One thing I noticed with my H-18 with respect to the curved beams was how the end caps played into the structure of the beams and boat. I think the H-17 beams are similar.

On those beams, the cast aluminum end caps are pop-riveted onto the end of the aluminum beam extrusion. The bolt for the beam to the hull then goes through the cast end caps. I found on my boat that over time the end caps work loose. You don't have a single stiff structure between the inner and outer hull bolts. I believe this also plays into the cracking you might find, as things can move around a bit. You can tighten the bolts and bed the beams all you want, but if those end caps are even a little loose the stiffness of the platform will suffer. On an older boat if that joint has worked a little bit probably the only way to get it solid would be to bed to the end cap into the beam with epoxy and then rivet it. On mine I simply re-riveted - it stiffened up for a bit but then worked loose again. On the 18 Magnum the problem is made a little worse by the fact that the supports for the wings push down on that end cap.

I always figured a way to make the boats better and cut manufacturing costs would have been to eliminate the end cap design for the beams. Anyway, no matter how tight the tramp on all of my Hobies I used to refer to them in my mind as Flexible Flyers. When I moved to the 6.0 I thought that was a stiff platform until sailing the N-20 with bedded beams - very nice...