We discussed this a few years ago. I have some new views on the subject and are interested in hearing from others. My mast was pumping so bad the other day i thought it was going to break the shrouds. I was out in a fresh breeze and he waves were building. I was under main only. I do think the jib helps keep the mast from pumping, but not much. When the main is near centered and pulled in tight that will stop the pumping, but that isn't possible on a reach. If you want to know if your mast can (will) pump; put up the mast on the trailer and walk forward and physically push the mast forward and backward. The only adjustment I found that helped was getting the shrouds very tight. tightening the forestay did not seem to help. I now believe that the tight shrouds somehow keep the mast from pumping. But I don't understand why tightening the forestay doesn't do the same thing. Maybe because they don't attach at the same point on the mast. I don't know. Any thoughts.