I too have thought about a spinnaker for my '91 30T. What Jackdaw said sounds spot on. Curiously, when I replaced the topping lift and main halyard sheaves on my '91 30T, there was a ~0.75" roller for a spinnaker (halyard..?). The roller was for sure used on older masthead Hunters, but may have been used for the topping lift if two main halyards were used on the fractional rig. It's an Isomat NG31 spar but no mention is given for a masthead spinnaker on a fractional rig on the Rigrite page, but there are several options for up to three halyard jibe boxes that accommodate a spinnaker. Rigrite might be able to help or try giving Hunter a call first. Eddie Breeden told me my boom spar was plenty strong enough to handle a loose footed main versus the factory boltrope footed sail when I had new sails made. But I know nothing about the lateral loads from a spinnaker. You could talk to the fellow who took over for Eddie, or talk to rigrite to see if your boat used isomat spar/parts and how spinnaker were rigged on the fractionals. Just a thought.
