I think there has been a change in terminology over the past while. Now there are three common rudders folks talk about.Flat or Slab Rudders
are just that, maybe a street sign on a frame. Flat as a pancake and no shape to it. These are the least effective but at one time the most popular.Foam Filled
(Used to be called airfoil) Rudders are a step up from the slab sided rudders. Much more effective and more rigid. Typically shaped like an aircraft wing too, wider at the top than at the bottom for clearance when sliding off banks. The leading edge generally
had a radius from very small to up to 2" and were shaped like an airplane rudder..I personally like these. They also had some foam filled rudders that had a "V" crease for the leading edge, these were not an airfoil.
This is what we used to call airfoil rudders and now are called foam filled rudders.Airfoil Rudders now
are oval in cross section and have no filling. Most I see have a very thin leading edge and are not a true airfoil. They are prestressed when welded to the end caps. Typically these rudders have a constant cord from top to bottom and are much taller than they are from front to back. This is what I think "most" manufacturers are selling now days cause they are easier to make than foam filled. My personal experience is that they lack the authority of the foam filled rudders and that was on a small boat as well. They are, however a real tangible improvement over slab or flat rudders. Do not confuse this terminology with the large single Glades Style rudders, which are actually an airfoil, they are nothing alike at all.
My next boat will have foam filled airfoil section rudders. I personally, like them better.