Ballast

January 2001. Bronze, river stone. Dimensions: 21 × 9 × 6 in (53 × 23 × 15 cm).
Private collection

bal·last /'bal?st/
verb; 

give stability to (a ship) by putting a heavy substance in its bilge.
"the vessel has been ballasted to give the necessary floating stability"

noun;
heavy material, such as gravel, sand, iron, or lead, placed low in a vessel to improve its stability.
"the hull had insufficient ballast"