system is now tail heavy without the rider. Like you pointed out, you can compensate for this by standing further forward. But as you unweight, the nose will have a tendency to rise since the system is inherently tail heavy. The further forward your foil is moved from the balance point, and/or the more you unweight the foil, the more aggressive this nose up pitch becomes.

The opposite is true for a foil that is too far back. That’s why it’s better to be a bit forward of balanced vs rear of balanced. A foil that is slightly forward of centre will pitch nose down slightly during the push phase of a pump and nose up slightly during the glide phase which is actually desirable. The opposite really sucks though! Not to mention you’d also have a tendency to pitch nose down when dropping down a big wave.

Too far forward and the foil will pitch up/down aggressively during the pump which is really inefficient and must be actively fought by the rider which is annoying, and although downward pitching while dropping down a wave is the worse, pitching up aggressively could result in a breach if the rider can’t get their weight forward fast enough.

All this to say, I do think there is a theoretical “perfect spot” where the system is in balance, but in practice advanced riders will prefer to be forward of this spot to varying degrees depending on how much they like to actively control the system during, pumping, turning, dropping down a steep wave, etc.

Personally I like to be just a touch forward of the balance point, around 1cm or so. But I definitely see the case for really skilled foilers to be further forward. Past a certain point though I think even advanced riders will be hurting their performance even though they can “get used to it”.

As other’s have pointed out, the popup is another matter. Since you’re more or less “stuck” paddling on the board at the centre of bouyancy, if you move the foil further forward the foil will lift really aggressively as soon as you speed up (ie. start catching a wave) and you’ll blow lots of popups by foiling out and risk some foil/face collisions, haha. Learning to prone foil I often recommend placing the foil a few cm back from the balance point to make popups less intense for their first few sessions. Even though it’s not ideal for the actual foiling part, this leads to a lot higher wave count and they can edge their foil forward as they get more comfortable.

Good prone foilers can execute the pop up with their foil really far forward but for the majority of us having the foil balanced under the centre of bouyancy or sligthly in front will be much more consistent.

As an aside, I suspect this aggressively “lifting” during the prone popup has led to the very common (and incorrect) belief that moving the foil further forward in the tracks will somehow increase the lift that the foil is producing.

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Thanks for the breakdown Greg.
When you say ‘‘Personally I like to be just a touch forward of the balance point, around 1cm or so’’, you mean your Center of Lift (CL) 1cm in front of your Center of Gravity (CG) ?

Happy to, didn’t mean for it to run on much though!

And yes, I personally find that position to be the best all around for pumping, turning, and dropping into steeper sections.

So i bought two boards of the same model

3ft11
3ft9.

Theres markings on both boards that estimate the location of the mast.

2 big mark ticks from the baseline tick.

THE LOCATION ISNT THE SAME
I felt like i was back in time 2months of learning to prone. My wing was unresponsive to input, i couldnt figure out why, so some of the session i was trying to pinpoint the standup again…took like 30mins

I took a measurement and the distance was moved forward 2inches on the 3ft9.

I said fuck it, ill bring it to exact same distance as my other board. Coincidentally, 2 inches…and it felt like normal again. I was popping up and pumping no issues.

Take a picture of both boards next to each other on the ground. Are the tracks in the same place? Estimate where you feet are when standing on each board. Are your feet in the same place?

Ya I did, thx though!

Theyre different by about 2 inches,

So the new board sweet spot… is… right a above the entryslot for the mounting screws. Basically 0 on the tick marks. Alittle further back even.

So new board tracks are moved up by 2inches

Your videos are great at explaining, keep them coming.
You say ‘perfect’ place is over centre of lift, but isn’t there also a torque from the stab pushing the nose up so the ‘perfect’ place is somewhere between the centre of lift from front foil and torque from stab?

It is very important. It is all based around the center of volume of any given board. When designing the boards and placing the boxes, it is very intentional where they go…it is also intentional as to where the balance point is. The stance you have will adjust to your liking, and that is where the fine tuning takes place once up on foil and adjusting small movements at a time.

You can consider the COL of the front wing and stab combined. The stab has the effect of moving the COL forward slightly.

If you’re in level flight, your COG is directly over the combined COL.