Following your suggestions and have noticed a number of interesting associations. If I analyze the mast from the D3 tang ( top wire conection ) to the mast step the upper spreader root is precisely at 3/4 of the distance from the mast step. 48'1" on the Forespar drawing and 48' 0.75" by calculation.

If I calculate from the partners to the mast head I get 28' 11" as half the distance. The Forespar drawing locates the lower spreaders at 29' 8". A difference of 9 " to the spreader root. Could this be close enough to be a concirn ? The D1 tang is at 29' 4", a mear 5 " off from this mid way point.

Both of these factors are telling me the harmonic nodes you mentioned could be at play here. I'm assuming the 28 deg swept spreaders are at play here as they do provide fore and aft staying in the swept spreader ( 28 deg ) rig design.

Thank you for your comments in advance.

Robb