Sailboat Part 4: Rigs and Sailplans

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.

gaff rig sails; the "gaff" is the pole at the top of
the sail which allows it to maintain a squarish shape

bermuda rig sails with a typical triangular shape

Fore-and-aft rigged sails owe their popularity to the fact that they are very versatile. Fore-and-aft rigs are the fastest rigs for sailing upwind and across the wind, which are common conditions for boats which do much travelling in coastal waters.

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 While square sails were very popular during the age of sail, in modern times they have largely been replaced by the more versatile bermuda rig and other, specialized sails.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

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.

flying a spinnaker

There are also a variety of special sails which can replace a standard jib in certain situations. The most common of these is a spinnaker, which is designed to improve downwind sailing performance on fore-and-aft rigs. A gennaker or genoa jib is a more general purpose headsail which functions much like a jib, except that a gennaker is typically larger than the main sail. A racing sailboat may have a variety of specialized headsails for beating, reaching and running, which are traded out when changing course relative to the wind. Not all sailboats use or require jibs or staysails or other special sails, however.

Sailplans

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.

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

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.

Sailboat Part 3: The Physics of Sails

With a hull you have a boat, but it isn't a sailboat until you add sails. While a sail may look like it's just a sheet, sails are actually a type of purpose-built wing, and share many physical concepts with aircraft. However, because the usage is also different from an aircraft wing there are also considerations for sails that aircraft do not require.

basic sail forces

Lift as Propulsion

A sail that is just a sheet can only generate propulsion via drag, which would limit sailing to travelling away from the wind. However, real sails always have camber, or in other words are curved, which allows them to generate lift. While the way a sail generates lift is similar to an aircraft wing, an airplane generates lift using the relative speed of the craft to support its weight, whereas a sail uses the relative speed of the wind to move the boat forward.

While sailboats generally cannot sail directly against the wind, because sails generate lift they can sail perpendicular to the wind and even upwind to some degree. In general a sailboat that can sail closer to the wind is faster, and the distance to the wind (in degrees) that a boat can sail at is known as its pointing ability. Pointing ability is important because as a boat accelerates it creates its own relative wind, which causes the apparent wind to shift toward the direction of travel. This means that a sailboat that is moving quickly will also effectively be sailing closer to the wind.

Points of Sail

the points of sail

The orientation of the boat relative to the wind is known as the point of sail. There are three basic points of sail, sailing towards the wind (called close haul or beating), sailing perpendicular to the wind (called reaching), and sailing away from the wind (called running). Generally sailboats are fastest when reaching since the speed of the apparent wind is maximized at near right angles, and sailboats can even sail faster than the true wind speed on a reach.

The point of sail also includes a tack, which describes the side of the boat that the wind is coming from. If the apparent wind is angled from the left side of the boat then the boat has a port tack, and if the wind is angled from the right it has a starboard tack. More generally, side of the boat that the wind is coming from is called windward and the side facing away from the wind is called leeward.

When moving from one tack to another, the sails must be rearranged to the opposite side of the boat in order to continue to generate lift. Furthermore, because sailboats are unstable (or unable to move) when facing directly into or away from the wind, the skipper must ensure that the boat has sufficient momentum to turn all the way through the wind when changing tacks. Changing tacks by turning against the wind is called tacking, whereas changing tacks by turning away from the wind is called jibing (pronounced "jai-bing"). Jibes are particularly dangerous because the sails can sometimes violently swing themselves around, causing injury due to bludgeoning or even capsize due to excessive changes in the forces on the boat. 

Forces on the Boat

Much like a wing, a sail is subject to aerodynamic considerations such as lift-to-drag, aspect ratio and so on. However while wings are symmetrically paired on each side of an aircraft, sails produce considerable imbalanced forces which a boat's hull(s) must counteract. Sails produce two main forces on the boat, pitching nose down due to lift, and rolling away from the wind due to drag. In order to transmit the forces effectively to the boat, masts are built as tensegrity structures with the mast itself providing compressive resistance which is then tensioned by cables called stays that are attached to various points on the hull.

sail forces pushing a trimaran to the limit

Due to the height of the mast(s), sails actually gain significant leverage on a boat, tending to tilt it rather than slide. If the aerodynamic forces on the sails exceed the ability of the hulls to counteract, the sails will literally drag the boat upside down in a slow, dramatic motion. Other considerations aside, a taller sail will generate greater forces than a shorter one. Even though a taller sail might have a greater aspect ratio and thus better lift-to-drag, the extra leverage caused by the height favors shorter, less efficient but higher area sails instead.

The camber of the sails are also important with regards to the forces on the boat. If a soft sheet sail is aligned too closely to the wind, it loses its shape and "luffs", which negates the lift. A sail with a smaller camber can point closer to the wind, but generates correspondingly larger forces. Most sails and sailboats are designed to point within 30 to 40 degrees of the apparent wind, however some racing craft (like the BMW Oracle, pictured) can point as close as 20 degrees with soft sails. As opposed to sail height, pointing ability is much more important to performance and thus "thinner" sails with less camber are preferred. A boat with greater pointing ability can take a more direct path when beating and travel faster when reaching which are major advantages in a race.

