I just got the Ray Vellinga: Hydrofoils book, I have plenty of reading to do, and it looks like the stability issues are addressed. I am a fluids engineer, and the definition of a drag equation has always been (at least in my experience) : Drag Force = Drag Coefficient x Velocity Head x Area, where Area is defined as the area as seen from the maximum cross section that is at right angles to the velocity vector. The hydrofoil sections will change this area as the hydrofoil changes it's angle of attack. It appears that, at least for hydrofoil technology, the area used in the drag equation may just be the area when the angle of attack is zero. This alternate convention may apply here, and I can see that using a constant area in the equation, (rather than re-calculating the area presented at right angles to the velocity vector for various angles of attack) could simplify evaluations. Does anyone know which method of evaluating drag force is correct for hydrofoils?
For both airfoils (as used on airplanes) and hydrofoils (as used on boats like Vellinga describes in his book) the reference area is the area seen in plan view. In other words, the chord multiplied by the span. For things like streamlined struts, the reference area is not so uniformly used as the span times chord -- and is likely to be frontal area, instead. (For example, Cd as used in automotive parlance, references frontal area.) You just need to be sure that the area you are using corresponds to the data you have obtained (from actual tests or CFD). This is all adequately explained in Vellinga's book, if I recall.
The data of interest for hydrofoils and airfoils is lift and drag. Both are referenced to the area spanXchord.
In addition to Vellinga's book, you might find Abbott and Doenhoff's "Theory of Wing Sections" to be of interest. Data and descriptions for the foil profiles often used for hydrofoils (4412 and 63-412, for example) are presented in A&D.
I have built airfoils and hydrofoils for various purposes, and would suggest that you start with a smaller boat, so that you can learn the techniques without spending as much time and money. As it happens, I am currently building a little hydrofoil boat that uses the hull of an ultralight pirogue I had not been using. I also happened to have some foils that were not at all optimum, but that cost me nothing. For less than one hundred dollars, I can play around with various approaches to steering and angle-of-attack control, and see what I like.
BTW, banking into turns is typical, but not a requirement (any more than it is a requirement in a car). Many sailing hydrofoil boats sail flat all the time. You decide what you want, and design accordingly.
"Velocity head" in your terminology corresponds to "dynamic pressure" or "kinetic energy density" as used in aerodynamics. (1/2 rho V squared.)
It may seem odd that after more than 100 years of foiling boats, you will still be experimenting. There are not many production hydrofoils that you can simply copy (other than the foil boards). Electric propulsion can simplify things a little, because you will not need to devise a system to keep the at-rest water level safely well below the power head of an outboard (while still having the prop in the water while flying). The experimental nature of this is another argument for making your first boat small -- then you are only putting yourself at risk -- there are good reasons that most of Vellinga's pictures in his book show him wearing a crash helmet.
