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**Sailboat Physics**
Published 2023-03-15; Updated 2023-06-30
This is a brief, gentle introduction to the physics principles that enable boats to move under sail power.
This is valuable knowledge for sailors who have reached intermediate levels of expertise. Beginners should sail on intuition and mimicry, and in the worst case on rules they have memorized for specific situations. Once they've built intuition, they graduate to an intermediate level. At that point, an understanding of some of the physics creates the opportunity to sail from insight and something closer to first principles.
Understanding the principles furthermore informs a sailor's choice of boat, foils, rig, and sails. This enables the sailor to adapt to new sea and wind conditions, including cases such as close proximity to another boat while racing. In the extreme, it also enables invention for sailing with a damaged boat. It is not unusual for sailors in boats of any size to eventually find oneself far from shore with a broken line, spar, rudder, or sail; or an unfortunate balast imbalance due to water taken on or ice buildup on deck. In these situations, previous experience with boats in normal condition is useless and perhaps dangerously misleading. Meanwhile, understanding the first principles can lead one to appropriate and clever ways of safely accomodating the state of the boat and environment.
 The principles of how sailboats work are generally the same across all modern sailboats, from wind surfers up to megayachts. Sailing is an application of fluid dynamics. A sail, keel, and hull are airfoil-like "wings" projecting vertically instead of horizontally. The boat accelerates through drag and lift forces that arise from relative motion of air and water over these surfaces.
For those just starting to sail, I strongly recommend waiting before learning the kind of theory expressed in this article. In my experience, beginning sailors benefit from on practice on the water until they have enough experience to relate to the situations described here and start to question what causes them.
A sailboat moving partly or fully under motor power relies on different forces, discussed in the motoring article.
To avoid delaying the explanations with a lot of definitions in this article, I define most key terms as they are used and then provide a more detailed Glossary section at the end for reference.
Everything about sailboat physics is described relative to the direction that wind is blowing from. This direction is "0 degrees off the wind" and usually drawn at the top of the page in diagrams.
Sailboats cannot sail directly into the wind, or within about 40 degrees of it on either side. The exact limiting angle depends on the characteristics of the boat.
 When sailing with the wind nearly directly behind the boat (near 180 degrees) in drag mode, wind pushes on the sail like a parachute, and that is what moves the boat. This is a relatively slow mode and avoided when possible.
From about 40 degrees to 150 degrees off the wind, the boat operates in lift mode, where lift force on the sail drives it forward. This is the desired mode for modern sailboats.
I summarize the key observations for the lift mode case are summarized below to prepare you with the big picture. The rest of the article then explains these in detail.
- As the boat speeds up, the apparent wind direction rotates towards the bow and no longer matches the true wind direction from when the boat was at rest.
- The sail produces lift because of wind motion with unequal pressure on each side of the curved sail. This lift force is generally oriented between forwards and sideways.
- Sailboats do not move only in the direction they are pointing (the heading). They also slide sideways in the lee direction (with the wind), so the net course travelled is both forward and sideways even in the absence of current.
- The keel (and rudder, and hull) also produce lift that cancels most of the sideways motion from the sail lift. This lift occurs because of water motion over the curved shape with unequal pressure due to the leeward motion. This lift, and some additional sideways drag from the underwater parts, cancel much of the leeward component of the sail's lift to prevent excess leeway.
- The above and below water lift forces act at points vertically far from the center of mass of the boat and in opposite directions, so they combine to create roll torque that heels the boat over.
- Ballast in the form of crew weight, mass at the bottom of the hull, and mass in the keel create an opposing roll torque called the righting moment that prevents a capsize.
- The buoyancy of the hull opposes the downward portion of the ballast force, so that the boat doesn't sink.
- The rudder intentionally creates unequal drag on each side at the stern, and turns the boat by slowing it on one side.
- The force on the mainsail acts behind (abaft) the boat's center of mass. So, it also creates some yaw torque. If not completely balanced by an opposing foresail yaw torque, this torque rotates the boat towards the wind. That is called weather helm and must be opposed by the rudder's drag to maintain a straight course.
- When the boat heels, the hull becomes asymmetric underwater. This creates additional torque towards the wind direction.
Whether a given force or torque is desirable or not depends on the goal at the moment. It isn't the case that certain ones are always good or always bad.
For example, consider a sailor who intends to turn towards the wind for a tack. They desire the yaw torque from a heeling hull and unbalanced mainsail to accelerate the turn. But if they wanted to sail in a straight line, those torques would be undesirable. Then they would seek to balance the sail forces and ballast. The balanced forces would minimize those torques, and reduce their need for excess rudder drag to correct the course.
You may be familiar with the depiction of an airplane wing with streamlines flowing over it. The angle of attack and teardrop shape of the wing produce several effects which enable flight. These are low pressure, fast moving air on the top of the wing, incoming deflected air on the bottom, and a net lift force from these that is directed upwards.
The same airflow phenomena and resulting net lift force occur on a boat's sail. In fact, a sail is just a vertical airfoil that produces horizontal lift forces. Moving true wind rather than the propeller pulling an airplane forward creates the flow in this case, which is why a sailboat needs no on-board power source.
Furthermore, underwater, the boat's keel, hull, and rudder are vertical hydrofoils. The flow of water over these hydrofoils is similar to the flow of air over the sail. This water flow generates additional lift forces. The water lift forces are on the opposide side of the boat from the sail's lift, and in a direction perpendicular to the boat's heading.
