A central rudder ventilating when pushing hard is definitely a problem, and always will be, but one has to be sure that the cure is not worse than the problem. I've also experienced rudder ventilation many times while pushing hard off the wind, but while it can be scary, I have never had a boat round up or spin out from it. Usually it will keep tracking reasonably straight, or start to head up or down, but always gradually, and steering control is usually restored fairly quickly. But having no control can certainly be a concern.
The key is to always try to keep the boat/sails well balanced so boat will still track well without a rudder, and to keep the stern sections in the water for as much as possible. This is one reason why I do not like very wide aft sections on floats, as they will lift the stern more, particularly in quartering waves, which means a greater chance of losing control.
Float rudders are definitely the ultimate answer, but they do add more expense, more weight, more drag, and more complexity, and will be there for the 90% of time when they are not required. Weight can be reduced by making them smaller, but then one can end up with not enough rudder in other circumstances, to where control will be lost and the boat will end up in irons. However, float rudders are a valid solution, and hence they are optional on the F-85SR and F-32SR, but they would not be my personal choice.
The other alternative is to use a much longer daggerboard style rudder blade, with a more effective section, and the F-85SR will have such a rudder as standard. Advantages include simplicity, less weight, lower cost, and while it can be deep when needed, it can also be lifted up when not, so wetted area is less for the 90% of the time where a deep rudder is not necessary.
Next step is to add water ballast to stern, so as to keep main hull stern down when required, and a stern ballast tank is standard on the F-85SR and F-32SR, with float sterns tanks optional. This weight again can be got rid of for the 90% of the time when it is not required.
The other tactic is to never fly the main hull much at all, or have it just skimming above the water, so that rudder is always kept in the water. This is the fastest point of sailing anyway, as any higher and one is only adding windage and reducing power, while just touching the water or planing on the top is not much slower, if at all. In theory it should be slower, but in practice I doubt if there is much difference at all.
We have the same situation with the daggerboard down versus daggerboard up debate when off the wind - daggerboard up should be faster - has to be! But we have never found any difference when comparing identical boats, on long downwind legs, with board up on one, board down on the other. I have instead lost far more time by forgetting to put board back down at leeward marks, so now the board stays down all the time, period. However, I still find myself lifting it on very long downwind legs in important races, just in case, as it just has to be better - right? But I have never seen an instance where it was. So I feel the same about flying center hulls, looks good, but likely no faster, slower if too high, and it increases capsize risk. Just my opinion.
A long board will always be better, and the problem with a vertical board on a tri is that it will have to be behind the mast, and get in the way of the boom when raised, plus make it very difficult to lower mast on a trailerable tri. I'm happy to go either way, as it is a 'six of one, half dozen of the other' situation, and I have vertical boards in my F-41 cat design, mainly because it is better for the interior, a little simpler, and there is no boom above to foul.
Designs that work...
Thank you very much for taking the time to answer my questions so completely. It is a rare opportunity to get to talk so candidly with the designer of boats you admire and sail on.