I was intending to find over different places on the Internet looking for a good understanding why some windsurf foils have the front wing closer to the foil mast on the fuselage ( foils like supercruiser ) and longer distance for the race and slalom foils. I learned that one of the reasons is that they are much easy to maneuver and they need smaller sails with the same wind power on the closer ones. On my understanding longer distances between wing and mast means more power with a small front wing ( is it that correct?). But on the other side you'll have that you will need bigger Sail for the same wind.
I have this two foils: Supercruiser is fine very smooth and easy and it works with small sails fine, and a neil pryde flight evo F4 full carbon. It is very fast but very unstable and nervous. I decided to make it longer on the back side of the fuselage in order to make it more stable. It was 75 cm. and I would like to increase this to 95. Is there anybody who can give me an advise if this would work. I like this foil because it is very simple and stiff but too physical for me, and I am wondering that if I make it longer on the aft part of the fuselage and leaving the distance from mast to front wing as it is, it may work?. I will appreciate your suggestions. Thank you.

I once lengthened a Naish fuse, added about 10-15cm at the back, it made it feel more slow to react...
old thread:
www.seabreeze.com.au/forums/Windsurfing/Foiling/Naish-HA-914-Different-Fuselage-Lengths?page=1

It's all about the position of the front wing with respect to the footstraps (or feet). This applies to all foiling sports--wind, wing, kite. The front wing is the lifting device and acts as a fulcrum on a big lever (or teeter totter) that everything else rides on. This is called BALANCE.

If you locate the front wing via fuselage length, sliding twin tracks, Slingshot switch fuse positions, raking, footstrap positions, whatever, the end result is that you have to locate the ONLY LIFTING DEVICE at a balance point that lets you foil successfully. This usually means positioning the front wing somewhere fairly close to the midpoint between your feet. BALANCE.

The difference between windfoiling and wing/kite foiling is the addition of a sail mast base into the equation. This then requires a bigger distance between the front wing and foil mast--in general. Wing and kite don't need this distance since rider feet are the ONLY downforce in their systems.

The length of the fuselage BEHIND the foil mast is another subject related to other things besides BALANCE.

murcielago said..
I was intending to find over different places on the Internet looking for a good understanding why some windsurf foils have the front wing closer to the foil mast on the fuselage ( foils like supercruiser ) and longer distance for the race and slalom foils. I learned that one of the reasons is that they are much easy to maneuver and they need smaller sails with the same wind power on the closer ones. On my understanding longer distances between wing and mast means more power with a small front wing ( is it that correct?). But on the other side you'll have that you will need bigger Sail for the same wind.
I have this two foils: Supercruiser is fine very smooth and easy and it works with small sails fine, and a neil pryde flight evo F4 full carbon. It is very fast but very unstable and nervous. I decided to make it longer on the back side of the fuselage in order to make it more stable. It was 75 cm. and I would like to increase this to 95. Is there anybody who can give me an advise if this would work. I like this foil because it is very simple and stiff but too physical for me, and I am wondering that if I make it longer on the aft part of the fuselage and leaving the distance from mast to front wing as it is, it may work?. I will appreciate your suggestions. Thank you.

I had that foil and we increased the distance between the mast and the rear wing. To put it simply, it gives you more stability: same as putting more angle on the stab or having a bigger one without the extra drag.
We didn't move the front wing as it would have been a much bigger job to do.

azymuth said..

I'm still not sure why we have a much longer neck (mast to front wing) on our fuselages than wingers - why are our masts always mounted so far in the rear of our boards?
It buys us directional stability but at the cost of excessive torsional stress at the mast fuse mount.
Some of the race guys must have tested moving the foil mast (Tuttle box) forward?


Amazing how the wing vs windfoil geometry is so different - gear below used in waves in the same conditions;

I tried windfoiling on a JP SUP Foil board with a GoFoil surf foil and it was almost impossible! (for me at least). The board keep pointing into the wind when off foil and felt like it wanted to spin around the foil mast when up on the foil. I assume that was because the foil mast and windsurf mast were only mounted a few feet away from each other.

I can't really explain why that works with a wing but perhaps because you can easily shift the whole wing forwards or backwards as required.