The combined air and water lift forces yield a net force forward and slightly to the sail's side of the boat. The sailboat thus accelerates forward (making headway), while also slipping sideways a bit (making leeway).
Two interesting asides before continuing on to other forces acting on a sailboat. First, the exact principle of how the self-reinforcing low pressure and lift emerge is complex and not satisfactorily understood. However, it is modeled nearly perfectly by mathematics and highly optimized through simulation in today's airplane and sailboat designs.
Second, while the keels and rudders are collectively called "foils" (most commonly by dinghy sailors), a "foiling" sailboat specifically refers to one such as a foiling Moth or the AC catamaran that flies over the water. These boats have "foils" that provide not just horizontal lift but also airplane-like vertical lift from additional wings under the water.
All of these uses of the term "foil" are consistent, although confusing because they change with context. The term "appendages" can be used instead, which refers to all protruding aspects of the boat below the waterline. In casual usage means the keel and rudders, but technically includes thrusters, measurement devices, or anything else that sticks out underwater.
As with an airplane wing, the sail produces effective lift only when it is adjusted at an appropriate angle to the incoming wind. The angle of attack is adjusted by pulling on a rope attached to the back corner of the sail or boom. In sailing jargon, the sail is trimmed by adjusting a sheet attached to the clew.
If the back of the sail is held too close to the boat, then the air flow over the leeward side of the sail will detach and the sail will stall. This also creates excess drag as wind is pressing on the windward side of the sail at the same time. The sail will appear taut and the boat will likely heel significantly and press hard on the rudder.
If the back of the sail is far from the center, then the pressure differential is lost and the sail will luff. In this case, the sail flops back and forth like a flag and makes a lot of noise. This also damages the sail over time.
It is easier to detect luffing than stalling. This leads to the common wisdom: "when in doubt, ease it out" for the sail, and then pull it back in from the edge of luffing.
The angle of attack is selected primarily using the sheet. Other control lines shape the sail in three dimensions, so that it has the right amount of curvature. I describe how to operate these in a separate sail trim guide that builds on this physics article.
The ideal sail shape 1,2 for upwind acceleration is the smooth teardrop foil curve, with the deepest part of the curve slightly forward of halfway across the sail, and a small amount of twist so that the top of the sail has a steeper angle of attack. Maintaining this shape requires adjustment as the true wind strength varies, as the heading changes, and as the apparent wind changes due to the boat's own motion.
When overpowered by strong wind, one can intentionally create a less efficient sail shape to maintain control without luffing the sail. This is done by flattening the sail to reduce lift, and creating more twist (even steeper angle of attack at the top) to move the center of effort lower and thus reduce heeling. In even stronger conditions, the surface area of the sail is reduced by reefing or changing sails.
The lift from the sail forward and to leeward and lift from the keel (and hull and rudder) to windward, and drag force from the rudder to prevent turning, sum to a net force that is forward and slightly to leeward when a boat begins moving. These are shown in the top view:
There are of course drag forces opposing the velocity of the boat, from the hull and foils in the water and from the sail in the air. If nothing changes, these eventually grow with boat speed to counter all acceleration to produce a net zero force, at which point the boat moves with constant velocity. In practice, the wind and water typically create a continually varying environment within which the boat is repeatedly accelerating and decelerating.
In the vertical direction, gravity pulls the boat down and is opposed by its buoyancy. These cancel and leave the boat floating on the surface.
The lift forces act away from the center of mass, so they produce torques. Looking at the boat from the bow end, the keel and sail lift roll in the same direction. That creates heeling motion as the boat accelerates. Opposing the heel is gravity, because the weight distribution shifts from centered to windward as the boat heels.
In a keelboat, substantial weight in the keel and ballast low in the hull
are acted on by gravitational force to windward that counters the lift
torque. In a dinghy, the crew shift their weight far to windward by hiking
(or perhaps with a trapeze) to produce the same effect.
Much of the discussion here was based on Bryon Anderson's The Physics of Sailing article in Physics Today, which uses terminology more familiar to engineers.
So far I've shown diagrams of a single-sail boat, that is, a cat-rigged boat, where the mast is fairly close to the bow. Except for small dinghies, most sailboats today are sloop rigged, with the mast further aft and a headsail in front of it as well as a mainsail behind it.
A headsail generally allows a boat to point slightly higher and accelerate faster than a boat that is cat rigged with similar mainsail area.
The forces on the headsail are the same as for the mainsail. Headsails also produce lift and drag. A jib or genoa headsail is primarily useful upwind. When sailing downwind, they are blanketed by the mainsail and do not receive much wind, so may be doused and replaced with alternative downwind headsails.
There are two major changes to the physics of sailing upwind when using a headsail in addition to the mainsail:
-
The forces on the headsail push the bow away from the wind and the forces on the mainsail push the stern away from the wind. This means that the boat can be steered primarily by adjusting the relative amount of lift and drag on these sails.
-
The airflow off the back of the headsail changes the apparent wind for the mainsail. This means that the main needs a steeper angle of attack than the headsail. That is, the mainsail is trimmed in tighter than the headsail or than it would be in the absence of one.
It is good idea to adjust both sails so that they are achieving maximum lift and the boat is balanced with no rudder forces required to sail straight. Then, slightly ease the headsail and turn the rudder a few degrees to compensate for the intentionally unbalanced forces.
