I think the caterpillar straps to the sides of the keel.
On a "shame on us" note. this whole thread has been about motors (well props which are pretty useless with out a motor).
We could be using our sails!!!. Clean, green, quite, long range, cheap to operate and can drive the boat at hull speed
So I'm thinking a shroud/duck would only slow me down under sail (more drag and no way to feather/fold). So why would we even be talking about such a "solution" unless we where closet motor boaters.
I feel i need a shower.