foil area for sailing yachts

These types of generalizations offer no information, except to prove the very small sampling and the fact that each yacht had a specific SOR and weren't designed to a "general" SOR. You can take each one of the yachts you've listed and go back, to find the first one and it's intended use, which will be vastly different for each. Production runs do generalize lots of stuff, but this is usually in terms of how they're appointed and equipped, as the hull form and it's appendages are already set. Occasionally you'll see a shoal option added or some other feature, but the hull form is still set.

These types of comparisons have been performed, though using a much larger sampling base, to level out the curves they'll plot and there's still no debate, as to newer sailboats getting fatter, shallower and having more freeboard, with smaller appendages of higher aspect and larger rigs, because of the increase inform/weight derived stability.

I've seen charts plotting these trends over the last half of the 20th century and it's very clear, if you use a big enough data base, to draw you conclusions from. It's also important to compare apples to apples. A modern 40' yacht has nearly plumb ends with maximized LWL and significant beam, while a 60's yacht of the same length might carry 30% or more in over hangs with 20% less beam, greatly diminishing the accommodations volume. So, if you're going to make these types of comparisons, it's important to discuss the same volumes, not just LWL, LOD, sail area to appendage ratios, etc.

 

That was my question in my first reply. I don't think there is any general way to compare different boats. Normal sailors most likely compare boats with similar LOA.

Which size of modern boat should e.g. S&S Swan 40 be compared to? Modern 40' boats with at least 20% longer LWL or with 30-33' modern boats with about the same LWL or something in between?

D/L ratio has certainly a lowering trend, since it uses LWL instead of LOA. What about D/L using LOA?

 

Getting back to the OPs original question, Many of us can calculate the area and span of the foils based on hull and rig dimensions, displacement, and design heel for the steady state conditions. That represents a sort of minimum value since seaway and gusts and maneuvering and accelerating the boat all require a bit of extra area in order to keep the average lift up to where it needs to be and the average drag low. Seaway and gust conditions are in play 100% of the time, so that is a necessary correction. But the importance of the other ones are negotiable.

If you really want to be able to ghost through an anchorage and pick up and cast off from a mooring under sail alone, then you need to deal with the maneuvering issues at low speed and the fact that there will be large perturbation velocities relative to the average velocity.

If you want to be able to tack up a channel that is only twice as wide as the boat is long, you need to deal with that sort of dynamics.

If you don't have an autopilot, you need a very well mannered hull. If you are running an autopilot and are power starved, you need that plus a rudder system that requires minimum control power and has a high degree of authority and high response rate. This generally means bigger is better. Modern cruisers all have autopilots and few are power starved these days. The designer doesn't need to build in the sort of straight line tracking that was typical of earlier cruisers.

And the installation has to be structurally sound, which means different things to different people. Having woken up to find I spent the night anchored by my rudder in a six knot current, I like big beefy rudders. If you decide you need a 3" rudder shaft, there is only so much you can do to limit the cord.

If you aren't using a bulb keel, you need to design enough volume in the fin to hold, say, three tons of lead near the bottom. That also puts a strain on the design. The use of modest bulbs on cruisers is a pretty good indication of the changes in the SOR of typical cruisers. Cruisers can expect facilities that can handle these boats. They have vastly better navigation support so the keels aren't expected to take the abuse that they once were. Dinghys and windlasses are better so anchoring out isn't as big a deal. Mooring fields are more common. Engines are better and bigger on average. Onboard power capacity has increased meaning less power starved autohelms. Basically, everything called a cruiser now is a motorsailer and the hulls have evolved predictably in that direction.

So the question is how much do you value the versatility of being able to sail and maneuver at low speeds and how willing are you to carry an extra amount of ballast, draft, and wetted area in order to achieve it. How important is the ability to sail for long periods without a lot of power assist. How meticulous are you willing to be in keeping the hull clean. Do you clean your hull with a sponge daily or with a pulaski every other year.