"Whenever I’m motoring with the wind about 10 degrees off the starboard bow, the mast starts pumping so badly that it shakes the whole boat. The same thing happens at anchor. The mast is deck-stepped, and I’ve beefed up the compression post under the deck to make it rock solid. I’ve also checked the mast with a plumb bob and found that the column is straight but has a slight rake aft. All the rigging is tensioned evenly. I’d like to know why this pumping action occurs when the wind is blowing onto one side. A professional rigger told me he had no idea. Is there anything else I might do?"
— Dan Smith, Fort Myers, Florida
Win Fowler replies: This problem is not uncommon, particularly with a single-spreader deck-stepped mast with just a single set of in-line lower shrouds. The best solution is to install forward and aft lower shrouds; usually this means installing additional chainplates and possibly reinforcing the hull. The tangs under the spreaders will also have to be modified.
To stop the pumping, try rigging a temporary babystay from a yoke at the spreader base down to the stemhead. Tensioning it when you are under power or at anchor should solve the problem. Be careful not to tension the stay so much that you pull the mast forward out of the column more than the distance of the mast’s diameter. Take the stay base back to the mast when you’re sailing; the mainsail should stabilize the mast and keep it from pumping. If this temporary stay works, you might think about adding a permanent stay.
I can only guess why the pumping occurs at one specific angle, but I suspect that the base of the mast is not bearing perfectly evenly on the step. A very small misalignment, just 1⁄64 inch or less, will be enough to affect the dynamic behavior of a deck-stepped mast. The problem might also be that the base of the mast is not perfectly square to the maststep. Or it could be a little of both. Unstep the mast to check the squareness of its base and see whether there is wear or corrosion on the step. When the step and the mast base are perfectly aligned with each other, the tendency to pump will be reduced; the more the base of the mast is supported by the step, the better. Of course, the step itself must not move.