Gary,
Today, we had the Captain of Pilar Rossi sign-up as a member, so he would be best qualified to answer the following question. Below are pictures of Pilar Rossi. The tri-hull configuration they are using for this yacht leads me to believe it is not only to add stability, but also to add additional lift and bouyancy... for what appears to be an added heli pad to the upper aft deck. This is only speculation on my part, but I thought it was interesting and somewhat relavent to your question.
Re: a 90 foot supporting a heli question... Van Mill's 143' Star Ship (link below) supports a Bell 206. Maybe some others can suggest some smaller yachts that do the same.
http://www.yachtforums.com/forums/sh...4&postcount=14