Tapping, or percussion testing, is an extremely helpful non-destructive inspection technique. It can provide information of a vessels structural integrity which may not be visually noticeable. Percussion tapping usually causes no damage, except for perhaps a small localized loss of bottom paint.
PDF file: Percussion Tapping.
This is a very practical article on thru-hulls and seacocks, which includes a brief overview of alloy composition and new products on the market which includes non-corrosive composite material types for those of you who want to minimize sources of corrosion on your boat. Bob
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Read the article on Sail Magazine: Thru-hulls and Seacocks.