# Hydrodynamics of hydrofoils, roll stability, input?
**URL:** https://foil.zone/t/hydrodynamics-of-hydrofoils-roll-stability-input/25051
**Category:** Build, Repair & Mod
**Created:** [September 29, 2025, 7:53pm UTC](https://foil.zone/t/hydrodynamics-of-hydrofoils-roll-stability-input/25051 "2025-09-29T19:53:24Z")
**Posts on this page:** 3
**Page:** 3
### Author:  [@IntuitiveEngineering](https://foil.zone/u/IntuitiveEngineering)
#### Post date: [October 12, 2025, 9:52pm UTC](https://foil.zone/t/hydrodynamics-of-hydrofoils-roll-stability-input/25051/41 "2025-10-12T21:52:30Z")
Lots of interesting discussion on the wing shapes and drag here.
Just curious if any of you had any feedback on my theory for the roll stability vs wing tip direction. I have noticed that this effect also works across the entire wing, roll stable wings will have significant anhedral. Seems counter-intuitive given when in air anhedral makes roll less stable. Cheers.
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### Author:  [@Willa1980](https://foil.zone/u/Willa1980)
#### Post date: [November 20, 2025, 12:19pm UTC](https://foil.zone/t/hydrodynamics-of-hydrofoils-roll-stability-input/25051/42 "2025-11-20T12:19:29Z")
**I think I finally found an explanation for anhedral making hydrofoils more stable on roll:**
If I want to turn left, I shift my centre of gravity (COG) to the left side of the board. This rolls the foil to the left. Because lift is always perpendicular to the wing, the lift vector now tilts to the left as well. A component of that lift becomes a **side force** , which makes the foil start to **slip (drift)** to the right relative to the water. In other words, the foil wants to keep moving straight ahead, but the tilted lift vector forces it into a curved path.
Once the foil is slipping sideways, the direction of the incoming flow changes: the water now arrives slightly **from the right**.
If the wing has **anhedral** , this sideways flow increases the **effective angle of attack** on the lower (left) wingtip in a left turn. The left wing therefore generates **more lift and more drag**.
- The **extra lift** acts to **level the foil** (restoring roll stability).
- The **extra drag** helps the system continue to **yaw left** , supporting the turn.
This behaviour mirrors what happens in aircraft. A dihedral wing allows an aeroplane to turn using rudder alone: deflecting the rudder left yaws the nose left and creates sideslip from the right. With **positive dihedral** , that sideslip increases the angle of attack on the **right** wing, rolling the aircraft **into** the turn. Roll and Yaw are therefore **positively coupled** for dihedral wings, and **negatively coupled** for anhedral wings (making anhedral wings stable on roll).
In the mean time, I made some more wings and stabs with proper vacuum bagging. This is really fun, and I am sooooo much looking forward to test them next summer.
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### Author:  [@javierru](https://foil.zone/u/javierru)
#### Post date: [November 21, 2025, 9:48pm UTC](https://foil.zone/t/hydrodynamics-of-hydrofoils-roll-stability-input/25051/43 "2025-11-21T21:48:18Z")
great work, looking forward for your results. I just published my webapp to calculate the power draw of foils, I believe you will like it 😁
> [@Foil calculator, a intuitive tool to calculate the power draw of front wings](https://foil.zone/t/foil-calculator-a-intuitive-tool-to-calculate-the-power-draw-of-front-wings/25429):
>
> Hi there, i have been working in a intuitive way to calculate the power draw of foils for almost a year and with help from a friend we finally got it to the point in which i think it is worth to share, give it a thy and let me know what you think. here is the [link](https://foil.jauer.it) The calculator is intended to be used for pump foil, as a few watts make a world of difference, but i believe for e-foil might be useful as well. I have many models of gong v2 as that is the setup i use, if you want more wings, let m…
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