Thanks for the answer Wouter,
Actually your answer makes me think more into it (trying not to..)
If you say flat strip is used for rigidity in the VERTICAL plane- well I don't understand why flat strip is more rigid than rod, I understand why flat strip is more rigid than rod in the HORIZONTAL plane, but why VERTICAL ?

To my understanding mainbeam rigidity is given by the stiffness of the Alu cross section and the aditional compression put into it by the straps @ tensioning pin. The longer the dolphin striker pin=more rigidity (considering the apropriate tensioning strap)

Considering that , the ultimate role of the straps is to handle tension loads, whatever it's cross section. Further more , a 1m long 7mm diam ss rod is much more rigid than a 30x3mm , same lenght ss strap - in the plane perpendicular to the 30mm side. Of course the opposite occurs in the other plane.

I understand it is easier to fix the strap to the Alu profile, but you can use an end fitting for the rod.

A 30x3mm ss strap has 90sq mm of cross section, while a 6mm diam rod has 28sq mm of cross section, hence lighter.

More to it: when the boat nosedives and the Dstriker enters the water it reacts pretty much as a wing, opposing resistance for the boat to resurface, while a rod has much less resistance for resurfacing.

Is this like a MythBuster case ?
6mm diam rod can take a lot of tension, we designed stair ramps hangin' from ceiling, supported by thin cables...
don't like the math of it but I assume even 5mm rod should do it. The tension in the rod should be higher or least equal to the compression and dynamic forces that the mast exerts on the beam.

Wouter can you clear this out for us, or at least explain where am I wrong, may be missing something.

by the time I wrote this you already wrote more to it.