This configuration is called slight weather helm. On a boat with a tiller, the tiller is pulled towards the windward (i.e., "weather") side of the boat. On a boat with a wheel, the wheel is turned towards the leeward side; in either case, the aft end of the rudder is pointed to windward.
In this configuration, the mainsail will be overpowered first in a gust. This will cause excess force aft of the mast, which causes the boat to round up towards the wind, slowing it down and reducing heel. That is the desired outcome. It also means that dumping the main by blowing the vang or the traveler can quickly depower the boat.
If the boat were instead balanced, it could be knocked down in a gust. If the boat had lee helm, then it would turn away from the wind in a gust and accelerate and heel further.
In strong winds, there can be excess weather helm, forcing the boat to point towards the wind even outside of gusts. In this case, the relative areas of the sails as well as their trim must be adjusted by reefing. In the extreme, a small storm jib and no mainsail at all may be required to keep the boat off the wind, as the mast produces a small amount of lift on its own.
When racing, it is a common tactic to make course adjustments partially by adjusting the balance of the sails or shifting balast instead of steering. This leverages relative lift forces instead of solely the drag from the rudder and can help maintain speed, especially when initiating a tack.
When running (sailing about 150 degrees from the wind or further, with the wind coming from behind the boat), the sails obviously cannot produce lift because wind is no longer running over them. Instead, they operate in a different mode, which is significantly less efficient. For running before the wind, the sails act like giant parachutes and create drag force against the moving wind. This is very slow, and even with large spinnaker sails, running is often the slowest point of sail. Most boats can only achieve a fraction of the wind speed when running, and no normal sailboat can exceed it.
It is often faster to sail deep broad reaches (about 140 degrees off the wind) and gybe (have the wind cross the back of the boat when turning) back and forth than to run straight downwind.
The sail angles I gave above were approximate. That is because actual points of sail depend on the boat, and are defined by the balance of forces. For example, the definition of running before the wind is when the boat is using the sail for drag instead of lift!
Many of the forces on a sailboat are self-limiting. This creates a potentially undesirable ceiling on performance. However, it also creates a desirable stable and self-correcting system for safety.
As a boat accelerates, the apparent wind flowing over the sails comes from a direction that is closer to the course of the boat. If the boat could accelerate without limit, it eventually would be moving directly into the apparent wind.
For a boat in lift mode sailing upwind, this means that as the boat accelerates, the crew must pull in the sheets to adjust the angle of attack and avoid luffing. The boat is progressively sailing closer and closer to the apparent wind even though its course is the same. As the sails are pulled in, their lift vector moves closer to perpendicular to the heading and the force accelerating the boat decreases. This causes the boat to slow down, which then changes the direction of the apparent wind. So, a boat moving slowly is able to point closer to the true wind than one which is moving quickly.
The apparent wind becomes a limiter on the speed of the boat.
When sailing in drag mode downwind, the apparent wind is initally from behind. As the boat accelerates, it is moving away from the wind. This causes the apparent wind speed to decrease, which decreases the speed of the boat.
The lift vectors from the sails and keel cause a yaw torque that makes the boat heel as it accelerates upwind. As the boat heels, several changes occur that reduce its speed.
On many boats, there is more wetted hull area when heeling, which creates more drag and slows the boat.
The lift vectors from the sails and keel begin to point vertically instead of horizontally, decreasing the amount of acceleration along the heading.
Also when heeling, the sail moves closer to the water. There is usually less wind close to the water than aloft, and that wind is disturbed by waves, friction with the water (which is what caused the waves), and other boats. In the limit, the sail hits the water and the aerodynamic forces that caused the heeling vanish. Most boats will snap back upright in this situation unless swamped by a wave or pinned down by drag force from extremely strong wind.
So, the forces that cause the boat to accelerate and heel will eventually slow it. This is why a boat moves faster when it reduces sail in strong wind by reefing.
The righting forces of balast in the bottom of the boat and the keel desirably increase with heeling, limiting the heeling itself. An upright boat experiences no yaw force from this balast, because the center of mass is aligned with the center of buoyant support. As the boat heels, the balast sticks out to one side, creating an imbalanced force that pulls the boat back upright.
A boat in displacement mode has a maximum hull speed relative to the water. This occurs primarily because the bow wake has a wave period relative to the hull that increases with speed. When the bow wake wave exceeds the length of the waterline (LWL), the boat begins moving uphill on its own wake. The hull speed is given by:
\begin{eqnarray}
\operatorname{hull speed} &=& \sqrt{\frac{\operatorname{LWL} \times g}{2\pi}},~~ g = 9.81~\operatorname{m}/\operatorname{s}^2\
\operatorname{hull speed}[\mbox{kts}] &=& 1.3 \sqrt{\operatorname{LWL}[\mbox{ft}]}\
&=& 2.4 \sqrt{\operatorname{LWL}~[\mbox{m}]}
\end{eqnarray}
For context, here's the hull speed of boats with varying waterline lengths:
| Boat | LWL | Hull Speed |
|---|---|---|
| Laser | 3.96 m | 4.8 kts |
| Dehler 30 | 8.97 m | 7.2 kts |
| ClubSwan 50 | 14.00 m | 9.0 kts |
Boats with longer waterlines are potentially faster because this effect occurs at higher speeds. The vortices created in water and air from the boat's passage also create significant drag at high speed and affect the maximum speed.
A boat can exceed its hull speed over ground when sailing with a current or pushed from behind by waves.
