I've used Ram Mounts for various applications and I have never been able to keep them from moving when the water gets rough. I always used the recommended size, but it didn't matter. I finally just quit using them.
They're probably OK for a kayak.
2015 Seaark 1872 MV CC Etec-90
Two Helix 12 CHIRP SI's, , MinnKota Riptide ST80/i-pilot Link, Bob's Hydraulic
2018 Robalo R180 CC, Yamaha 115
Bitter Gun Owner