I use a 14lb Delta Fast-Set with about 30' of chain for my SeaSport 24 (6000#+) and stick without a problem anywhere in the lower river below Bonneville. For a sure set I use the rule of thumb that you need chain equal to at least half the length of your boat. I use a windlass, so I have 30' of chain to make sure the anchor is laying flat. If I was pulling by hand, I'd probably go with 12-15' of chain for my boat. I used a 24 lb rocker anchor with about 12' of chain in my 19' sled and never had a problem anywhere.
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Timbermans motto: The only good tree is a log.