C-22 mooring
For my C-22, which is both larger and heavier than your 20 foor O'day, I use a 100 pound musroom anchor. This is in Long Island Sound (CT) with a very muddy bottom. At low tide there is barely three feet (yea, I ship the rudder), and at high tide there is about twelve feet of depth. For scope, I use fifteen feet of 3/8 inch chain, a jaw to jaw stainless steel 5/16 inch swivel, and then 1/4 inch chain on top. I then have a 5/16inch shackle attached to the 1/4inch chain and then a half inch shackle attached to that which holds the mooring bouy and a three foot piece of half inch three strand line with a thimble in the eye on the part on the half inch shackle. There is another spliced eye in the other end of the three strand through which I clove hitch about six feet of doulble braided 7/16 inch line with about 8 to 12 inch eyes that attach to the bow cleats in bridle fashion. This pennent is about six feet. This set up has survived a hurricane, albeit not the most powerful of hurricanes. So it really is not weight that you need, but something like a mushroom anchor that will grip the bottom. The weight part is actually in the chain that you use.
