Dismasting occurs to a sailing ship when one or more of the masts responsible for hoisting the sails that propel the vessel is broken or overturned, but the hull itself is still upright and seaworthy. During the age of sail, dismasting usually occurred as the result of high winds during a storm acting upon wooden masts, sails, rigging, and spars. Dismasting does not impair the vessel's ability to stay afloat, but rather its ability to move under sail power.
Modern masts are usually made of fiberglass, aluminum, or other high-strength materials, and so are largely immune to dismasting. The energy required to break a modern mast would be likely to capsize or wreck the vessel. Most present-day sailing vessels are pleasure craft that only navigate lakes or coastal waters, rarely sailing in heavy weather on the open sea. Additionally, modern sailing vessels usually carry an internal combustion engine as a secondary means of propulsion, so the loss of sail power is often not a serious concern. Dismasting, then, is an historical term which normally only applies to large ocean-going ships prior to the advent of steam power. But see Stars & Stripes (America's Cup syndicate).
A sailing ship uses its masts primarily to unfurl sails used as a means of propulsion. Just as important, however, is the use of sails, in combination with the rudder, for directional control. When a vessel is dismasted, its ability to navigate is seriously compromised. If the vessel has been completely dismasted (all of its masts destroyed), it will drift out of control. Also, the broken masts, sails, rigging, and spars will fall overboard and act as an enormous sea anchor. This creates heavy drag on the hull which, in combination with high winds, might capsize the ship. If possible, rigging would be cut free from a dismasted ship in order to stabilize the hull.
A ship dismasted on the open sea would likely be lost with all souls aboard. Under such conditions, the crew might fashion makeshift masts and sails from spare materials carried aboard. This would allow limited propulsion and navigational control. If the ship managed to make landfall near forests with suitable wood, new masts could be constructed from the locally available material. The masts of a sailing ship were regularly replaced during its lifetime due to storm damage and normal wear. Most ocean-going ships would carry a large supply of rope, sailcloth, and even spars for ordinary and extraordinary repairs. A man-of-war would expect to carry out additional repairs due to battle damage.
In Herman Melville's seminal novel, "Moby Dick," Captain Ahab is said to have been, "...dismasted off Japan," alluding to Ahab's leg having been taken off by the white whale and replaced with a polished whale-bone peg-leg. "but like his dismasted craft, he shipped another mast without coming home for it. He has a quiver of 'em."