For winging, having the front wing close to the mast allows you to have the mast closer to the center of the board. If the mast is too far back, the board will turn downwind when you lift the wing, making starts harder. The newer skinny "downwind" boards that some wingers like for light wind can have the mast almost in the center of the board.

Unlike winging, there is some sideway pressure on the board windfoiling, so the mast also functions like a fin. The further back it is in the board, the better that works. Large sails and racing setups have more sideway pressure, so they use the tuttle box all the way in the back.

Which fuselage length works well is a matter of leverage. In windfoiling, the board angle is affected by pressure on both feet and on the mast foot. In a race setup with large sails and full commitment to the harness, there is a lot of pressure on the mast foot, so the effective distance between the front and back "lever point" is quite large. That works best if the "lever points" below the board, the front foil and stabilizer, are also far apart - hence the long fuses. In a more upright freeride setup with smaller sails and less pressure on the mast foot, the fuses can be shorter. In winging, the mast foot is not there at all, so a much shorter fuse works well.

murcielago said..
I was intending to find over different places on the Internet looking for a good understanding why some windsurf foils have the front wing closer to the foil mast on the fuselage ( foils like supercruiser ) and longer distance for the race and slalom foils. I learned that one of the reasons is that they are much easy to maneuver and they need smaller sails with the same wind power on the closer ones. On my understanding longer distances between wing and mast means more power with a small front wing ( is it that correct?). But on the other side you'll have that you will need bigger Sail for the same wind.
I have this two foils: Supercruiser is fine very smooth and easy and it works with small sails fine, and a neil pryde flight evo F4 full carbon. It is very fast but very unstable and nervous. I decided to make it longer on the back side of the fuselage in order to make it more stable. It was 75 cm. and I would like to increase this to 95. Is there anybody who can give me an advise if this would work. I like this foil because it is very simple and stiff but too physical for me, and I am wondering that if I make it longer on the aft part of the fuselage and leaving the distance from mast to front wing as it is, it may work?. I will appreciate your suggestions. Thank you.

Dear murci?lago.
Please follow this thread.
www.seabreeze.com.au/forums/Windsurfing/Foiling/Highly-modified-RS-Flight-F4-Foil--warning-pic-heavy-?page=1

I did the same and it worked perfect.
Regards
Ignacio

BullroarerTook said..
I love this topic. Here's another related question: assume I have a 90cm fuse with the front foil 20 cm in front of the mast and everything feels balanced. Now shift the fuse forward 10 cm without making it longer. It seems to me that the front foil has more leverage and should get smaller, but the stabilizer has less leverage and should get larger. Is that right? Or should the stabilizer stay the same because it's the same distance to the front foil?

Thought experiment: imagine you moved your footstraps back 10 cm instead. What effects would it have? What would you need to do to rebalance your rig? Think in terms of what forces each component exerts. Also remember that the glider (wing/fuse/stab combo) still need to deliver the same net upward force to keep you above the water. Not trying to be a PITA but I think some of the terms you are using suggest that it may help to look at things from a different perspective.

I think, by your reply, you thought I was asking what happens if I slide the mast forward 10 cm. I was not. The mast stays in the same position, but a new fuse is used which moves both the front and rear wings forward. This changes the torques the foils exert on the mast.

BullroarerTook said..
I love this topic. Here's another related question: assume I have a 90cm fuse with the front foil 20 cm in front of the mast and everything feels balanced. Now shift the fuse forward 10 cm without making it longer. It seems to me that the front foil has more leverage and should get smaller, but the stabilizer has less leverage and should get larger. Is that right? Or should the stabilizer stay the same because it's the same distance to the front foil?

Interesting question! The windfoil setup is amazingly complex, with different levers to consider.

One way to look at front foil and stab is that the stabilizer determines the angle of attack. For this, we can ignore where the mast is attached. In stable flight, total lift is constant, so a larger AoA means stable flight can be reached at lower speeds. Therefore, we use + shims in light wing, and - shims in high wind (although some companies use the +/- in reverse).

