MBounds, where did you find that graphic?
That graphic was one I created for an issue of the Hobie class magazine, the HOTLINE
I had been wondering the same thing, I'm surprised that Hobie didn't go with a standard NACA foil shape, considering the amount of research that went into them... I'd be surprised if Hobie put more research into their rudder/daggerboard shapes than NACA did.
Most of the research that Hobie Cat did in the '60s and '70s was empirical - they built what looked good and could be easily produced - and if it didn't work, they tweaked it and tried again. After all, both Hobie Alter and Phil Edwards (creators of the Hobie 18) were surfboard shapers and went with their gut feelings.
NACA was a federal agency (precursor to NASA) with ~500 employees in the early '40s when they did the research on foil sections. The amount of money and time spent was order of magnitudes larger that what Hobie did on the foils for what are essentially recreational toys.
Then again, a lot more research and advancements have taken place since the H18's debut, the daggerboards are pretty outdated and inefficient.
They are what could be easily and inexpensively produced with the materials available in 1977. The high-aspect ratio boards being used on modern catamarans would be impossible without carbon fiber and advanced construction techniques.
There's also a trade-off in convenience - the 18's boards are relatively shallow and very robust - an ideal combination in a recreational boat. An F-18s high aspect boards require a lot more care, and a momentary lapse in attention results in a broken ($$$$) board. I've never seen an 18's board break (Banged up and delaminate - yes. Broken off at the hull? No.)
As I've mentioned before, it's difficult to execute a NACA section in a high-quantity production part, especially the trailing edge.