Mast rake is the angle at which a mast slopes aft from vertical as viewed from the side. In the rare event that a mast is raked forward it is said to have forward rake. For reasons that aren't fully understood, most boats seem to sail best with a bit of rake. They also look better, to most eyes anyway.
The majority of modern boats have their mast raked between 0.75 and 1.5 degrees, with 2 to 2.5 degrees being the conventional upper limit. Some references suggest that fractional rigs should have about twice the rake of a masthead rig, say, 2 to 3 degrees vs. 1 to 2 degrees. Rake is determined and set during design and can help in getting the right lead (pronounced “leed”), or the correct distance between the rig’s nominal center of effort (CE) and the hull’s center of lateral plane (CLP), for proper helm balance. After launching, mast rake can be used to adjust the lead should helm balance be a problem.
Many modern traditional-looking boats have considerable rake, over 3 degrees, simply because it looks right. Old-time fast coasting schooners and yachts often had a lot of rake, because it was associated with speed. This is where we get the term “rakish” from—fast and stylish. Additionally, for boats carrying cargo, rake was sometimes used so that the mast could be positioned forward of the hold, but the halyard would still be over the hold for hoisting.
On rigs with more than one mast, rake should be slightly greater on each mast as you go aft. If two or more masts have the same rake, they will appear to converge at the top when viewed from down low, which looks wrong. The drawing above is of the 52-foot, aluminum center-cockpit ketch Magic Moment, designed by my office and built by Kanter Yachts. The mainmast has 6.5 degrees of rake with 7.5 degrees of rake on the mizzen. That's extreme and about as much rake as is practical for an ordinary boat.
A dead vertical mast will work acceptably, as will a mast raked forward a bit, but this usually doesn't look right. That said, some traditional boats have masts with tremendous forward rake. The foremast on some Chinese junks, for example, is raked forward as much as 15 degrees. (Note that in this case, being lug rigged, the sail hangs off the yard and the luff is well forward of the mast, as opposed to being entirely aft, as is the case with a Bermuda rig.)
As a practical matter, rake affects the swing of the boom. Specifically, a raked mast lifts the end of the boom as it is eased out. This is good for offshore work, as it keeps the boom from dipping in the water and possibly breaking when the boat rolls deeply. On the other hand, because the boom has to lift as it swings out, its weight on a well-raked mast will tend to keep it inboard and thus make it harder to fill the sail in light air.
Rake also has an important effect on staying. A 2-degree rake means that the swept-back lowers, uppers and intermediates, all anchored to a chainplate located slightly aft of the mast, will have a strong backstay effect. Combined with double lowers and an inner forestay, this means there’s less need for runners. At 3 degrees or more of rake, such a boat could usually dispense with runners completely, or use them only in very heavy weather.
The forward component of mast-compression load from rake is minor. Even at an extreme 6-degree rake, the resultant forward load is just 10 percent of total mast compression. (The sine of the rake angle gives the forward component of force at the mast step.) On a standard modern rig with 1 or 2 degrees of rake, the forward component is just 3.5 percent of overall mast compression.
On traditional coasting schooners, the foremost mast would sometimes be set up so that it would have a substantial rake aft—say, 4 or 5 degrees—as it sat freely in the partners and mast step. The top of the mast would then be bent forward through headstay tension. The resulting bowed mast would minimize the amount of fore-and-aft staying that was required. Though this was effective on such traditional craft, it’s not recommended for modern yachts.
Rake does not increase forestay, backstay or shroud load on modern sailboats, because the mast is installed at the proper rake, not bent there from vertical after it’s been stepped.