Water Sheer and Hydrofoils

[lightandfrost]

Wind sheer in aviation is an abrupt change in wind speed and/or direction. The two most common types are temperature inversions wherein the abrupt change occurs at the temperature inversion level. Micro bursts from thunderstorm activity can also cause wind sheer. Both these types of sheer occur in water although the causes are different

If an aircraft wing is near stall and the sudden change in wind velocity and/or direction causes the air across the wing to fall below the stall speed of the wing then the wing will stall. This can cause aircraft accidents especially during landing when the aircraft is slow and near the ground. Abrupt changes in water flow speed and/or direction occurs in water. Two common water sheer causes will be outlined here however, there are others.

Surface wave action: Wave and chop produces temporary changes in water speed and direction near the surface much like a thunderstorm microburst. If the water flow is counter to the direction of flow across the hydrofoil, the foil can stall. When a stall occurs many riders think the stall was caused by poor handling, cavitations caused by the hydrofoil exiting the water or wing tip vortices sucking air from the surface. However in many cases, water surging in directions counter to flow across the hydrofoil is the cause.

Water flow surge across sandbars and other shallow areas: Water flowing from deep water to a shallow area then again to deeper water due to wave action or tidal flow can cause a venturri like effect where the water flow temporarily increases across the surface of the sandbar. Hence unlike wave and chop this effect can occur deeper in the water. If this increase in water speed is counter to the flow across the wing then a stall can occur even though the hydrofoil is not hear the surface.

Like wind sheer for pilots, countering this effect can be difficult. Many aircraft can pass thru a wind sheer area in the vicinity of thunderstorms and not be affected, while one aircraft crashes. Riding hydrofoil in wave and chop is the same. You can fly your hydrofoil nap-of-the-wave for a period with no stall when a wave suddenly stalls the foil. When passing thru shallow areas off the coast, maintaining speed and not trying to go too hard upwind might be the best solution.

When learning to ride foils it is best to not launch into the up-slope of an advancing wave. Let the wave pass and then try and start.

[ronnie]

What it looks like down there.

[ronnie]

Wave vortices.

[jumptheshark]

After getting pretty comfortable in flat water on a foil, it can be humbling to take it to the surf. The first season in waves its sort of natural to slow down a bit when the water is big and that can lead to a lot of drop outs and frustration. Just riding along at constant speed in waves that reach shoulder to head high can have multiple rollers of turbulence below the surface that will unexpectedly leave you in the water if your not careful. I think that some wind swell situations are the worst, and would imagine that some of the guys trying to downwind foil on sups experience it. The repetitive three to four wave sets that get pushed along to the point they only break a bit, but never really curl like those in the video above make for hidden traps for your wing like pot holes in the road. You can be feeling rather secure, heading out and just plop, you drop right out as your wing goes through a roller and stalls.

Ways to mediate this are to ride with the wing rather deep, and traverse the swell instead of going through it head on. Just making sure you are almost always at a decent tangent to the swell direction works well enough, but you can still feel them on the wing at times like little bumps.

Speed is the real answer, but for many of us, slowing down is a big part of the fun. The carbo foil crew here don't seem to identify much with this issue. They are always locked and loaded and out past the sets Im talking about. Happens more at the beach with relatively shallow long fetch sand shelf.

Have sized up my wing to help.

[Kamikuza]

I can't see the videos

Have sized up my wing to help.

Yep. Pimping the Axis 720 again, but it was so much fun in waist to head high surf. Once or twice I contemplated just releasing the kite and surfing in but it was my new one so...no.

[ronnie]

I can't see the videos

Have sized up my wing to help.

Yep. Pimping the Axis 720 again, but it was so much fun in waist to head high surf. Once or twice I contemplated just releasing the kite and surfing in but it was my new one so...no.

https://www.youtube.com/watch?v=cFRHgk6niIc

Vortices.
https://www.youtube.com/watch?time_cont ... eVXXij_tIg

[Kamikuza]

Thanks. That second one is fascinating

[ronnie]

Thanks. That second one is fascinating

At Page 319, they seem to be concluding that the Rib Vortices are strongest in plunging waves and propagate from the leading edge downward, but they are weaker (maybe not visible?) in spilling waves, but last longer and go to a greater depth.

https://eprints.lib.hokudai.ac.jp/dspac ... JFM545.pdf

[NYKiter]

Interesting article...appears to only be a simulation....

[TomW]

For me, this is a constant fun challenge. I have only windwaves and chop. Even when gybing /carving I have now learned to read the water and can reduce the ventilation and instabilities caused by water turbulence. Sometimes it's quite surprising. I've found my bigger 590 moses wing to be more sensitive, so I still prefer my smaller 550 wing.
In addition we have what I think you call eel grass in strands, and small clumps that you hit. There's other organic debris floating around too.