.. the structural reliability of the fin is part one. Part two is the design of the bulb. I think this is where much of the "ad hoc" comes in. In the IMOCA 60 class, some agreement seems to be there. ...... ....
It may be so that the performance difference between different keel/bulb designs is small compared to other factors and therefore almost everything can be accepted. And it is good to have the flat bottom of the bulb when your boat is taken out of the water?
That is an interesting academic work:
Today almost all NA use Computational Fluid Dynamics to design the hulls and keels. Some programs cost a fortune and smaller firms use them in time share.
I saw the keels on that study you mention and then went to the conclusions to see if there were some surprises, but no. What he found confirms what is common knowledge regarding keel performance and shape.
Regarding keels the most innovative study that I know is this one, by Lucas. Probably it will be the next step in what regards top racing boats keel design, not in what regards fluid Dynamics but in what regards the possibility of changing slightly boat CG. On the Open 60 they do that changing the quantity of water on the several water ballast tanks but this would provide the same effect without increasing the boat weight and that will bring an obvious advantages in performance...with some disadvantages in reliability due to the greater complexity of the system.
They are testing it on Mini class racers but I think the system would work better on bigger boats:
Another interesting approach is the one by Defline with two canting keels:
2qp double quille pendulaire - defline yacht architecture
Even if here I cannot see the advantages regarding substituting the vertical keel by a foil, except maybe to cruising boats. Probably this system will also increase the motion comfort, again in what regards cruising boats. They tested it on a small boat and they are using it now on bigger cruising designs:
However, you can find almost everything among 30-50' feet racing/cruisers available on the market today. For example the new Far East 31 bulb with chines, the J-boat L-keels with flat bottom, T-keels with maximum thickness in the same position as the maximum fin thickness, very flat, beavertail etc. And all claim that they have an optimized design
Max RM with minimum drag is not the only think that counts here and I believe that flat shapes on Bulbs or a larger format regarding the shape of a torpedo was nothing to do with the minimum possible drag but with create that lift that way. If a boat has a force pushing him up it would be like if that boat had a reduction in weight and that has obviously performance advantages that can (or not) be more advantageous than a slightly bigger drag. I believe the calculations to calculate if that is advantageous or not will be far more complicated than the ones to access the drag of a given bulb