Jessica wrote:
I have a 15' 500 pound sailboat. How much weight would be required for a
secure mooring in a small lake?


Besides dead weight you also have to worry about a few other factors.

Here's a link to get you started if you are unsure.

http://www.boatus.com/boattech/casey/34.htm

My 21 ft sailboat moved a barrel weighing over two hundred pounds.
Wind, waves and a line that was too short.

LdB