Hull speed only applies when there is a bow wake. Under the right conditions, a boat can hydroplane (plane) and lift out of the water. At that point, it slides along the top of the surface and does not create a wake. This requires sufficient speed relative to the weight of the boat, as well as a conducively flat hull shape. Dinghies frequently plane in moderate wind, cruisers seldom plane, and racing keelboats will plane under strong wind and careful sailing.
Planing has the further advantage that it reduces drag from the hull in the water. So, it not only allows exceeding hull speed by eliminating the bow wave, but also allows the boat to operate more efficiently.
The very fastest boats eliminate even the relatively low drag of planing by foiling. The IMOCA 60 offshore class, Nacra 17 catamaran, and a growing set of foiling dinghies and small catamarans leverage this technique.
The wind off one sail affects another by changing the direction and amount of turbulence that the latter sail experiences. This doesn't just apply to a headsail and mainsail. It includes the case of one boat sailing near another.
When sailing behind another boat going upwind, the airflow is turbulent and less effective for generating lift. It is very hard to pass an identical sailboat when slightly aft and to leeward of it. When racing, the overtaking boat will often try to head above the front boat or tack to be more to leeward in clean air.
A sailboat also casts a significant wind shadow downwind of itself, often on the order of ten boat lengths. Within that shadow, there is less wind pressure as well as more turbulence. This can be felt when one sailboat passes in front of another, causing the second boat to momentarily experience a lull.
When sailing downwind, the shadow is cast in front of the boat. This is why an upwind headsail is not very useful when sailing downwind. It is within the shadow of the mainsail.
In a race, a trailing boat sailing downwind has a chance to catch up because it casts its wind shadow over the leading boats. When flying a spinnaker the volume of this shadow increases and is a major tactical tool.
Turbulence also causes a sail to affect itself. The trailing leech edge of a sail sheds vortices, and those vortices produce drag on that edge. The head and foot of a sail are also discontinuities that create disruptions in the smooth flow, and experience reduced lift per area compared to the center of the sail.
Visualizations in virtual wind tunnels reveal these effects between sails on the same boat and different boats. The general strategy is to be aware of these and attempt to sail in clear air away from other boats.
A triangular mainsail has control lines to adjust each edge of the sail. These are essentially the same regardless of the size of the boat, although certain controls (such as the cunningham or leech line) may be absent on specific boats.
The corners and edges of the mainsail are:
-
Head -
* -
/| -
/ | -
/ | -
/ | -
/ | -
Leech / | Luff
-
/ | -
/ | -
/ SAIL | -
/ |
- Clew ---------- Tack
-
Foot | -
.------------+-----+--
- /| HULL /
- | '+-----+----+---'
- |__/ | /
-
|__/ - RUDDER
-
KEEL
The control lines for the mainsail are:
- Angle of attack:
- Mainsheet
- Traveler. Absent or not controllable on some boats
- Luff:
- Cunningham. Absent on roller furling mainsails
- Halyard. Not available for adjustment with in-mast furling
- Leech (primarily for top of sail twist):
- Vang
- Mainsheet when close hauled
- Leech line as a secondary adjustment. Not available on most small boats
- Foot:
- Outhaul. Not available on boom furlers
The draft is the deepest part of the sail's curve and the center of effort. For best performance in moderate wind, the mainsail draft should be slightly forward of the vertical center of the sail. That both makes the sail efficient in shape and keeps the helm balanced.
Battens help preserve curve and draft shape under both very heavy and very light air.
Increasing luff tension moves the draft forward and decreasing it moves the luff aft.
Increasing mast bend both flattens the mainsail and moves the draft aft. This is done via backstay tension, or mainsheet and vang tension in an unstayed dinghy. This is why increasing cunningham tension upwind helps a dinghy point higher: it counters the mast bend pushing the draft aft and allows the boat to be balanced, reducing weather helm.
Increasing outhaul tension flattens the sail, reducing both lift and drag. A flatter sail is better at high apparent wind speed, where drag is a concern and excess lift leads to heel. A deeper sail is better at low wind speeds, where more lift is required to accelerate.
Three important notes about this. First, drag increases with the cube of wind speed, so it is a dominant concern when moving fast but insignificant when moving slowly. Second, the apparent wind speed is what matters. So, in the presence of a constant true wind, a boat benefits from flattening its sail as its own upwind speed increases. Third, this applies to a sail operating as an airfoil in lift mode, upwind. When in drag propulsion running downwind (or even in a broad reach, when the apparent wind is low), a very loose outhaul and deep sail can be advantageous.
Leech tension, primarily controlled through the vang, adjusts the angle of attack towards the head of the sail separately from the foot end, and also flattens the entire sail. The leech tension affects a both the differential airfoil shape at the bottom versus the top of the sail and the angle of the trailing edge of that airfoil, which acts in a similar manner to an airplane's flaps. This is visible as a twist of the sail.
Increasing tension on the leech will increase the angle of attack near the head, where the angle of attack would otherwise lag the foot end. Wind is stronger higher off the water. For upwind sailing in moderate to strong wind, increasing tension will power up the sail, until the point where the sail begins to stall.
As a first approximation, you can think of the vang as a mainsheet for the top of the sail and the true mainsheet as a sheet for the bottom of the sail, and then trim both based on telltales.
