Boston Harbor Sail Club
Advanced Sailing version B
Lecture Notes
Date: 7/7/1999
Amended 8/30/99
Version B date: 7/30/2005
By: Captain Tim Johnson
From: Lecture by Ken Woods, A.C. on 10/24/98
INDEX
Sail Trim ......................................................................................................................................... 2
Airflow ........................................................................................................................................ 2
Sail shape .................................................................................................................................... 4
Telltales ....................................................................................................................................... 6
Sail Cloth .................................................................................................................................... 7
Wind Controls ................................................................................................................................. 8
Cunningham ................................................................................................................................ 8
Boom Vang ................................................................................................................................. 8
Outhaul ........................................................................................................................................ 8
Backstay ...................................................................................................................................... 9
Traveler ....................................................................................................................................... 9
Jib sheets ................................................................................................................................... 10
Downwind Sailing with a Spinnaker ............................................................................................ 11
Nomenclature ............................................................................................................................ 11
Raising a Spinnaker .................................................................................................................. 12
Controlling the spinnaker .......................................................................................................... 13
Gibing with the spinnaker ......................................................................................................... 13
Taking down the spinnaker ....................................................................................................... 14
Stability ......................................................................................................................................... 15
Metacenter................................................................................................................................. 16
Heavy Weather.......................................................................................................................... 17
Hull Speed ................................................................................................................................. 18
Miscellaneous data for the J-29 ................................................................................................ 18
Racing techniques ..................................................................................................................... 19
Polar plot ................................................................................................................................... 19
1
Sail Trim
Airflow
One of the core ideas behind sailing is airflow over the sail surface. For this visual, image a
flag on a flagpole. The flag will hang limp when there is no airflow and then stream out in the
direction that the wind is blowing. One characteristic of the flag is the ripples that form over the
surface of the flag. Eddies are created and push these ripples down the flags length eventually
spilling out behind the flag. This is turbulent airflow over the flag, which contrast with the laminar
airflow before the air strike the flagpole. On a sailboat, the flagpole is the mast. By adjusting the
positioning of the sail, we can take advantage of the turbulent airflow to provide lift and forward
motion. Advanced Sailing is about sail shape, airflow and the forces that a sailboat experiences
while underway.
2
3
When the sail is trimmed so that the leading edge of the sail points directly back into the
wind, the sail splits the air flow into two parts, one passing along the front side of the sail and the
other passing behind the sail. As the air streams along the back of the sail, the sail curve turns
away from the direct of the airflow and causes the air to fill in behind the sail, bending the airflow
into swirls or eddies. This creates a vacuum that pulls the sail taunt. Figure 2 shows the effect of
the air behind the sail. If all the forces along the sail are summed up, the sail could be represented
by one vector arrow, seen in Fig. 3. The location of this vector would be at the intersection of the
bisectors of the sail’s corners were the sail perfectly flat. It is by sail trim that we can change the
sail shape and thereby change the direction and force of the sail’s vector.
Since there are two sails on a sloop-rigged boat, the jib has forces acting on it similarly to
the main sail. Both sails can have the forces on each sail represented by one vector called the
center of effort (CE), Figure 4. There is an additional effect of the jib sail acting as a scoop to force
air over the back of the main sail at a faster rate of speed than the air moving over the front of the
main sail. This turbine effect is created because of the venturi of the slot between the main and the
jib. The sail plane vector can be represented by two composite vectors: one parallel to the ships
direction of sail and one across or perpendicular to the ship. The smaller vector is what pulls the
ship forward and is resisted by drag through the water of the hull and through the air by the ship’s
sail rigging. The idea behind sail trim is to maximize the forward motion by changing the sail
shape to point the CE as far forward as possible.
The perpendicular component of the center of effort vector causes leeway and is resisted by
the center of lateral resistance (CLR). The center of lateral resistance is the geometric center of the
portion of the boat underwater. One measure of the CLR is the aspect ration (AR), which gives the
boat owner a clue about the shape of the keel. The area included in the aspect ration is the keel.
The span is the average depth of the keel. The formula for AR is the span squared divided by the
area. In the example shown, a full-length keel can have an AR of .5 while a fin keel of the same
area can have an AR of 5. Fin keel vs. full keel: fin keels have high aspect ration of height to
length. This allows fin keels to point higher into the wind due to a greater CLR than boat with a
full keel. Full-length keelboats are considered blue water boats because if they were to go aground,
the keel structure is more likely to remain intact.