Wing Sails vs Soft Sails

a soft wing sail, showing parts of the 'skeleton'

Normal cloth sails are not very aerodynamically efficient for various reasons. Because they are more or less loose they cannot perfectly hold an aerofoil shape and "luff" if pushed too close to the wind. The "wingtip" of a soft sail also tends to have a truncated shape, which leads to greater induced drag, and soft sails cannot generate lift downwind.

To counteract these disadvantages, several varieties of "wing sail" have been developed which can maintain a more complex shape more rigidly. The original wing sails were made of stiff composite materials which can be quite heavy, however later designs include sails that have a soft, stretchable covering over a rigid "skeleton" which can change its shape to tack. Wing sails in general can not only produce greater lift-to-drag when compared to soft sails, but they can also point closer to the wind, averaging 15 degrees as compared to 20 for the best soft sail. Wing sails can also be swiveled backwards to generate lift when the wind is coming from behind the boat.

a hard wing sail with a hinge for adjusting the camber

Wing sails do have several disadvantages, however. Wing sails are universally heavier and more complex than their more traditional counterparts, especially for rigid composite sails. Large wing sails require heavy machinery in order to adjust the camber of the sail, even more so with rigid wing sails which can grow to become small skyscrapers on top of the boat. The extra weight requires balancing but provides no advantages otherwise. The extra breakable parts also constitute a significant cost liability.

The greatest disadvantage, though, is that for most wing sails there is no way to reduce the sail area in cruise. Soft sails have what are called "reef points", which allow the sail to be partly folded to reduce the sail area if there is excessive wind or if you want to slow down without stopping. Without the ability to reef, strong winds force you to take your sail down completely (which is also difficult to do with a wing sail), or else the sail can actually rip the mast down or even flip the boat, both of which are quite dangerous. Soft wing sails can usually be reefed, but at the cost of extra complexity.

Finally, wing sails tend to be ugly, which along with their other disadvantages (cost, weight, complexity) makes them relatively unattractive in spite of their potential advantages. I note them here mainly as a potential possibility, not as something I would use for my own design.

Sailboat Part 2: Hulls

Before I can say anything about sails there is a more important part of the boat which makes the boat a boat: the hull. The hull is the part of the boat that floats, the part you sit or stand on or in. A single boat may be composed of one or more hulls, for which there exist many different designs.

To start with, I will only be covering some basics about hulls here. Hull design is an incredibly complicated subject, with different concerns for different hull configurations and a good deal of trial and error in the process of making improvements. One thing I will say, however, is that a boat's speed is highly dependent on hull design. The surface area of the hull that is directly exposed to the water is the single largest source of drag for a boat.

Monohulls

 When most people think of a "boat" they typically think of something like what is shown in the picture on the right. This type of boat is the most common, has only one hull and is called, accordingly, a "monohull". With a single hull that is typically much longer than it is wide, monohulls tend to roll considerably, as can be seen in the picture. They also expose a large surface area to the water, and so tend to be slower than multihulls. In races there are often separate classes for monohulls and multihulls for this reason. In order to counteract the rolling tendencies of monohulls, most are built with a "keel", which is basically a large, lead-filled fin under the center of the boat. Both the weight and the hydrodynamic action of the keel help keep the boat righted.

Hydroplaning

At a certain speed where the drag forces on the boat equal the propulsion forces, a boat can no longer accelerate further. This is called "hull speed". While there are several ways of circumventing this limit, the most common method for monohulls is to force the boat to climb over its own wake, thus reducing the surface area exposed to the water and thus drag. This is called hydroplaning, and requires a special hull design. Hulls designed for hydroplaning tend to perform worse in non-hydroplaning usage, and additionally a considerable amount of force is required for the boat to climb over its own wake, which makes hydroplaning less efficient than displacement mode (the normal way a boat travels). Sometimes air bubbles are pumped under the boat to further reduce friction. It should be noted that most high-speed boats like jet boats and even racing sailboats use hydroplaning hulls.

Hydroplaning hulls cannot use keels like regular monohulls do, since the keel and the shape of a keel hull interfere with hydroplaning. Instead they use a "daggerboard", which is an unweighted, retractable fin. Multihulls also commonly use daggerboards, although for somewhat different reasons.

Self-righting Design

When a ship, for whatever reasons, finds itself upside down it is called "capsizing". Capsize is considered a disaster for a boat, since a capsized boat cannot be controlled or propelled intentionally, and because the crew can become trapped inside the hull. For a monohull the most common mode of capsizing is rolling over, consistent with their weakest counterforce.