The amount of twist desired varies with apparent wind speed when sailing in lift mode (upwind and reaching):
- Light wind or low boat speed: low leech tension to get a deeper sail and acceleration
- Moderate: moderate leech tension, low twist to point high and reduce drag
- Strong: high leech tension, almost no twist. The flattening factor is dominant in these conditions to reduce drag.
- Overpowered: zero vang leech tension, with high twist to spill air and bring the center of effort down and reduce lift, to reduce heeling (and speed). In an upwind broach, blow the traveler and vang to quickly spill wind without losing all power at the foot, needed for steering
Downwind in drag mode, the leech can be much looser to produce a very full curve in the sail. Gusts will cause the boom to rise and remove tension, undesirably spilling air. Use the vang to keep the boom from riding up in gusts, but do not over flatten the sail until the conditions are strong and the boat becomes overpowered.
When overpowered and gybe broaching downwind, do not release the vang. It will not prevent the broach on that point of sail. If running with spinnaker, releasing the leech tension will depower the main and force the center of effort forward. That turns the boat away from the wind and will trigger a gybe instead of preventing one. (If already capsized donwind, then release the vang to lose leech tension and prevent the mainsail from pinning the boat down.)
A triangular headsail such as a jib, genoa, or code zero has the same three corners and edges. The corners are the head (top, attached to the halyard), tack (front, attached to the bow), and clew (back, attached to the sheets). The edges are the luff (leading edge between head and tack), leech (trailing edge between head and clew), and foot (bottom edge between tack and clew). The control lines are:
- Angle of attack:
- Sheet, which may be on a sideways traveller for a self-tacker
- Luff:
- Halyard. Not available for adjustment on a furling headsail
- Leech:
- Sheet
- Leech line, not available on all boats
- Jib leads, on fore-aft travellers or rings. Not available on all boats
- Foot:
- Sheet
Compared to the mainsail, there are many fewer controls on most headsails. On many boats, the only control is the sheet, which prevents independent adjustment of the edges. In that case the headsail's role may be less about power and more about balancing the overall center of effort of the sailplan to manage weather helm, and changing the aparent wind direction across the main.
The jib leads control the twist of the sail, adjusting leech tension and changing the angle of departure separately for the top and bottom of the sail.
Not shown in the points of sail diagram is sailing by the lee, where the sail is on the "wrong" side and the boom is pushed forward in front of the mast. This is only possible on boats that lack swept-back side stays supporting the mast, or headsails that would stall.
Sailing by the lee is typically employed only when racing in small catboats. Sailing by the lee can be faster than running dead downwind or on a deep broad reach on the other tack because it improves flow on the forward face of the mainsail. It also avoids the need to gybe when surfing off waves or encountering a puff.
There are strategic advantages to sailing by the lee in certain cases in competition. A boat with its boom over its port side is defined to be on starboard tack, regardless of its heading with regard to the wind. That boat has right of way over port tack boats, regardless of the direction from which the wind is coming. Second, sailing by the lee allows a boat to swivel its stern at an angle that would normally be impossible on the current tack. This may allow it to create or break "overlap" in terms of racing rules.
It is generally not a fast point of sail, so in the absence of these strategic considerations, gybing and moving off on a broad reach is faster for most boats than sailing by the lee.
Once you understand how a sail works as an airfoil, a natural question is how square rigged ships sailed by the British and southern Europeans worked during the age of sail. In fact, they indeed had terrible upwind performance.
 Square rigged ships were developed to exploit trade winds. They sought to sail downwind most of the time. They could sail the North Atlantic in a clockwise direction by following the current and prevailing winds, sailing west at low lattitudes and east at high lattitudes.
On other courses, square rigged ships were hampered by their upwind performance and could only point maybe 65 degrees off the wind. They could only make upwind progress slowly by changing tacks frequently. There were also "jib" headsails, "staysails" between masts, and "spanker" sails at the stern with more airfoil shape that helped provide some acceleration upwind through lift forces.
Tacking itself was hard with such a large angle within which the ship could generate no forward acceleration. They often either gybed around, even when going upwind, or used a rowboat to tow the bow sideways through the wind to complete a tack.
The risk of "missing stays" and failing a tack in battle or off a leeward coast was a serious liability.
There's a lot of terminology for boats. Every part generally has a unique name. This is important for clear communication. However, beware that the names themselves vary regionally and sometimes between racing and cruising sailors within a region.
Spars are poles. A mast is a vertical spar and a boom is a horizontal spar that holds out the foot of a sail. The universal joint where the main boom is attached to the mast is called a gooseneck. Spreaders are the horizontal bars that push shrouds away from the mast.
The rig is the collection of spars and lines that adjust and support it. The rigging is the lines.
Most modern sailboats are Bermuda rig sloops, with a single mast and boom and two sails. A catboat has only one sail and today is usually a single-handed dinghy, although a handful of catboat yachts are in production.
A ketch has two masts (with the aft one in front of the rudder post). These are great for shorthanded and heavy weather cruising because of the low center of effort and flexibility of sail plan, but fell out of favor in the 1980s as cruisers adopted a more racing aesthetic and favored the reduced cost of a single mast.
A yawl has two masts, with the aft mast behind the rudder post, and is extremely rare to encounter today.
Sunfish are one of the few common modern dinghies with an unusual rig. Their lateen rig has a lower boom that crosses past the mast instead of ending at it, and a second, upper boom at the top that pulls the top of the sail up.
