**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](../beginner/index.md.html) 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](../motoring/index.md/html).
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.
Key Observations
===================================================================
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.
1. 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.
2. 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.
3. 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.
4. 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.
5. 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.
6. *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.
7. The buoyancy of the hull opposes the downward portion of the ballast force,
so that the boat doesn't sink.
8. The rudder intentionally creates unequal drag on each side at the
stern, and turns the boat by slowing it on one side.
9. 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.
10. 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.
Lifting Bodies
===================================================================
Airfoils and Hydrofoils
-------------------------------------------------------------------
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](https://www.scientificamerican.com/article/no-one-can-explain-why-planes-stay-in-the-air/).
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](https://youtu.be/z2sT8hLoTmQ?t=54) or the
[AC catamaran](https://youtu.be/wbJBk7MlG0k?t=420) 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.
Angle of Attack
-------------------------------------------------------------------------
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.
Sail Shape
----------------------------------------------------------------------
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 [^NorthSails],[^SailZing] 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.
[^NorthSails]: Gladstone, [Upwind Sail Power](https://www.northsails.com/sailing/en/2018/08/upwind-sail-power-by-bill-gladstone), North Sails News, 2018
[^Sailzing]: [Sail Shape Upwind: Six Things to Look For](https://sailzing.com/sail-shape-upwind-six-things-to-look-for/), SailZing
More Forces
-------------------------------------------------------------------------
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](https://physicstoday.scitation.org/doi/10.1063/1.2883908)
article in _Physics Today_, which uses terminology more familiar to
engineers.
Headsails
-------------------------------------------------------------------
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:
1. 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.
2. 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.
### Weather Helm
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.
Drag Propulsion
===================================================================
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!
Self-Limiting Effects
===================================================================
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.
Apparent Wind
-------------------------------------------------------------------
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.
Heeling
-------------------------------------------------------------------
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.
Hull Speed
-------------------------------------------------------------------
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.
Planing
-------------------------------------------------------------------
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](https://www.imoca.org/en) offshore class,
[Nacra 17](https://nacra17.org/) catamaran,
and a growing set of foiling dinghies and small catamarans leverage
this technique.
Turbulence and Shadows
---------------------------------------------------------------------------
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.
Shaping the Sails
==========================================================================
## Mainsail
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.
### Luff
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.
### Foot
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
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](https://youtu.be/iP4yK4mIr3E?t=183),
do not release the vang.
It will not prevent the broach on that point of sail. If [running
with spinnaker](https://www.youtube.com/watch?v=iUC7lVnL31I), 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.)
## Headsail
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.
Sailing by the Lee
===============================================================
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](https://www.youtube.com/watch?v=Y10zGG2nHjs)
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.
Square Rigged Ships
===================================================================
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](https://scijinks.gov/trade-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.
Glossary
===================================================================
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
------------------------------------------------------------
*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](https://laserperformance.com/wp-content/uploads/SunfishRiggingGuide_2022.pdf)
at the top that pulls the top of the sail up.
Appendages
------------------------------------------------------------
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](https://www.practical-sailor.com/blog/keel-bolt-inspection-and-repair) 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.
Standing Rigging
------------------------------------------------------------
*Lines* are ropes (or cables) on a boat. *Rigging* is the set of lines
associated with the sails and spars.
[Standing rigging](https://rigworks.com/standing-rigging-or-name-that-stay/)
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](https://goodoldboat.com/freestanding-rigs/)
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.
Sails
-------------------------------------------------------------------------
*************************************************************************
* Head
* . .
* /| |
* / | |\
* / | | +
* / | | |\ Head
* Leech / | Luff | | \
* / | | | \
* / | | | \
* / | Leech | | \ Luff
* / | | | \
* /_________| Tack | |______\ Tack
* Clew Foot | Clew | Foot \
* .-----------+----------+-- .-----------+----------+--
* | / | /
* '-------------------' '-------------------'
* 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](https://www.yachtingworld.com/video/skip-novak-storm-sailing-part-3-using-storm-sails-517) 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
Running Rigging
-------------------------------------------------------------------------
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.
Mechanical Advantage
---------------------------------------------------------------
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](https://www.youtube.com/watch?v=irpD7eABfbc)
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](https://www.youtube.com/watch?v=LepMcTJuFfI),
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](https://www.youtube.com/watch?v=BBqsF72xNSU) 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.