I was in the water arguing with someone. I told him that in order to pump longer (with no wing) you need to get as high as possible without breaching and stay high during your pump. I figured this out SUP and prone foiling. I can assure anyone you will pump longer and farther with less effort like this and you will feel the foil lift much more (like 50% or more). It's quite a dramatic effect.

He responded that foils are less efficient near the surface based on the studies he read and I just must be feeling the effect of having less mast in the water.

WHO IS RIGHT?!?!

I looked up a recent study which agrees with him:hal.science/hal-03963200/document#:~:text=Hydrofoils%20typically%20operate%20in%20proximity,alters%20the%20dynamics%20of%20the

Note: "h/c" : h = depth of foil, c = chord depth, so 1 h/c = .1 m depth and .1m chord length, for example. I think we're interested in "ii" below which is between .91 and 2 h/c, or where the foil is between 1 and 2 times deep as the chord is long.

Quote from paper linked above:

"The evolution of the hydrodynamic coefficients with the different submergence depth shows three distinct regimes for the influence of the free-surface:

i) for high immersion, [... no difference]

ii) for moderate immersion, corresponding to 0.91 > h/c > 2, the forces are affected by the free-surface; the amplitude of the waves above and downstream the foil increases as the immersion is reduced, but without wave breaking. The lift moderately increases (+12% between h/c = 2 and h/c = 0.91), while the drag drastically increases (+143% between h/c = 2 and h/c = 0.91);

iii) for low immersion, corresponding to h/c h/c > 0.55, the lift drops while the drag still increases, then for h/c < 0.55, both lift and drag drop.

The theory your friend in the water believes sounds like one of many the outspoken local mast producer of your area espouses.
You two (posters) are both correct, the difference is like night and day. Even a beginner with no pumping skills can feel this

Taeyeony said..
Low immersion in your paper is so shallow For the foil with 10cm chord h/c = 0.55 l, it means your foil is at 5.5cm underwater. It is not practical to ride that way. So in most ride high or low on the mast, the foil operate in high immersion mode so no difference in lift to drag ratio.

The real difference is the drag from the mast. Ride high reduce mast drag, simple.

I think you are wrong about the significance of mast drag - but you are correct the lift of the foil remains the same, whether immersed deep or not, as does the drag over the foil. What changes is the amount of water the foil is pushing out of the way. The higher you ride the sooner the water you are pushing away meets air which is way less dense to displace - that is why you are more efficient the higher you fly.

Another way to understand the power of ground effect is to paddle a 14ft race SUP at full tilt in deep water and then go do it in less than 0.5m water depth. 0.5m depth will feel like you are paddling in mud because the water your board is displacing downwards (opposite to what the foil does) is hitting the bottom and refracting back up and hitting the bottom of the board and slowing you down.

With the foil, where the water is displaced upwards the opposite occurs - it meets air, so there is no refraction and less mass to displace so the foil is faster the closer you get the foil to the surface.

If you look close at guys pumping in smooth water you will see the wake the immersed foil is pushing to the surface.

As well as a 'reverse ground effect ' from being high on the mast near the surface, there is definitely a normal ground effect when pumping in shallow water close to the bottom too.

That's what I told the po-lice, I was just ridin' high, it's less of a drag

JohnnyTsunami said..
I was in the water arguing with someone. I told him that in order to pump longer (with no wing) you need to get as high as possible without breaching and stay high during your pump. I figured this out SUP and prone foiling. I can assure anyone you will pump longer and farther with less effort like this and you will feel the foil lift much more (like 50% or more). It's quite a dramatic effect.

He responded that foils are less efficient near the surface based on the studies he read and I just must be feeling the effect of having less mast in the water.

WHO IS RIGHT?!?!

I looked up a recent study which agrees with him:hal.science/hal-03963200/document#:~:text=Hydrofoils%20typically%20operate%20in%20proximity,alters%20the%20dynamics%20of%20the

Note: "h/c" : h = depth of foil, c = chord depth, so 1 h/c = .1 m depth and .1m chord length, for example. I think we're interested in "ii" below which is between .91 and 2 h/c, or where the foil is between 1 and 2 times deep as the chord is long.

