What is this called and what does it do?

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[font "Times New Roman"][#000000]Okay, I think I just found out what it is. Here look at this:[/#000000][/font]
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[font "Times New Roman"][#000000][url "http://www.sterndrive.info/outboardmotor/id21.html"]http://www.sterndrive.info/outboardmotor/id21.html[/url][/#000000][/font]
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[font "Times New Roman"][#000000]I think it is the prop nut keeper. How big is the one you have in your pictures?[/#000000][/font]
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