Masts are held up with a system of wire or rod called Standing Rigging. Masts must withstand many forces, but the system of standing rigging is primarily concerned about forces from the side, since the wind will always be hitting the sails on one side, one side will be loaded and the other will not. Masts are set up much like suspension bridges. If we support the rig with only an Upper Shroud, it will break in the middle when it is loaded. So we introduce a Lower Shroud to support the middle of the mast. If one of these shrouds is much looser than the other, then they will not work effectively, so the goal is have the mast supported evenly from top to bottom.
Shroud angles are very important where they intersect the mast. The wider the angle of incidence at the mast is, the less load it takes to hold the mast up. This is easy to visualize if you look at the guy wires on a large radio antenna. However boats don’t give us that much real estate so we we have to get clever.
The minimum angle of incidence for a reasonably sized wire or rod to be able to support the load is about 12 degrees. Most angles of incidence are closer to 15 or 16. If the wire came out of the mast at a 15 degree angle and went straight to the deck, your boat would have to be very, very wide. This is fine on a catamaran or a high performance ocean racer with outriggers, but doesn’t work for most sailboats. This is why we have spreaders.
Spreaders help open up the angle of incidence for the upper shroud so that it can be effectively tightened and then redirect the wire down to the chainplates, allowing us to support the mast without a boat that is 20’ wide.
Each of these spreaders is under compression pushing the mast sideways so you will notice that below every spreader is a diagonal shroud. That diagonal shroud supports the mast in that location by also supporting the spreader and taking some of that lateral load.
In this side view of a mast, you see the two additional Stays that hold up the mast. These are the Forestay in the front and the Backstay in the back. Some boats have multiple forestays for multiple jibs or simply to help support the mast if it is prone to pumping.
Some boats will have twin backstays to accommodate cockpit layout and many race boats have running backstays which must be trimmed on either side every time the boat tacks or gybes to support the mast. These allow larger mainsails to be carried and give the racing crew more control over the trim of their sails.