Building a simple Hydrofoil

[fredrosse]

The Wing Guy indicated...."the reference area is the area seen in plan view. " ALSO "The data of interest for hydrofoils and airfoils is lift and drag. Both are referenced to the area spanXchord." REPLY: I think that area (plan view) is used to determine the lift force, "Cl" not the drag force, "Cd". Is that correct?
For angles of attack from, say 0 to 10 degrees, the plan area does not change in any way significantly, however the area facing the velocity vector does change significantly, hence my question in the previous post.

"Velocity head" in your terminology corresponds to "dynamic pressure" or "kinetic energy density" as used in aerodynamics. (1/2 rho V squared.)" REPLY: Yes, that is the definition I use

"The experimental nature of this is another argument for making your first boat small " REPLY: I only have so much resources, so I am going to make the 19 ft boat, as light as prudent, but with a 14 horsepower power plant that costs several times the cost of the hull, I would still have a boat that I could enjoy, even if the hydrofoil project is a complete failure.

Thanks so much for your response here, but I still have clarifications that need solid resolution.

 

[Heimfried]

10 years ago I read a lot about hydrofoils. The above mentioned publication (Abbott von Doenhoff) will show you the usual dealing with airfoil data. Lift and Drag are not given as forces but as coefficients. Their curves, depending on angle of attack are shown in diagrams. The data are measured, not calculated, I'm sure for calculating the forces only one area is used for lift as well as drag. The paper will answer it.
You will find it online e. g. searching for "NACA Abbott". ( https://ntrs.nasa.gov/api/citations/19930090976/downloads/19930090976.pdf )

 

[montero]

And don't drink 2 bottles of vodka at helm :

 

[The Wing Guy]

The Wing Guy indicated...."the reference area is the area seen in plan view. " ALSO "The data of interest for hydrofoils and airfoils is lift and drag. Both are referenced to the area spanXchord." REPLY: I think that area (plan view) is used to determine the lift force, "Cl" not the drag force, "Cd". Is that correct?

No that is not correct. "Wing area" (in plan) is the reference for both. You should watch some wind tunnel tests being performed. Lift and drag are simultaneously being measured, and lift is always measured perpendicular to the free stream, and drag is always measured in line with the free stream. These are conventions that are always observed, and they make comparisons of wing sections possible. The actual summed lift vector direction changes with angle of attack, but for aircraft and hydrofoil design, and for virtually all discussions regarding wings, the conventions are very helpful to avoid total confusion. I don't recall Vellinga's book being particularly helpful re the history of wing testing, but A&D certainly describes things well, as will any good aerodynamics textbook.

For angles of attack from, say 0 to 10 degrees, the plan area does not change in any way significantly, however the area facing the velocity vector does change significantly, hence my question in the previous post.

This is also generally untrue. Imagine a symmetrical section like a 0015. You can print one and cut it out of paper. If you are careful and hold it in front of one of your eyes, you can see that in the range of -10 to 0 to + 10 the leading and trailing edge do not extend beyond the maximum thickness point that you are seeing. You can also lay the paper cut out section shape on a piece of graph paper to see that for small angles of attack, neither the leading edge nor the trailing edge extend appreciably beyond lines that define the maximum thickness point. Of course at very large angles of attack (outside whit is usually seen in hydrofoils, then the frontal area increases and drag increases greatly.

(For significantly asymmetric sections, (say 4412) the trailing edge drops below the maximum thickness point at low angles of attack, so what I just wrote does not apply precisely, but the convention holds: the wing are as seen in a plan view is the reference area. )

As I said, for cars, buildings, flat plates, and various other things, the reference area is frontal area. Wing Cd's are extremely low as compared to automotive Cd's partly for this reason.)

Imagine how cumbersome it would be to have to calculate the frontal area for each angle of attack, just to get a drag number.

"Velocity head" in your terminology corresponds to "dynamic pressure" or "kinetic energy density" as used in aerodynamics. (1/2 rho V squared.)" REPLY: Yes, that is the definition I use

When talking with aerodynamicists you will want to use their terminology instead.

"The experimental nature of this is another argument for making your first boat small " REPLY: I only have so much resources, so I am going to make the 19 ft boat, as light as prudent, but with a 14 horsepower power plant that costs several times the cost of the hull, I would still have a boat that I could enjoy, even if the hydrofoil project is a complete failure.

Thanks so much for your response here, but I still have clarifications that need solid resolution.

After you have read Vellinga, feel free to come back with additional questions.

Also remember that Google is your friend. This is Gemini's take on the matter:

"Wing area is used as the reference area for both lift and drag because lift is primarily generated by the wings, using the wing planform area simplifies math compatibility, and it provides a consistent baseline for comparing different aircraft designs. [1, 2, 3, 4]

Why the Wing Area is Chosen

• • Direct Proportionality to Lift: The main lifting force of an airplane comes directly from the wings. Using the planform area (the surface area seen from directly above) matches the physical source of the force. [1, 2]
  • Equation Consistency: Total aerodynamic drag on an aircraft includes induced drag, which is a direct byproduct of lift. Using the same wing reference area for both lift and drag allows engineers to use direct mathematical relationships—like adding lift and induced drag coefficients together cleanly. [1, 2, 3]
  • Standardized Comparison: It turns complex force measurements into clean, dimensionless coefficients (\(C_{L}\) and \(C_{D}\)). This lets engineers easily compare the efficiency of different wing shapes and sizes on equal terms. [1, 2, 3]"
  
  

 

[The Wing Guy]

AI is getting better quite quickly, but it sometimes screws up in the oddest ways. What Gemini said re reference area for lift and drag is generally correct... but then it goes off the rails when it implies that engineers would add together the lift and drag coefficients. Certainly, engineers compare those coefficients but adding them together is not something I have ever seen done.

