I use a might-mite anchor. It is rated for a smaller boat but it holds well. Also the tines bend back so if you have to rip it out you can bend it back. After about 10 seasons a tine may break but it works well.
Some places are so rocky that its like dropping an anchor into a rock jetty...it goes in opens and there is no way of getting it out.
I saw a really nice rock anchor on craigslist a year ago and I am kicking myself for not buying it.
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