Do I need to sine to go deep downwind with hydrofoil?
How can I go upwind / downwind more than speed of wind?
Why can I sail very deep downwind with a foilkite?
Please read following longish explanation with care. It may be helpful
Answer: Apparent wind speed + Apparent wind ANGLE (especially downwind)
When one is hydrofoiling with a race foil (very low hydrodynamic drag) and flying a sophisticated race kite (Elf, Chrono, Sonic etc.) with high performance (low aerodynamic drag, high lift from sailplane-like high-aspect ratio wing, low drag thin flying lines + bridle) then one is sailing upwind faster than the wind. How/why? At take off procedure, due to kites airspeed(sine/dive/loop)-produces power to pull rider + board through/out of the water. There is a lot of hydro drag from board / rider body when waterstarting, this needs a lot of pull to come over. Once starting to fly, the drag goes to minimal, perhaps to only 2-3% of the starting drag.
What happens then? The speed will accelerate, due to diminished hydro drag. Then, the kite will have increased airspeed. This results in increased lift=pull. This results in increased speed on water (boatspeed), which results in increased airspeed again, and increased lift and we are off!!! The acceleration stops, and max speed found only, when the hydro drag of the board and airdrag of rider, lines, bridle and kite will prevent further increase in speed, and therefore further increase in lift. The lift equals the drag, and topspeed is there.
Then one turns to sidewind and then continue downwind. What happens?
When one turns towards downwind, one turns to a bigger true wind / apparent wind angle (the moment of reaching, before downwind). This is the key moment to one's coming downwind performance! What happens? The angle of attack on the kite's leading edge increases dramatically due to changing course, and the lift it produces, shoots up in great amount. Speed through water (boatspeed) increases greatly, one accelerates like a sportscar. What happens during that acceleration? Since TRUE windspeed / windangle stays the same, our apparent wind angle will move again to smaller numbers, much closer to upwind angles. Why? Because, we are already sailing faster than the wind, and the kites leading edge will meet the airflow from the front, rather than from very high angle of attack, that some people call "drifting". These some people also say, that a wave kite can be good for hydrofoil, since they "drift" downwind well, and one can sail very deep angles downwind. Yes. It is true. But only if you sail SLOW! Then the apparent wind angle comes from "behind" = very big angle of attack. The kite "drifts" (sits deep in the window, near stalling).
Now back to one's reaching moment, with proper high performance kite. A lot of acceleration made (in just 1-2 secs, now one starts to turn more and more downwind:
When the speed increases, the lift will increase, and tolerate then this new lower angle of attack, and we turn down more, and same happens again. More lift, more speed. Ok, so when one keeps the high performance kite going downwind, the kite is actually sailing almost upwind. This means, that one can turn more downwind, since one wants to sail as deep as possible, straight to the mark, if possible. When one sails deeper, and deeper steering angles, in the end, the apparent wind speed decreases on the wing, and the increased hydro/aero -drags again meet the lift, and then we have found our max downwind performance. This performance is a mix of many, many little things that matter ( how deep is the foil in the water=less mast in water= less drag, how is the bar? More bar pressure = more aerodynamic drag on kite, if pulled beyond optimal profile for downwind. How is the steering angle, compared to wind speed / boatspeed, kites angle of attack and lift produced?
All these little things matter, and they can be summed up as a skill of sailing fast.
Sine, in general is only working when boatspeed = speed through water is very low. By flying the kite in sine wave style up and down creates airspeed and additional lift. A high performance hydrofoil does not benefit from this, since at highspeed the steering movements on the wing to produce the sine create aero drag that partially ruins it, and the rest is ruined due to general decrease in speed when kite flown to a direction which is not forward, and therefore diminishing boatspeed, and airspeed, and again lift. Sine is needed, if one is shitting the pants with high speed transition from upwind to downwind. If there is no speed going downwind, then one has to sine to produce some airspeed and lift. One can accelerate only by sailing higher angles fast. There is no acceleration possible, if driving deep angles dead downwind.
So, how Do I maximize my light wind hydrofoil downwind angles/performance? Use a BIG high performance kite with a lot of lift against the aerodynamic drag it produces. More lift with low airspeed is the definite key here. It is a big foilkite btw, as long as someone reports, that a LEI can outperform a foil kite in light downwind performance.
My very longish 2 cents. Please bear with me. But, this is not easy to unfold with just 2 lines. This is quite complex, and combines both aerodynamics and hydrodynamics. It has many things in common with flying a seaplane.