Giant kelps (which may reach lengths of 45 m) are a prominent exception to the general rule that wave-swept organisms are small. The ability of these kelps to maintain their large size in the presence of ocean waves has been attributed to their extreme flexibility and the concomitant tendency to 'go with the flow', a tendency that reduces the hydrodynamic forces imposed on the plant. However, the flexibility of giant kelps carries with it the potential for the organism to apply an inertial load to its own structure if the blade mass reaches the end of its tether. Here, we examine the complex trade-off between flexibility and inertial loading using a simple computational model of the bull kelp Nereocystis luetkeana. In field and laboratory tests, the model accurately predicts the forces and motions imposed on flexible structures in wave-induced flows. Subsequent predictions from the model suggest that mature N. luetkeana can indeed benefit from moving with the flow, but that the forces imposed on juveniles are actually increased by the plant's flexibility. Furthermore, the benefit accrued from going with the flow is sensitive to the shape of the plant. If the bull kelp were to grow while maintaining a juvenile shape, the stress placed on its stipe would be drastically increased by dynamic loading, and these inappropriately shaped plants would be subjected to a high risk of breakage. For certain combinations of wave height, wave period and stipe length, the increased stress in hypothetical 'small'-shaped plants may be associated with chaotic motion of the blade mass.

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