Most of us want the CoG to be ahead of the CoL - it's more stable that way. en.wikipedia.org/wiki/Longitudinal_static_stability#Center_of_gravity For someone starting out, it seems strange since many windsurfers are accustomed to leaning back. Even if you disagree with that, you can still see that by your method, it goes out of whack as soon as you start to put any pressure down through the harness/boom. As well, as you go faster, you have more lift to deal with and you'll be moving your CoG to compensate (see second Sam Ross video below)

Segler's video which he did several years ago (which I'll give him the honor of posting) shows how to approximate your stance over the wing similar to what you've done.

Watch this vid by Sam Ross and notice how his CoG indicator shows how much pressure he has forward. At the end, when he's overpowered, note how he moves even further forward to balance the forces.



Edit: More Sam Ross

Looking at the board while trying to foil is a recipe for a much longer and harder learning curve.
That's why I never recommend removing the front straps, they are there to give you a consistent reference point for foot placement without looking at the board.
That allows you to keep your vision outside the board which makes height control much easier.

segler said..
OK, I'll bite.

Thanks!

stehsegler said..
Thanks for the video. The more I learn about how the foil and board work together the more I am starting to wonder about custom foil boards. Given the foil setup has a sweet spot but the height and weight of the rider generally will be different it would seem a board specifically build to match the rider makes a lot of sense.

That's why almost all boards have some variability built in - multiple footstrap positions, (sail) mast track. It's not an exact science even for the same person on the board if you change wings, stab angle, sails (size/draft position), etc. On one of my boards, I've sailed both a 5.4 and 9.0. I've sailed from 800 to 1500 wings over three four brands of foils. I made 90% of the adjustments with mast base, boom height and moving the back foot around.

Funny enough, it's a custom.

One last thing to consider is that most new models of foils will be designed to work within the existing paradigm of foil boards. There is very little incentive to design things way out of bounds of the current set ups of fin box/mast box/foot straps. It's been refined for a good 5 years or more and works reasonably well. Even something at the edge like the Starboard 115++ fuse is still intended to solve a problem within the common general parameter and works pretty well with Starboard and other brands of boards.

That's not to say a custom doesn't have some benefits, especially the more you are toward the ends of any bell curve of height, weight or intended use.

The center of lift (COL) underwater is always JUST the front wing, since nothing else down there lifts UP. The stabilizer lifts DOWN to counterbalance the weight of the rig. The whole shebang is a teeter-totter with the front wing acting as the fulcrum.

When the teeter totters, that is the pitch changing.

Foil wings generally have their COL at about 1/3 chord back from the leading edge. This varies with planform and aspect ratio, of course. Some wings, like the i76, have a VERY LONG COL. This makes the i76 very easy to balance on different boards and setups. I have one.

My high aspect ratio wings (AFS-2, LP, Moses Race) have a VERY SHORT COL. These require detailed balancing.

You always balance the COL under your feet first. Only then do you fine-tune with mast base position.

A COL under the back foot will not fly. I know this from experience. The famous original F4 was that way. That is why San Francisco foilers in 2017 had to build stern extenders on their boards, or have their finboxes moved forward.

Foil manufacturers have now figured all this out. Most modern foils will fit onto most boards with just a minimum of messing around with balancing via footstraps.

If you want to know whether your board will balance easily, just look at the back straps. If the position of the front screw is at or near the midpoint of your foot in the back strap, it will balance easily. Purpose-built foil boards or foil-ready boards all have this. If the front screw is way aft of your foot, you will probably need a foil like the i76 or Moses 790 to get it to balance. Many previous boards had the finbox too far aft for easy balancing.

Case in point. I own two Mike's Lab formula boards, the model L6. On one I had the finbox moved 3 inches forward to balance the high aspect ratio wings. On the other one I left the finbox stock, which is too far aft for high aspect ratio wings. However, it balances just fine with a i76. Done and done.

