I thought it was necessary to clarify a few points.

The Nacra 6.0 was 'flying' the spinnaker when it flipped, not just rigged for one. The factory offered a new mast at a greatly reduced price (at the urging of the dealer - me) even though the mast was being used with a spinnaker. Dave commented to others on site that he was not using any downhaul while flying the spinnaker which is asking for problems. Did he let the mainsheet go to save them from going over which is a common mistake when someone has limited spinnaker experience, who knows? That often will cause a mast to snap. If he was using a Model 'A' Loos Gauge then 35 is only about 365 lbs and 15 is only about 150 lbs. which is way too little tension. His previous mast didn't fail while in use it was rolled into power lines and got zapped.

I agree with Jake's assessment about the spreader rake. I caused my mast track to squeeze slightly at the spreaders in the same race. Before the race I moved my spreader rake forward to 1 1/4" (same as Dave's) which I have never done before. I've always had it at factory specs. 1 3'4" and never had a problem. It was a windy, rough and wild race. I downhauled the daylights out of the sail like never before and the combination of that and too little spreader rake caused the mast to arc like a banana. I noticed the excessive downhaul too late. My fault, no one else's.

Te 6.0 mast hula was pretty much resolved several years ago. I haven't had it happen more than once or twice in recent years and only in extreme conditions. It used to happen all the time. Wide lower battens, move the jib cars outboard and most critical, don't bring the clew traveler car too far forward. As the wind would pick up I would usually move the traveler car way forward. I eventually realized the different angle of pull on the sail from the mainsheet system was inducing the hula. I found this out from experimenting in light conditions a few times with the car all the way forward and sheeting in hard.