As elements that project from the hull below the waterline, keels and rudders are collectively known as appendages. They are also known as foils because of their shape and they way they operate. They are specifically hydrofoils (analogous to aerofoils a.k.a. airfoils, but for the water).
The hydrofoil appendages serve to create lift and drag forces for controling direction. Rudders provide direct control over the direction of the boat. Keels are necessary for monohulls to travel upwind. Multihull boats can move upwind without them due to the force from their hulls, but benefit for upwind performance. Windsurfers do usually have small fins, but the edge of the flat hull also digs in and acts as a keel.
A daggerboard is a keel whose underwater area can be adjusted by moving it vertically through the hull to reduce its force when sailing, reduce draft when launching and retreiving, and for storage. Daggerboards are primarily found on dinghies, but are also used on some catamarans.
A centerboard is a keel whose are can be adjusted by pivoting around the attachment to the hull. These have the advantage that they will kick up when accidentally running aground, reducing damage compared to a daggerboard.
In general sailing jargon, a "keel" usually refers to the keel of a larger boat, which has significant ballast (weight) added to increase the righting moment of the boat.
A fixed keel is a keel that cannot be adjusted. A bolt on keel has been attached to the hull by bolts that must periodically be inspected for structural stability. This is the most common kind in modern boats. An encapsulated keel is molded into the hull and stronger in the face of grounding or striking submerged objects at sea, and generally less vulnerable.
A swing keel has such balast and it swings up like a centerboard for reducing draft. The ballast is less effective when raised.
A centerboard keel is more like a dinghy centerboard. It keeps the ballast fixed in the hull or near the center of rotation and leading edge so that it still has righting moment when the keel is raised.
A lifting keel is a weighted keel that moves like a daggerboard and raises vertically.
In addition to allowing a boat to moor very close to shore, some lifting, swing, and centerboard keels allow a boat to dry out with the tide and sit flat on the bottom without external support. This is also referred to in some regions as "taking the ground".
The chord is the length of the keel. A full keel runs nearly the full length of the boat. It provides good protection for itself and for the rudder and propeller and may have shallow draft, and helps provide a wide slick when heaving to. Full keels give a more comfortable boat motion that reduces rocking and pitching. Full keels have lower performance head upwind and may turn more slowly, and are primarily found on older cruising boats.
A cutaway forefoot on a full keel means that the front begins midway through the hull instead of at the bow to increase maneuverability and some upwind performance.
A fin keel projects downward near the center of the boat like a fin. These are the standard on modern racing boats and dinghies. They are also easier to maneuver backwards under power when docking. A long chord fin keel is relatively long fore to aft, to increase stability at the expense of performance. Racing keels have a short cord and long depth.
A canting keel can have its angle adjusted side to side. These are found on some high performance racing yachts.
Bulb and wing keels shorten their draft by placing a large weight at the base. Because they lack length, they provide righting moment from the ballast but have reduced lift and drag and are less efficient than other types. The bulb has a hydrodynamic advantage over both a flat keel and a wing because it reduces the drag from vortex shedding at the bottom of the keel. Some high performance airplanes have similar bulbs on their tips. Wing keels are generally poor performers (and create extra vortices and snag on things), so have fallen out of favor.
Twin keels, also called dual keels and (incorrectly) bilge keels are a pair of side by side keels, tilted slightly outwards to their respective sides. They reduce draft without sacrificing area for lift and drag force. They also allow a boat to sit without support when drying out or on the hard (placed on shore by a crane). Unlike swing and other adjustible keels, they also provide access to the boat bottom for maintenance in this position because the hull is not flush with the ground. However, cleaning and painting between the keels is more difficult than for fin and full keels because of the reduced space.
Some boats have leeboards, which are matching boards or swing keels on the outer sides of the boat instead of in the center. These are relatively obscure and appear on some historical boats, especially from China and Holland, and on sailing canoes. Even more obscure are bilgeboards, which are swinging bilge keels.
Lines are ropes (or cables) on a boat. Rigging is the set of lines associated with the sails and spars.
Standing rigging comprises lines that are not adjusted frequently while sailing and are generally run from the deck to the mast. The most common ones on modern boats are:
Forestay : At the bow, pulls the top parts of the mast forward. Some headsails are attached to the forestay. Not present on catboats such as Lasers. A full length outer forestay runs to the top of the mast. A fractional rig outer forestay runs a fraction of the way up the mast. On some small boats such as 420s, the forestay is only used for mast retention without sails and the jib halyard becomes an inner forestay that carries the load when raised.
A solent rig has multiple forestays for different headsails close together. A cutter rig has cutter stays or baby stays set significantly further aft so that the headsails can tack without obstruction.
Backstay : At the stern, tensions the top parts of the mast backward to adjust the bend and adds support for boats with inner forestays. Not present on all boats. Boats with running backstays have one on each side of the boat and they must be adjusted for each tack. (Since they run, these are not "standing" rigging.)
Shrouds : Also called "sidestays". These run from the mast to the sides of the boat. Shrouds are not present on some small boats such as Lasers that do not have a trapeze or headsails. A few unusual large boats have freestanding masts without shrouds.
There are several kinds of shrouds on larger boats. These are a combination of continuous shrouds that run from the mast to the deck and discontinous shrouds that are shorter pieces that terminate at spreaders.
Shrouds are named cap if they connect to the masthead, intermediate if they connect mid-mast, and lower if they connect below the spreader base.
