A chainplate is a metal plate used to fasten a shroud

Shroud (sailing)

On a sailboat, the shrouds are pieces of standing rigging which hold the mast up from side to side. There is frequently more than one shroud on each side of the boat....

 or stay

Stays (nautical)

Stays are the heavy ropes, wires, or rods on sailing vessels that run from the masts to the hull, usually fore-and-aft along the centerline of the vessel...

 to the hull

Hull (watercraft)

A hull is the watertight body of a ship or boat. Above the hull is the superstructure and/or deckhouse, where present. The line where the hull meets the water surface is called the waterline.The structure of the hull varies depending on the vessel type...

 of a sailboat

Sailboat

A sailboat or sailing boat is a boat propelled partly or entirely by sails. The term covers a variety of boats, larger than small vessels such as sailboards and smaller than sailing ships, but distinctions in the size are not strictly defined and what constitutes a sailing ship, sailboat, or a...

. One end of the chainplate is normally fastened to a turnbuckle

Turnbuckle

A turnbuckle, stretching screw or bottlescrew is a device for adjusting the tension or length of ropes, cables, tie rods, and other tensioning systems. It normally consists of two threaded eyelets, one screwed into each end of a small metal frame, one with a left-hand thread and the other with a...

which is connected to the shroud or stay, whereas the remainder of the chainplate normally has multiple holes that are bolted to the hull. This distributes the load across the hull, making it possible for a somewhat lighter hull to support the load of the shrouds and stays.