I use two 18lb river anchors and they hold my 20' boat in moderate winds. I did make a bungee system that is attached to the anchor which does help a lot. I took a 6' piece of marine bungee cord and put it inside of 1/2" hollow braided Poly line. The bungee stretches until the bunched up line straightens out and gets tight. Like REV said, it stretches when a wave hits it vs. just moving the anchor. A heavier anchor might hold better, but don't forget you will have to pull it up multiple times in a day of fishing.
2013 Lund Pro Guide 2010,150 Vrod w/Simons Flash, Helix 12 Chirp SI GN1 & MG2N