Discontinuous shroud parts are labeled with "D#" or "V#", where numbers increase from 1 at those that touch the deck. The "V" shrouds are vertical and the "D" shrouds are diagonal.
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Head -
. . -
/| | -
/ | |\ -
/ | | + -
/ | | |\ Head -
Leech / | Luff | | \ -
/ | | | \ -
/ | | | \ -
/ | Leech | | \ Luff -
/ | | | \ -
/_________| Tack | |______\ Tack
- Clew Foot | Clew | Foot \
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.-----------+----------+-- .-----------+----------+--
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| / | /
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'-------------------' '-------------------'
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Mainsail Headsail
[Names for the corners and edges of the sail]
Most modern sails are triangles. Some performance mainsails technically have four sides, although are thought of as oversize triangles with a bit of the top cut off to get more sail area up high in clear air. Some gaff rigged sails are true quadrilaterals, but can also be thought of as triangles that are bent over at the top.
The corners of a triangular sail are called:
Head : Top corner, where the halyard attaches to raise and hold it to the mast.
Clew : Aft lower corner that moves in and out and where the sheet acts for trimming.
Tack : Fore lower corner fixed relative to the boat about which the sail turns.
The edges of a triangular sail are called:
Luff : Fore edge between the head and tack.
Leech : Aft edge between the head and clew.
Foot : Bottom edge between the clew and tack.
The roach is the part of the sail between the leech and the direct head-to-clew line. The term is used almost exclusively with mainsails.
A mainsail is behind a mast, with the tack attached to the mast. The foot of the mainsail is attached to the boom. In extreme storm conditions on the ocean, a mainsail may be replaced with a tiny trysail that has a free foot. Sloops have a single mainsail.
A mainsail may have either vertical or horizontal battens, which are thin beams of fiberglass that help it maintain a curved shape.
The foresails or headsails are in front of the mast. In the simplest case, you're daysailing and the headsail is just going to be referred to as the jib.
If you're racing or cruising, the kinds of headsails you might encounter are:
Jib/Genoa : The typical working headsail. Described by numbers based on the fraction of J, the perpendicular forestay/luff-at-deck-to-mast edge that they cover. A 90 jib is 90% J and thus clears the mast. A 110 is 100% J, and thus overlaps the mast slightly. Technically, a jib or "working jib" is at most 100 J and a genoa is a sail larger than 100 J. In practice, up to about 115, the term "jib" is often used.
Genoas are also referred to by a single-digit number, which is generally "#1 Genoa" ≈ 150 and "#2 Genoa" ≈ 135.
Jibs are referred to by single-digit numbers, but depending on the boat these can vary from "#1 Jib" being the largest to it being the smallest.
On a solent rig, the genoa is usually furled on an outer forestay and cannot be tacked without being furled. The sheets can be run around the outside to allow them to gybe, however, if there is not a Code Zero or other sail in front of them.
On a cutter rig, the genoa can usually tack in front of the cutter stay with some care.
Staysail/Storm Jib : A very small jib, possibly on a baby stay or other inner stay. This is for extreme conditions, primarily for keeping the bow off the wind when a gust causes the boat to turn up, or for tracking straight into the wind when motorsailing.
Asymmetric Spinnaker : A large, curvy lightweight sail for downwind sailing that is attached at the tack to the boat, like a gigantic genoa. Often very colorful. These are less efficient than symmetric spinnakers for running, so typically used on a deep reach. These cannot tack without being doused.
(Symmetric) Spinnaker : A large, curvy lightweight and often colorful sail for downwind sailing that is attached to a spinnaker pole. The spinnaker pole is a boom that must be manually switched between gybes. This cases the spinnaker to alternate which corner is the tack and which is the clew depending on the gybe. These are more difficult to manage than asymmetric spinnakers, but more efficient dead downwind. These cannot tack without being doused.
Code Zero, Drifter, Reacher, Gennaker : Various names for lightweight, oversize genoas-like sails that are flatter than asymmetric spinnakers. These are good from a close to a deep reach. They are usually rigged on an outer forestay and cannot tack without being furled.
Old ships had other sails you will not typically encounter, such as the "square" (actually rectangular) sails, topsails above the main, the yankee jib with a very high clew, and downwind studding sails that stick out far and low from each side.
Furling sails can be rolled up onto the forestay, inside of the mast, or inside of the boom.
The spinnakers and code zero sails are often referred to a "downwind sails". The main, jibs, and genoas are sometimes called "white sails" regardless of their color because they are traditionally white and the downwind sails are often colored.
Some reference sources on sail terminology:
- https://www.boatsnews.com/story/39116/sailing-boats-find-your-way-among-the-names-of-the-sails-and-their-uses
- https://improvesailing.com/guides/sail-types
Sheet : Sheets are attached to the clew, possibly indirectly via a boom. They pull the clew towards the centerline of the boat and also flatten the sail a little by pulling back or down, tensioning the leech, and on a headsail, the foot.
Headsails often have two sheets, one around each side of the mast so that they can be adjusted from the cockpit. In this case, the working sheet is the one that is currently under tension and the lazy sheet is the one that is loose on the other side.
A self-tacking headsail has a single sheet and its clew is attached to a jib boom or a traveler. It automatically slides to the correct side.
The mainsheet attaches to the main boom. On a small boat may be adjusted directly ("boom sheeting") from the boom or run to a block mounted in the middle of the cockpit. On a larger boat it may be attached in the cockpit, run through a series of blocks on the coachroof, or attached behind the helm.
