Term: centerboard (maritime)
A metal or wooden slab housed in a casing or trunk along the centerline of a sailboat; may be lowered to increase the boat's resistance to sideways motion and raised when the boat is in shallow water or beached.
View pictures related to boating at Wisconsin Historical Images.
[Source: "Glossary." Wisconsin's Great Lakes Shipwrecks (http://www.wisconsinshipwrecks.org/)]