Yes, the hulls do experience different forces. You can actually see, hear and feel certain components move on the boat in a seaway. Catamarans are specifically built to both resist and accommodate (depending on what is appropriate) these forces. They have whole structures like main beams and tied bulkheads devoted to it. They also, like our boat, often use uni-directional fiber layups in specific places, as well as intelligent use of coring and resin-infusion or vacuum bagging. Catamarans like Wharrams are actually designed to allow the hulls to work almost completely separately from each other. These boat really move about in a seaway.

The rigging is designed likewise.

The effects are not more dramatic than I intuitively or experientially know. I appreciate that you are just understanding these differences for the first time, but those of us sailing these boats are very aware of them.

As I have agreed, the rudders do see differential forces. I don't agree with you that this has much bearing on anything at all. While they may experience different forces at different times, this isn't any different than a single rudder experiencing these forces. In fact, given that the rudders are connected and share the forces, the differentials are much, much less than you are presuming them to be. The hulls are a completely different story - they are long spans in both vertical and horizontal directions connected by variously different multiple components, experiencing multiple parts of a seaway at different times and moving in and out of the water. The rudders are dimensionally small, both are located in a fixed small area that is in the same relative place to each other, always submerged and tied together by a single component.

For example, I can grab one of our rudders and turn it - causing the other rudder to turn in synch. In this example, one rudder is experiencing all the force, but transferring a good amount of it to the other rudder. Try this - grab one end of a stick and have another person hold the other end. Push and pull on the stick. Does the other person share any of that force?

As for the prop-rudder distance, this doesn't vary too much among catamarans - probably much less so than on monohulls, where props locations range from within an inch of the rudder to being located 15' away on the back of a fin keel. The space on ours is pretty much normal for most catamarans. I can't really tell what it is on the Alpha 42, because the pictures don't give a good perspective. Certainly, the drawings indicate the props almost hitting the rudder. I doubt that is the case in practice because it would preclude folding props - which are very important on catamarans. On one picture of the boat in the air, the distance looked just like ours. Lagoon catamarans actually have their props aft of their rudders.

Besides the obvious differences like daggerboards vs LAR keels and skegs vs spades, most of the catamaran underbody differences are rather subtle - much more so than on monohulls. The hull section shape, rocker and prismatic are the big players in catamaran hull design, but changes in these are not readily obvious. Hull L/B ratios are the more obvious differences.

Mark