Traveler : A track with a block on a car, typically for the mainsheet. This allows moving the attachment point for the mainsheet from the center of the boat towards the side the boom is on. The traveler offers two features: the main can quickly be eased by releasing the traveler to slide outward, and the main can be pulled down (to flatten it) via the mainsheet by positioning the car directly under the mainsheet. Not present on all boats.
A self-tacking jib uses a traveler in front of the mast that allows it to move to the correct side of the boat without requiring manual intervention on a tack.
Halyard : A line that pulls a sail up. The halyard adjusts tension on the luff. There is one halyard per sail.
Outhaul : A line that pulls the mainsail clew back (out) away from the mast. The outhaul adjusts tension on the foot. On a boat with a mast-furling main, it also is used to unroll the sail.
Cunningham : This also often called "the" downhaul, however technically any line that pulls down (opposing a halyard) is a downhaul. The cunningham attaches to the the main just above the tack and adjusts luff tension.
(Boom) Vang : The vang runs between the foot of the mast and the boom. It adjusts leech tension on the mainsail by angling the boom down, and prevents the boom from riding up in gusts. This is called the "kicking strap" or "kicker" in the UK, especially by racers. The vang may be hydraulic in larger boats instead of a line and support the boom as well as pull it down.
An inverted vang that uses a gas strut above the boom performs the same role while leaving more room for crew to pass under the boom in a small boat. This is sometimes called a "gnav"--vang spelled backwards.
Some small boats lack a vang and must rely on the mainsheet for luff tension.
Running backstays : These connect from the middle of the mast to the back corners of the boat. Only one is working at a time, and the other is lazy. Running backstays must be swapped for each tack, and very carefully in a gybe to avoid the boom colliding with one under tension. Few recent boats have running backstays because of their complexity.
A winch is a drum that a line is wrapped around to make it easier to tension. Dress a winch by running the line clockwise around it at least three times. Sometimes four wraps are needed for very high tension lines. Be careful not to let the line overwrap itself or to get fingers caught inside. Winches accept a handle at the end of the drum, which is a lever that provides mechanical advantage. Some are also electrically powered and can be turned by pressing a button.
Some winches provide further mechanical advantage by having the drum rotates more slowly than the handle is turned, so that less force is required but for more time when tensioning. Rachet winches tension clockwise and have no effect counterclockwise. Dual-speed winches give higher mechanical advantage counterclockwise. To ease a line that is on a winch, put your hand over the wraps around the drum and slowly let it out. To completely release a line that is not under tension, swirl the line counterclockwise around the drum.
Many modern winches are self-tailing and incorporate a round clam cleat at the top of the winch. For traditional winches, you must maintain tension on the line by hand while rotating the drum.
Very large professional racing boats have pedestal winches where the drum is separated from the handles by a gear and chain system and the handles look like an upside-down bicycle pedal mechanism. These typically have dedicated crew positions called grinders.
A block is a pulley or set of pulleys. The part that turns is called a sheave. Blocks change the direction of a line. When a line passes through the same block multiple times, it gives mechanical advantage by allowing the block to be pulled on using less force but requiring more line to be pulled through it.
A cleat holds a line under tension so that it does not have to be held. Some small boats have only one or two cleats, such as for the halyard. Large boats are covered in cleats for their many high tension control lines.
The kinds of cleats commonly encountered in small and midsize boats are:
Cam Cleat : Has two cam (teardrop) shaped, toothed jaws that swivel. Cleat the line by pulling it between the cams while tightening. Release by tensioning the line and pulling away from the cams. With practice, it can be released by sending a fast wave of slack down the line.
Clam Cleat : A valley of fixed teeth that the line is pulled through. Cleat and release in the same way as with a cam cleat.
Horn Cleat : These are used for lines that are seldom adjusted, such as mooring (docking) or halyards. Has a tapered bar forming two horns, attached by two legs to a surface. Cleat by tying a cleat hitch or OXO, uncleat by untying the hitch. With some practice, the cleat hitch can be tied at a distance for docking by flipping loops down the line.
Many people tie cleat hitches insecurely, especially in North America. See this video for details on the number of wraps based on thickness and the locking loop direction.
Jam Cleat : A wedge with a rounded end, which looks like half of a horn cleat very close to a surface. Cleat by wrapping the line around the smooth part and jamming it under the horn. Release by pulling out of the horn. Not as common as the other cleats.
V-Jam Cleat : An arch with a V shaped cutout that a line can be jammed into. Cleat by pulling into the V, release by pulling out.
Clutch : Technically not a "cleat", but it performs the same function. The line runs through a hole and then a lever can be pressed down to lock jaws against the line. cleat by pressing down with the lever. Release by first tensioning the line and then pulling up the lever and flipping it fully open.
Used extensively on larger modern boats to lead multiple lines to a single winch. Although you can physically tighten a line when it is in a clutch, do not do so as it separates the sleeve from the core and puts wear on the sleeve. Likewise, do not release when the line is not under tension or it will create the same kind of wear.
Bollard/Bitts : Single (bollard) or paired vertical posts used for mooring lines on large ships, sometimes with crossbars. These never appear on small boats, and are only used with small boats when temporarily mooring to a ship or large-ship dock.
<script src="../sailing.js"> </script>Footnotes
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Gladstone, Upwind Sail Power, North Sails News, 2018 ↩
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Sail Shape Upwind: Six Things to Look For, SailZing ↩