Mast pumping?

[vyv_cox]

My impression is that it is commoner on older masthead rigs than modern ones, which are more likely to be properly rigged. We have a 19/20 rig with double spreaders that are not swept. Mast flogging is not a big problem but occasionally occurs with a moderate wind from the beam. I sometimes see a small triangle hoisted on the forestay to allay a similar problem but a mast is harder. I have seen people hoist a fender halfway up to break the vortex but haven’t tried it myself. I believe that spirals are built into tall structures for the same reason, and although I haven’t tried it, it looks as if the spiral winding described above might be successful, but presumably less so with double-spreaders.

That is certainly true of Sadler 34. The babystay to mast angle is very acute
with the result that tensioning it does not provide a lot of damping. Fore and aft lowers might be more successful.

The point about cap shroud tension seems to be misunderstood. Imagine pressing a thin stick lengthways between your two hands. As you increase compression the stick comes closer and closer to buckling. A small lateral force will easily bend it, far more so than without the compressive force. Exactly the same happens to a mast as cap shroud tension is increased.

 

[Daydream believer]

Change the frequency of the offending part of the rig. Experiment with shrouds & backstay. Unlikely to be forestay if you have a furled jib on it. As suggested earlier a line stretced between opposing shrouds, as high as you can reach, or just a large fender hoisted up the backstay, may solve the issue. I have a line with a hook fitted to the backstay so I can hook it to the boom & this takes the weight off the topping lift & changes frequency of the backstay. I leave the mainsheet slack.