COL clearly not in the middle of the footstraps. Yet, all the time I hear that its where it must be :) Supercruiser and 125 Freeride board.

That photo is basically the same as my stock L6. The finbox is clearly aft of the back footstraps. And, yes, the front wing in the photo is directly under the back straps and right next to the strut, like a kiteboard foil. I say, good luck getting that to fly, but apparently people do.

When I mounted my AFS-2 to the stock L6 (with far aft finbox like the photo), I had to basically hang my back foot off the stern to get it to lift at all. Moving the finbox forward 3" completely fixed it. I still ride that modified L6 today, with AFS-2, LP, and Moses Race. It is so good that I am not motivated to switch to a custom foil board I have.

My non-modified stock L6 (with far aft finbox) rides just fine with the i76 in the B position since the i76 wing has a very large center of lift. And, yes, the i76 wing is more forward of the strut than the Supercruiser.

Many many pre-foiling boards had finboxes aft of the back straps like this. Users should go with what works and mount the Supercruiser to these boards. It will apparently work.

Good article about stab adjustments. Use Google translate :)
marseille.glissattitude.com/blog/windfoil-calage-des-stabs-en-foil.html

www.boldmethod.com/learn-to-fly/performance/what-effect-can-and-does-cg-have-on-your-airplane-performance/

Observe that the center of gravity (CofG) is ALWAYS ahead of the center of lift (CofL) in a conventional design.
As you move the CofG rearward or the CofL forward you require less downward force from the stabilizer. This can allow you to downsize the stabilizer for less drag (Starboard Race) and/or reduce the stabilizer angle (Race + fuselage) which also reduces drag and both together increase efficiency.

The downside is that stability is reduced as the CofL is moved forward but that can be compensated for by adding fuselage length. Also the setup requires constant mast base pressure otherwise the CofG can quickly travel behind the CofL (we actively control CofG to trim the foil).

Race foils are designed for minimum drag, to support a large/heavy rig being run quite far forward and to produce maximum windward VMG. This can make them awkward for sailing anything other than upwind because off the wind small variations in rig pressure can shift that CofG reward (because the mast base is so far forward) and as a result you get a high speed breach.

There is no free lunch.

On the other side of the equation:

You can setup the Supercruiser to be very front footed without ever moving the wing relative to your feet. All you have to do is change the stab angle (this setup has LOTS of adjustment here) and since the stab is both very large and quite far behind the front wing it has a huge effect.
The downside is ever increasing drag as you increase speed so the whole setup tops out quite quickly. Also the CofG has to be brought forward via foot pressure very noticeably as speed increases. Its similar to the I76 in this regard.
On the plus side just like in the aircraft example you get lots of stability and an extremely gentle stall.

In this configuration the foil is designed to use a far rearward mast base and lightweight sails which create very little mast base pressure (opposite to the race foil above). Changes in sail power effect the CofG very little which makes it an easy foil to use on different points of sail.

Choosing a setup is very dependent on what YOU want it to do and that takes the ability to look at the complete setup rather than focusing on one oversimplified data point like wing position.

Grantmac said..


Observe that the center of gravity (CofG) is ALWAYS ahead of the center of lift (CofL) in a conventional design.

Some one needs to define "Centre of Lift" for this discussion or it will keep going around in circles. Is it the centre of lift of the front foil or the combined centre of lift of the whole lot of the underwater bits?

Djungelman said..

Ian K said..

Grantmac said..


Observe that the center of gravity (CofG) is ALWAYS ahead of the center of lift (CofL) in a conventional design.

Some one needs to define "Centre of Lift" for this discussion or it will keep going around in circles. Is it the centre of lift of the front foil or the combined centre of lift of the whole lot of the underwater bits?

