Pumping Mast

[Robbb]

Good day Brian,

We spoke earlier this year about my haunting mast pumping problem. Forespar built the rig but the standing rigging was supplied by a local rigging company. My spar pumps fore and aft in the most moderate of conditions. In a one foot chop the upper two panels pump and is over all unstable in heaver conditions. In true off shore conditions I would expect the mast would be in danger of failing if not certainly wearing its self out by the time a passage was completed.

The boat is a mast head sloop designed by Bob Perry, model Passport 470. The tube section is Forespars section 5890 with moments of Iyy 54.68 and Ixx of 22.19. The rig design uses discontigious wires and two aft swept spreaders at nominally at 26 degrees. Single aft lower and V1 is attach to a single chain plate.

The wire is 1 X 19 ( 316 ) 7/16 in on V1,D1 and 3/8 everywhere else. Spreader dihedral design is 3 degrees up on the lowers and 5 up on the upper spreaders. The boat is equipped with an removable inner stay and runners for heavy conditions. A hydraulic back stay adjuster is installed. When the inner stay is set up hard the pumping generaly eliminated. The runners alone have minimal effect. The boat is main sail driven with a small fore triangle so running the inner stay in all but heavy conditions is highly undesirable.

Forespar had recently increased the length of the lower and upper spreaders by nominally 8 inches and 2 inches respectively and installed a turnbuckle on the V2/D3 wire for additional adjustment but this action had not solved the problem.

Visually the rig looks good. Spreader dihedrals are up, a nice prebend with the exception of the top panel being a bit flat and over all is laterally stiff. A pull on a V1 wire will cause a rotation about the mast tube axis. A tug on a D1 wire will deflect the mast and actually set up a slight oscillation fore and aft. This has been my dock side tuning ruler in that if I can make it oscillate at the dock then it will do the same in any sea way. So far this has proven to be true.

Additional wire tension does not improve the stability but instead will distort the hull to the point where doors will no longer close. I have concluded that there is either a geometry problem ( less likely ), too much wire stretch in the 3/8 in V2/D3 wire or simply the tube moments are too low for its height of mast ( 66+ feet including the burry) and weight of boat. ( 32,000# )

Do you or any of your spar talk members have an opinion as where to take this exercize next. Three experienced riggers have worked on the rig and all have concluded the mast section is undersized and without additional stiffners or a new tube,tuning will not solve the problem. I wondering if optomialy stayed, stability might be acceptable. The other suggestions seem extreem unless indeed this is all about low moments.

Robb