For those who have not fallen asleep yet, Abbott and Doenhoff, (on page 3 in my version) provides the expressions for lift and drag. Aside from the coefficient referenced, the expressions are identical, with "S" (wing plan area) in the same place in each. Vellinga doesn't not make this so crystal clear, and provides some drag data for shapes other than airfoils without explaining that the Cd in those cases, has a different reference area.

Also, for most people, I suspect that the difference between 2D flow and 3D flow may be confusing. The several hundred pages of data from wind tunnel testing provided in A&D relates to 2D testing. In other words, what is being tested is the shape of the section as seen in a 2d view (a cross section). The third dimension would be span, but that is not tested for section data. The test sample spans the tunnel, so acts like a wing with infinite span (there are no wing tip vortices, and there is no spanwise flow, and there is no consideration of aspect ratio. When you design something with airfoils or hydrofoils, you must take all of that into consideration. So the thrill of seeing that, for a 2412 section at 4 degree AoA, Cd is .005 when Cl is about .5 (WOW! 100:1 L/D) evaporates once you take into account the other sources of drag.

Nevertheless, when you carefully design something for high L/D, (which translates directly to glide ratio for a sailplane) the L/D ratio can still be 60:1 (in the very high performance sailplanes.) In powered aircraft, 17:1 is good for clean configuration, and half that is typical for dirty (flaps deployed) configuration. Good hydrofiol craft can be about 15:1. The little microfoiler I am building right now will be worse than that: therefore not much good for human power. OK for outboard motor power, possibly OK for sail power.

Although a sailplane can actually operate at 60:1, even including the drag on the fuselage, a hydrofoil boat typically has a far less streamlined hull and superstructure, causing significant aerodynamic drag. The aero drag must be added to the hydrodynamic drag to get a feel for how much power is required for a desired speed.

More required reading: Marchaj's Aerohydrodynamics of Sailing is useful for understanding what makes a sailboat go fast or slow, and much of what is discussed can be applied to power boats.

Lastly, this page by Mike Waters is useful for deciding whether to foil or not. I have a catamaran with unusually slender hulls (18:1). I might wonder if it makes sense to put it on foils. From his chart (modified from an SNAME chart) I can see that at 10 knots, I could expect 250 lb per tonne resistance from foils and about 150 lbs from slender hulls. I could have a hard time getting my cat to takeoff, without perhaps, using motor power assist. (My water line is a little more than 25 feet, but so much more to bother rescaling the graph. )

 

[FlyingMo]

I think the confusion results from this: By convention, the reference area for the aerodynamic drag coefficient of a car is the projected frontal area, and for an airplane it is the vertical projection of the wing area. There are some details, such as the drag coeffecients of equipments on an airplane, such as pylons, cameras, bombs, etc. are also initially calculated with the frontal area, and their contribution to a specific airplane drag is recomputed for the wing area in a late step.

For 2D airfoils the reference is the chord length.

But generally I can recommend to not loose too much time with the selection of airfoils. Just pick any one you like, and that you can easily manufacture. For any reasonably good airfoil, their contribution to drag is much smaller than induced drag or drag from ventilating the airfoil, and the most difficult thing to get right as a beginner is stability. For example for an airplane I am working on the peak lift to drag ratio of the 2D airfoil of the wing is more than 130 and of the tail is 80, but the lift to drag ratio of the airplane as a whole is only 20 (which is still considered good in its size class).

So if you want to DIY it first think about stability:
The easy solution is surface piercing hydrofoils. for exampe two V-shapes in the front and one V-shape in the rear. the tip of the v can be rounded of, like an U. The three tips go under water, and form a tripod. More weight on one of these three legs means that a wider section of the hydrofoil is submerged which leads to more lift. It is therefore self-regulating. The only thing you need to worry about is dynamic instability, i.e. it bobbing up and down, but that can be fixed with changing the center of gravity.
The reason not everybody is going this easy route is twofold: it builds realtively wide, and a large part of the hydrofoils is ventilated (i.e. there is an air bubble trapped in its wake) which creates a lot of drag. So these are not very efficient.

The more efficient way is essentially building an underwater airplane, with a straight main wing in the front and a tail in the back, both fully submerged. But then it is much more difficult to get the stability right.

There is the free tool called flow5, which is made for airplane design, but it allows putting in the density and viscosity of water and an open boundary (i.e. air), so it could be used for hydrofoils as well. I really like it, because it is comparatively easy to use, but nevertheless it is a tool for engineers.

 

[The Wing Guy]

For 2D airfoils the reference is the chord length.

This may confuse the OP. He was asking for a reference area, whereas the cord length is one dimensional.

The more efficient way is essentially building an underwater airplane, with a straight main wing in the front and a tail in the back, both fully submerged. But then it is much more difficult to get the stability right.

This is the standard configuration for foil boards, which are something of a marvel of efficiency. Pitch, roll, and even yaw stability are heavily dependent upon the skill of the rider. There are those riders who say its no big deal, of course, but many people have attempted to foil and found it too difficult. I used to fly a delta wing hang glider, and the natural stability of the cg being below the wing, instead of above, was one thing that made learning to fly those extremely easy.