I have sailed a lot of different cats but I must say that the rubber 'bung' system on the Nacra and Inter boats is the tightest most sensitive rudder system I've ever helmed. I disagree that it numbs the feel of the helm and coming from a Hobie 18, Nacra 5.2, and Nacra 6.0 to an Inter20 and a Nacra F18 I find the latter two extremely twitchy. While it does retain some return force at full extension, while near center it has practically none.

Somebody touched on it before but you'll find the biggest difference between a boat that tracks straight without a hand on the tiller and a boat that has a good bit of helm is rudder rake. Without getting into a disertation on boat design, the modern boats put a lot of weatherhelm into the design of the underwater foils. By countering this helm with the rudders, it puts the daggerboards at a very slight angle and loads them so they help provide additional lift to windward. However, this load has to be counteracted by the rudders. Now, enter the spinnaker where the underwater foils need to be adjusted to counter a significant change in the sail loading around the course and the rudders have to work even harder.

All this loading on the rudders is counteracted by having the center of lift of the rudders closely centered near the pivot point of the steering system. The closer these two are, the less helm you will experience. If you actually move the center of lift of the rudders forward of the pivot axis, you will experience lee helm (i.e. the boat wants to turn downwind when you release the tiller).

Any catamaran can be adjusted so that it tracks in a straight line when you put the tiller down. It is easy to improperly interpret this as meaning the rudders are not working as hard but in reality they are experiencing the same loads - you just don't feel it in the tiller. I think the Inter/Nacra components allow their rudders to be adjusted a little closer to the pivot axis of the steering system allowing for a more nuetral helm. I am able to tilt my rudders so far forward on my F18 that I get lee helm.