I think one of the aspects of this discussion that has not been mentioned to this point is that the direction of the center of effort from the sails when flying the spinnaker is different than when not flying a spinnaker. Because the speed increases with the spinnaker, the apparent wind is changed (coming from further forward), the sails are trimmed to this apparent wind causing the center of effort to move from a more forward direction to a more lateral direction. The force on the platform is more lateral, allowing the crew to put that force to good use to propel the boat. The appearance of lifting the bows may be more a function of changing the direction of the force than the vector of force around the center of resistance actually lifting the front of the boat. Just thinking about the force diagram and relating it to sailing the F17 for several years, this is at least part of what is going on.