For the racing setups, moving the front wing further forward has resulted in using "less powerful" stabs - smaller and/or lower angles. The goals of the "foil forward" design changes were take off in less wind and faster speed. Both are accomplished because a lower AoA also means less drag. In your thought experiment, with everything else being kept equal, moving the mast forward should then also be accompanied with using negative shims on the stab (for a flatter stab).

What's missing from the discussion above is how the downward forces from the "three feet" influence angle of attack. Shifting weight forward, from the back foot to the front foot or the mast foot, reduces the angle of attack. So an ancient race fuse with the front wing close to the mast needed more powerful stabs (angle and area), since it had less leverage. Effectively, you're buying the balanced feeling with extra drag.

All that raises questions about control. Why can racers control the faster foils with a lower angle of attack if the front wing has more leverage? Not sure what the answer is, but one candidate is the position in the lift vs. AoA curve. If we assume that a certain weight shift changes the AoA by a given amount, then the effect of this weight shift will depend on where we are in the AoA curve. Lift curves then to be close to straight lines in the relevant range. If your AoA in the initial setup is 5 degrees, and you shift weight forward to reduce the angle by 1 degree, that will lead to a reduction in lift by about 1/5th. But if you started out at at 3 degrees and reduce the angle by 1 degree, the drop in lift is about 1/3rd, or 33% vs. 20%. In other words, to get the same reduction of lift, a smaller change of angle in attack would be needed, so the foiler can get away with less leverage over the front wing. Just a theory, but perhaps it is correct.

BullroarerTook said..

Paducah said..

BullroarerTook said..
Respectfully, I believe your thought experiment works if we are only trying to balance the forces, but we need to balance the torques as well.

No, it's the same thing. Torque is a (rotational) force. That's why I'm suggesting the experiment because the net result is the same.

en.wikipedia.org/wiki/Torque

Move the whole fuse forward 10 cm under you is the same as you moving 10 cm back on the same fuse. The mast doesn't matter (for these purposes) and I did not mention the foil mast I'm not trying to be difficult. fwiw, if you have a mast (tracks) that slide - the net effect is still the same in terms of vertical forces. It will make the board handle a bit differently due to sideways forces but your question was about the vertical forces.

If you disagree, perhaps you can share your reasons why it's not functionally the same thing. Having used + and ++ fuses which pretty much do the same thing, I can tell you that the answer is not complex. And, probably should have thought of this in the first place since the Starboard Plus 115 is essentially does the same thing by about 7cm, iirc.

Edit: added pic of fuse equivalent to original Starboard 115 and ++ version. Foil by small US builder. The stab is about 5 cm forward, the front wing is 13cm forward.

You're right. I spent the morning with free body diagrams and ended up agreeing with you. Was just coming in here to say it. I'm still struggling to believe now that the mast position doesn't matter though. (I know it doesn't from a pitch point of view, but it seems wrong.)

thx

Cool! Thanks for being gracious.

Your mast comment had me thinking, though, and I did some quick sketching. If one looks at the fuse and mast in isolation (which I did for a moment), it seems that the mast being way back allows the wing to exert more leverage. This puzzled my lizard brain until I realized that the change in leverage is matched by an equivalent change in leverage from the rider+sail.

The first time I used the red fuse above, I only pushed the mast base up a few cm and made no change in the stab angle. Once I got flying, I had to step in front of the straps to get the board back down. A very unsettling feeling as one might imagine. Ultimately, what worked in light wind was to push the mast base forward about 15cm and take a degree off the stab. That still left me with added front foot pressure which made the board go upwind and jibe much better in light winds. Similarly, the IQFoil Plus fuse is run with stab angles generally from 1-2 degrees less than the original 115 fuse used.

boardsurfr said..
All that raises questions about control. Why can racers control the faster foils with a lower angle of attack if the front wing has more leverage?

I'd posit another thought. The longer race fuse (many of the ++ are closer to 120cm long) acts as a "dampener" ie a longer range of motion for the stab for an equivalent change in AoA. In simple terms, how tightrope walkers use long balancing poles or why airplanes with longer wings roll slower than those with shorter wings. My physics background isn't sufficient to throw around words like moment of inertia and angular momentum with any confidence.