Rigs
The shape of a sail and the way it is attached to the mast is called a sail rig. A single mast may have more than one sail, and the sails may be of different rig types. There are many different types of sail rigs with varying performance in varying situations, but they can be divided into two broader categories, fore and aft rigs and square rigs.
Fore and Aft Rigs
In modern times all of the most common sail rigs are fore-and-aft. Fore-and-aft rigged sails are aligned with the lengthwise direction of the boat, and are typically triangular but may also be trapezoidal. There are several types of fore-and-aft rigged sails, but the most common in modern use are the bermuda rig and gaff rig (most other types are archaic). Gaff rigs are a bit more complicated, and bermuda rigs are the most popular rig on modern sailboats.
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| gaff rig sails; the "gaff" is the pole at the top of the sail which allows it to maintain a squarish shape |
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| bermuda rig sails with a typical triangular shape |
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| square rigged sails with "yard" extensions on the sides |
Square Rigs
Square rig sails were once found on many oceangoing vessels and military sailing vessels, and are still familiar today. As opposed to fore-and-aft rigged sails, square sails are aligned width-wise with the boat. While their shape and orientation suggest that they would be drag propelled, they are instead curved from top to bottom, and actually generate lift from wind coming from behind the boat. Square sails are only effective for sailing downwind (running), which is a common condition for boats that sail along trade winds, but much less so for coastal waters (since the wind typically blows toward the shore). While square sails were very popular during the age of sail, in modern times they have largely been replaced by the more versatile bermuda rig and other, specialized sails.
Staysails and Headsails
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| a boat with only staysails flying |
In addition to the main sails that are flown directly from the mast(s), sailboats also commonly have extra sails which are flown from the stay cables forward of the masts. These extra sails are generically called staysails, although when flown at the front of the boat they are instead called jibs. Jibs may be attached to the bow, or to the bowsprit, which is a rod that protrudes out from the bow to allow the sails to be set further forward. Staysails and jibs may be used to increase speed, or in the case of square rigged boats to supplement upwind performance. Note that staysails and headsails are always fore-and-aft rigged.
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| flying a spinnaker |
Sailplans
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| a model sloop sailing 'wing and wing' with a spinnaker |
A single sailboat may have one or more masts, with one or more sails and staysails which may each be rigged differently. The arrangement of all the masts and sails is called the sailplan. As there are very many possible sailplans, and because not all of them are interesting, I will only cover the basics here. Additionally, fore-and-aft rig will be assumed since square rigged boats tend to have complicated sailplans, and because they aren't all that practical in general.
Sloop Rig
A boat with a single mast is called a sloop. In modern times sloops are the overall most popular sailplan, and additionally the fastest configuration for a monohull boat. While sloops are indeed fast, they tend to use tall masts and sails which can produce excessive unbalanced forces on a boat. Additionally, when sailing downwind a sloop rig produces even more unbalanced forces that require flying a spinnaker on the opposite side to balance, a configuration known as wing-and-wing. Unfortunately, spinnakers are tedious and management intensive to fly, and the wing-and-wing configuration with a spinnaker is unstable on a sloop, which makes the sloop rig possibly the worst configuration for sailing downwind. A sloop with one or more jibs was traditionally called a cutter, but since jibs are now ubiquitous the term is rarely used anymore.
Ketch Rig
One of the simplest and most common two-mast configurations is the ketch. A ketch has a taller forward mast called the main mast and a somewhat shorter rear mast called the mizzen mast. The masts in a ketch configuration are lined up lengthwise with the boat, which causes the pitching forces of the sails to be converted into a sliding force, and combined with the shorter masts compared to a sloop, ketch rigs tend to be very stable. The two sails of a ketch can also be set wing-and-wing without using any extra/specialized sails, and ketch rigs are much easier to sail downwind than sloop rigs are.
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| a ketch with a jib |
Ketch rigs are unique among sailplans in that they are one of the few types which can sail directly into the wind, a property which was originally exploited for fishing activities. Normally, when pointing into the wind a boat will drift backwards due to aerodynamic drag. By setting each of the sails to a small angle on opposite tacks, this force can be cancelled to keep the boat stable in place without an anchor, or with a higher sail angle even to slowly sail forward directly into the wind. Furthermore, a boat that is facing directly into the wind usually cannot be steered, but with a ketch rig even if the boat isn't moving the sails can be adjusted to aerodynamically steer the boat out of 'irons' and onto one tack or the other. Finally, ketch rigs offer a great deal of flexibility for reducing sail in rough conditions; both sails can be reduced, or the main sail can be cut and the (perhaps reduced) mizzen can be balanced with a jib.
Other sailplans
Other (3+ mast) sailplans do exist, and there are many possible combinations, however their performance is generally similar to a sloop or ketch. Most such rigs are also not as stable or versatile as a ketch, nor as fast as a sloop, and managing extra sails is additionally a pain which makes such rigs generally uninteresting. Even very large boats tend to use simple rigs like sloops and ketches nowadays, although not universally.
The Biplane Rig
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| a biplane rigged catamaran |
While sloops are fast, their disadvantages make them an onerous choice engineering wise. As I will show in more detail later on, the rolling forces created by the tall sail can be tremendous, and are perhaps the leading culprit for capsize in high-performance sailboats. However, for multihulls there is a more recent and lesser known sailplan which is equally fast (perhaps even faster) and which negates the disadvantages. This rig places two equal-sized sails lined up side-by-side rather than front-to-back, and is called a biplane rig.
For a given sail area, a biplane rig uses shorter masts and sails than a sloop or ketch, which leads to less extreme pitching and rolling forces overall. At the same time, the side-by-side alignment of the sails further reduces rolling forces by converting them into a sideways drift. Drifting itself is an issue since it shifts the boat off course, but it is still infinitely preferable to capsize, which rolling forces can cause.
Biplane rigs also share many similarities with ketches. A biplane rig can sail directly upwind like a ketch can, and has similar but superior performance when sailing downwind. The only weakness of biplane rigs is that on a beam reach the lee sail falls into the wind shadow of the windward sail, however this only occurs over a narrow range of angles and has little consequence in practice. Otherwise they dominate virtually every other rig upwind, and downwind their performance is high enough that specialized sails are more of a hassle and a hazard than they are worth. For this reason biplane rigs only rarely ever use jibs or other headsails.
Owing to their overall high performance, suitability for multihulls, and general ease of management, the biplane rig is my sailplan of choice for the concept designs that I will introduce later.
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