Is this the anchor you're referring to?
An anchor's holding is dependent upon its being able to hold onto the bottom. *The weight of the anchor alone is not sufficient to hold the boat. *For mud or sand bottoms, you want the largest surface area possible to dig into the bottom, and you want the anchor to set quickly and reliably. *The Navy anchor pictured above might be good in a rock bottom, though getting it out might be problematic, but for mud or sand there just isn't a large enough fluke to hold onto the bottom without plowing a furrow in the sand or mud, and its shape will cause it to lever itself out of sand or mud as the boat moves around.
IMO, the navy anchor is useless for anything except a last-ditch effort to save your boat when everything else has been thrown at it, or when you are anchoring in a rocky bottom, or as a lunch hook in calm weather.*