Hi If we are going to talk anchors there are a few terms we will need to use.You are probably aware of these but some might not be.
The line or chain connecting your anchor to the vessel is a RHOAD
The length of line between the anchor stock and your vessel you employ is the SCOPE.
The Shape of the curve of your rhoad is a CATENARY
A rope rhoad has a scope reqirment of 1/6
A chain rhoad has a scope reqirement of 1/3
ie for each foot of water depth you requirw 3'of chain and 6' of rope
These values will give a catenary which ensures maximum holding power of the anchor.
Solution to your problem go to 1/4"chain ie the diameter of the bar stock forming your links.
One type of anchor which will hold in any typoe of bottom from rock to mud is the Northill It was used to anchor flying boats You would need a Northill 5 you might find one at a war surplus outlet. This is a folding pattern achor which stows well when not in use.