There are some monohulls which have been designed to automatically flip themselves right-side-up again following a capsize in order to prevent the crew from needing assistance, so called 'self-righting hulls'. I mention them here because, while such a design sounds good on paper, in most situations where a capsize happens conditions are bad enough that it is likely to happen again. Normally a boat that capsizes will stay that way and remain fairly stable, however with a self-righting hull a boat can instead end up doing barrel rolls through a storm, which produces a sort of cat-in-the-dryer effect on the crew (pardon the mental image). Any (non-self-righting) well designed boat is livable while capsized, so at least the crew can have stable living conditions while awaiting rescue.

Multihulls

Thousands of years ago, the ancient Polynesians crossed the pacific to Hawaii and Easter Island on boats that were little more than multiple canoes tied together. The most primitive of these was the proa, pictured left. A proa has two hulls which are asymmetric, with one much larger than the other. The extra hull provides additional (buoyant) support against sail forces, which allows a larger, more aggressive sail to be used, but they can only turn away from the wind and handling a proa is quite complicated.

As a result proas are rather uncommon and exotic today. I should note, however, that there are some modern, overpowered, high-tech sailboat designs which use the proa configuration in order to achieve maximum leverage on the sail, but at the expense of only being able to sail in one direction. In general, though, proas are not very practical and most multihulls use other designs instead.

Unlike a monohull, the wide base created by multiple hulls makes well designed multihulls very resistant to rolling over. Instead, multihulls have a greater tendency to bury their bow into a wave, causing the boat to flip end over end, called "pitchpole". Also unlike monohulls, no well-designed multihull ever uses hydroplaning. The hulls can be so efficient that the hull speed of a racing multihull can exceed the hydroplaning speed of a monohull. However, like a hydroplaning hull, when hit from the side by large waves, a multihull skipper (captain) can retract the daggerboard and simply let the wave carry them sideways without flipping. The high resistance to capsize makes multihulls particularly attractive for rough conditions and open ocean use.

Catamarans and Trimarans

a catamaran

a trimaran

Most modern multihulls use symmetric hulls as opposed to the proa configuration. Boats that have two equal hulls are called catamarans, whereas boats that have three (symmetric but not necessarily equal) hulls are called trimarans. Note that higher numbers of hulls are possible, but basically irrelevant for engineering purposes. Catamarans and trimarans are fairly common and even familiar in modern times, seeing applications in both civilian and military settings.

While both create a wide base that helps to balance out various forces on the boat, trimarans in general have a higher total stability than catamarans, and are known for having higher performance overall. However, depending on other design factors that difference may not be very important in practice.

Between the two, catamarans actually offer a surprising amount of interior space when used for a large (30ft+) boat, whereas a trimaran is the opposite since its outrigger hulls typically don't provide enough space to be livable. For a smaller boat, a catamaran offers few advantages since the interior of the hulls are too small to be usable, whereas the main hull of a trimaran can potentially seat several people comfortably. Additionally, most multihulls feature nets stretched between the hulls to provide safety and (perhaps extra) walking space on the boat.

Due to their various advantages I will only be considering catamarans and trimarans for the design that I will propose later. Additionally, any disadvantages they carry are easily mitigated with few and small modifications, which is advantageous both for design and construction.

Sailboat Part 1: Why a Sailboat?

A while ago, for whatever reason I became interested in boats, and particularly sailboats. Eventually I found some really interesting stuff and came up with a basic design that could outperform everything that currently exists in a number of ways. Before that, though, it might seem odd that I would care about sailboats, since when most people think of sailboats they think of rich people and 18th century pirates, and how could something like that turn into interesting engineering?

For starters I think it would be good to cover what a sailboat can't do. A sailboat will probably never be the fastest boat (although there are people who try). Jet propelled boats properly hold that title and will probably never lose it. A sailboat also cannot carry a heavy load like a cargo ship or oil tanker. Sail power grows with the square of the sail area whereas boat weight increases with the cube of the volume. In other words sails scale poorly.

However for small and medium boats, sails can be surprisingly efficient and powerful. In light winds and smooth water a motor powered boat might well be able to outrun a sailboat, but in heavy wind and chop the motorboat slows down dramatically while the sailboat takes off. Of course, sails require a good deal of skill to manage, but under unfavorable conditions a skilled sailor has a big advantage over their lazier, motor lubbing counterparts. Some of the best sailboats are much faster than their motorized counterparts even under more average conditions.

Besides speed and handling under the rough, sails also carry some additional advantages. The most obvious is that all they require is a sheet and some wind to make the boat move, and the wind part of that is free. You can never run out of fuel, and assuming that you generate power for other uses by renewable sources, then you can avoid the toxic and dangerous costs of storing fuel onboard (and spare the water from those things as well).

Finally, sailing is fun. There's something undeniably romantic about the stark triangular outlines and the freedom you get with sails. You feel closer to the ocean and the wind when you're adjusting and navigating by paying attention to them, and the quiet of sails has a whole different feel from the constant drumming of an engine. With an engine you fight the ocean, with sails you ride it. Of course that has nothing to do with engineering in particular, but if engineering (or anything else) isn't fun then you're probably doing it wrong.