Quote from paper linked above:

"The evolution of the hydrodynamic coefficients with the different submergence depth shows three distinct regimes for the influence of the free-surface:

i) for high immersion, [... no difference]

ii) for moderate immersion, corresponding to 0.91 > h/c > 2, the forces are affected by the free-surface; the amplitude of the waves above and downstream the foil increases as the immersion is reduced, but without wave breaking. The lift moderately increases (+12% between h/c = 2 and h/c = 0.91), while the drag drastically increases (+143% between h/c = 2 and h/c = 0.91);

iii) for low immersion, corresponding to h/c h/c > 0.55, the lift drops while the drag still increases, then for h/c < 0.55, both lift and drag drop.

Hi Johnny,

hope you got a good session yesterday, I couldn't make it but it looked like it was nuking

I don't disagree that that being high on the mast helps (it does, a lot)... I do disagree with the theories you hear like reverse ground effect or that you are lifting water above the foil or that the water is denser deeper. These have no basis in any research that I've found, it's usually really good foilers interpreting a feeling they get and making stuff up to explain it. I am always ready to be proven wrong, but aero/hydrodynamics is a mature science and there's very little that hasn't been well studied over the past 100+ years.

The paper you cited isn't really applicable since they are using froude number 0.571 for the simulation, this is less than 2 knots speed for a foil with a 120mm chord. We operate at fn = 3.0 or above.

This paper has tank test results that are applicable to the speeds and foil sizes that we use: ntrs.nasa.gov/api/citations/19930092241/downloads/19930092241.pdf

At the speeds our foils operate at there is some degradation of L/D ratio near the free surface but it's very small, maybe negligible.

On the other hand, the difference between 40cm of mast wetted and 10cm of mast wetted is significant, something like a 20% reduction in total drag on a 800cm2 foil/fuse/stab. A knock-on effect of reducing the drag that much is that the negative (nose down) pitching moment caused by drag is reduced and this will give a feeling of extra lift/front foot pressure as the cp/deck intersection point moves forward.

you really notice it dockstarting/pumpfoiling... riding high on the mast with the wing just below the surface the drag seems really reduced and each pump feels effortless... the nose of my board moves much less with each pump. my friend who was videoing me once said he could see the difference like night and day in speed from the dock.

of course also winging and trying to glide in small bumps you really feel the difference but probably a combo of at least two things going on.

JonahL said..

Thatspec said..

JonahL said..

On the other hand, the difference between 40cm of mast wetted and 10cm of mast wetted is significant, something like a 20% reduction in total drag on a 800cm2 foil/fuse/stab. A knock-on effect of reducing the drag that much is that the negative (nose down) pitching moment caused by drag is reduced and this will give a feeling of extra lift/front foot pressure as the cp/deck intersection point moves forward.

Your estimate of surface area reduction riding higher on the mast seems spot on (though my foils need to be submerged more than 10cm in almost every riding situation). But there is clearly more at work here. "At times" (admittedly vague), there is a significantly greater benefit to riding high than a 20% difference. Certainly the smaller the swell, the higher the foil needs to be in it to derive the most benefit. But can we say that even in larger swell the lifting action increases as the foil gets closer to the top of the wave (as that's what I'm feeling)? It must be a combination of a number of the aforementioned explanations.

Since nobody can be bothered to prove any of this, I'll continue ride'in high

Circulation is greatest at the surface, so you should get the most boost close to the surface as well as the reduction in wetted area of the mast. There might also be an effect of the stabilizer getting some extra downforce due to riding further back in the circulation but probably only on small swells or very steep sections. I think we've all had the the sensation of hooking into a steep swell and the foil feels like it wants to launch out of the water...

I love this gif, but the last frame "raises" a question for me. I've heard the most energy is right at the crest of the wave, like the red/yellow/red idea. But the particle is moving upwards fastest what looks like 2/3 from the trough to the crest. That seems like the place to be right?