Fortress FX-37 Anchor - 21LB [FX-37]FX 37 Anchor 21lb For over three decades, the much lighter Fortress Anchor has out performed substantially heavier steel anchors in holding power tests conducted all around the world. In the US Navy tests, not only did the Fortress out perform heavier steel anchors, they were also able to withstand pull loads that averaged over 200x their own weight. Fortress Anchors are constructed using a high tensile, corrosion resistant, aluminum magnesium alloy.