Mast pump

[David W]

Hello all, We have a Crown 34, aka San Juan 34.It has a double spreader rig, fore and aft lowers, two side stays, one to the mast head, one to the lower spread (I think my memory fails me...)

On the weekend, we were on a dock, beam on the wind. I noticed the mast oscillating (pumping?) fore and aft, maybe an inch or so. I checked the stay tension, comparing side to side and fore and aft stays.

The two aft lowers were not the same tension, one was noticibly but not dramatically tighter than the other, the two foreward lowers were about the same...the windward stays in general were tighter which I attributed to them being on the windward side of the boat.

Where can I find some basic info on tuning this rig to minimize the oscillating or pumping?

Cheers
David

[Bob Pingel]

The classic answer to this question is, get the tuning video -- check in the catalog.

If you are seeing a tension difference from just wind loading on a spar, I would guess you are tuned far too loose.

There is a phenomenon called the Carman Effect, where even a properly tuned mast can oscillate due to the wind -- basically resonation is built up as the structure sheds wind vortices. The solution to this is to break up the wind flow to cause uneven turblent flow - a quick test is to hoist a fender aloft.

I would definitely start with a good tune, you can get there with a good rigger or Brion's video. If you still have problems, you can explore the Carman effect.

Bob