Ellison's challenge also specifies a catamaran, although nobody involved really wants to depart that far from the Cup tradition of racing in monohull boats.
New Zealand banker Michael Fay sued defending champion Dennis Conner in 1987 after Conner refused to consider his challenge to race in a 90-foot monohull.
The original start scheduled for Sunday was postponed because of force nine winds and huge seas but Saturday's start for the 17-strong monohull fleet went ahead as planned.
Fay sued and won a court order requiring a match for the Cup, but Conner outsmarted him by building a speedy catamaran that sailed circles around Fay's 90-foot monohull.
The rest of the multihull fleet will finish in Boston over the next few days, while the Open 60 monohull class leaders are expected to arrive from 12 June onwards.