From what I understand the only Forces acting upwards are the Ones generated by the front Wing. The sum of the Forces are acting in the CoL or center of pressure CoP. Its aprox One third of the chord length in, messured from the leading edge. That is, a Little bit aft of the leading edge. To make things compli camplicated, the CoP moves back and forth depending of angle of attack. This makes regular wings unstable by default/nature. Im not an expert so dont hang me if Im wrong
Good videos about CoP and stability

The stabilizer also plays a part in up force, When angled to induce a higher angle of attack over the whole foil. Same as when a plane changes the angle on its tail wings.

Starboard came up with the ++ fuse on their race foil to reduce the required angle of attack on the stabilizer and get the same amount of lift by shifting the main wing forward.

im not sure the kites do the same with their stabilizers though. They havent got a rig to carry around either.

All I know from SB presentation is that standard 255 stab has an angle of 2.7 degrees.

To try to answer Ian's question, the center of lift is at about the 1/3 chord point back from the leading edge of the wing. This applies to almost all wings, be they airplane, glider, or hydrofoil. There are variations, of course, based on planform and aspect ratio, but this is a good generalization.

The force vector diagram above has it right. Look at all those vectors. Note that there is only ONE vector that points up. All the others point somewhere else. The UP vector is created by the wing and by nothing else in the whole system. That wing is doing ALL the lifting. Nothing else is.

Since the wing also acts as a fulcrum for the teeter totter, the stabilizer pushes down to counteract the down force of everything in front of the wing. This is the sail and rigging and that part of the board forward. (The stab also counteracts drag forces, which is why kiteboards, with no rig to lift, also have a down force stab.) I suppose the stab could also counteract the down force of the front foot, but why bother with that? Just balance the wing with your feet and let the stab take care of everything else.

Back to the photo of the Supercruiser. I have to wonder whether it is configured for kitefoiling instead of windfoiling. If you look at the many pictures of foils out there in the world, you notice that kitefoils have their wings right next to the strut, while windfoils have a space (from 3 to 8 inches) between the strut and the trailing edge of the wing. Start by looking at the Slingshot website to see this. Kitefoils do not have a rig to lift, so the ONLY way to balance them is feet. Nothing else. Yesterday I was foiling with friend who got onto his kitefoil for the first time. After thrashing and crashing for the first hour, he finally got the hang of it. He came in and told me it's ALL IN THE FEET. Amen.

Sailworks sells and supports windfoils. They won't even let you out the door until you have heard their balance lecture. They also say all this balance stuff on their website. Until you get the wing balanced between your feet, you will just struggle and never get flight. Start there, get it about right, THEN fine-tune with the mast base position to tune the stabilizer. They want their customers to succeed. Obviously. Their emphasis on balance is a big part of this.

The measuring tape is a starting point, never an ending point.

I have helped a number of new foilers who just could not get started with any kind of flight. In looking at their gear I could see that their geometry was all wrong. Wing too far aft or too far forward, and mast base to front screw too long or too short.

If they had gear that allowed for adjustments of wing position--power plate, twin tracks, Slingshot ABC, or movable footstraps, we **STARTED** by first getting their wing to the mid point between their feet.

Then we **STARTED** with their mast base at something like 43" in front of the front fin screw or strut leading edge.

Now they could fly. It was always amazing to them.

With time on water and making small adjustments over ten sessions--that's ten, as in 10, sessions--they always succeeded. Always.

We always had to do this when mixing brands of boards and foils.

If the new foiler had all one brand, it was usually balanced out of the box since the geometry was already correct. (Slingshot is a great example.) Even then they still had to make small adjustments by feel, over the space of ten sessions.

Most of them were quick learners. Ten sessions got them to the point that it took me 50 to achieve.

Lecum said..
...Please tell me if I've missed any important issue.

Yes, moving everything on top of the board (sailor, mast base), boom height and stab angle.

Having tracks does offer adjustability. There are other means at our disposal, too. For other foiling sports (kite, wing) where you don't have the option of moving the sail around and changing the amount of mast base pressure via the harness, it's a more significant issue to move to the CoG around.