Sail shape
The desired sail shape is a curved surface much like an airplane wing. The sail’s leading
edge should point back into the direction the wind is coming from. Wind when it blows over the
surface of the earth is affected by the earth’s rotation. As a result, there is a wind sheer the higher
up the mast you go. The sheer affects the wind speed and direction. Basically, the higher, the
stronger and more consistent the wind while the wind direction rotates clockwise. What this means
is that the sail should twist to accommodate the wind sheer. Sails being made of cloth easily
accommodate this need when properly adjusted. Every square inch of sail cloth raised should work
the wind to achieve maximum effort. In general, when the wind is strong and you are working
upwind, the sails need to be flat and trimmed in. When the wind is light along the same course, the
sails can be a bit fuller giving a deeper curve to work the wind. Sails have a sweet spot just like a
tennis racket. By adjusting the sail controls discussed later, the sweet spot can be moved.
4
5
Telltales
Wind indicators we used on boats are called telltales, light yarn or magnetic tape. They
indicated apparent wind because they show the summation of the boats forward speed added to the
wind speed. The only indicators of true wind are flagpoles ashore or pennants from boats at
anchor. Off shore, all you have is the apparent wind so the observant sailor will learn to set and
trim their sails accordingly and note the courses held when close hauled on port and starboard
tacks.
There are three different telltales on
a sailboat located on the shrouds, along the
sail luff and on the sail leech. Each telltale
tells the sailor different information about
the wind and how well the sails are working
the wind for power. A telltale attached
along the shroud stays will indicate wind
direction. This telltale will point to the
leech of the sail when the boat is close
hauled to the wind. Depending on the
direction this telltale points, the sailor can
determine what point of sail they are on and
set their sails accordingly. When sailing
downwind, a telltale attached to the backstay
gives a better indication of the wind
direction and should be check often to prevent accidental gibes. The reason not to trust the telltale
on the shroud is wind spilling off a main can redirect the telltale on the shroud but a sudden change
can tell you to check the backstay telltale.
Telltales attached along the luff
of a sail will tell us what the shape of
the sail is doing to the lift used to
power the boat. The J-29 is outfitted
with three sets of telltales. Telltales in
the luff are paired together with one
opposite another at the same point
along the luff. The sailor can only see
one at a time from the helm and the
shadow of the other on the opposite
side. From the helm, when the sail is
properly trimmed, the inside telltale
should be streaming back along the
canvas if there is enough wind. If there
isn’t enough wind, they will hang at an
angle based on the wind speed. If they
hang straight down, you’re becalmed.
The only behavior you don’t want to see is the telltale lifting off the canvas and waving about. This
tells you that the sail isn’t trimmed in enough or that the helm man is sailing too close to the wind.
The outside telltales can also be streaming back or it can be lifted by the eddies created by the
airflow filling in behind the sail.
6
Telltales attached along the leech of the sail indicate how the airflow is behaving as the air
leaves the working canvas while beating upwind or reaching. The desired behavior is for the air to
resume laminar flow after leaving the canvas. This behavior is indicated if the telltales stream
straight out behind the sail. If the telltale bends around behind the sail, the leech of the sail is too
close to the wind and the sail twist needs to be adjusted. These telltales provide no information
when sailing downwind.
Sail Cloth
Finally, sails no longer are just made of cloth. Fiber sails are laminates of polyester film
layers laid down over specially, designed worktable that provide the 3-D surface to form the sail.
Over the initial layer(s) are laid the carbon fiber threads that absorb the load followed by the final
layers of film polyester. The sails are heat treated to seal the layers together. These sails are
expensive and last for a season or two. Cloth sails are very durable and while over time they
stretch, well design boats will accommodate the changes. Cloth sails are distinguished by different
weights (meaning more threads per inch or heavier threads) with heavier weight sails meant for
higher wind loads. Sails are numbered with the heavier sails with the higher numbers being raised
when wind speeds are up. A fully outfitted racing boat has a variety of sails stowed below decks.
A spinnaker is a nylon sail of very lightweight and really isn’t meant for winds about 20 knots.
7
Wind Controls
Cunningham
The Cunningham is a block and tackle that is attached at the foot of the mast to drawn down
the luff of the mainsail. By stretching the luff, wrinkles in the sail that develop when the main is
raised are pulled out thus clearing a path for laminar airflow. The reason the Cunningham does this
trick and not the main halyard is due to the increased purchase of the Cunningham block and tackle
as opposed to the single whip tackle of the main halyard. In a single whip, the line is run over one
sheave (at the top of the mast) thus giving a 1 to 1 advantage to the line. In block and tackles, the
strength multiplier is the number of falls that you count between blocks.
Boom Vang
The boom vang is a block and tackle arranged to pull down on the boom. It is attached at
the foot of the mast to the boom partway out. Sometimes the runner is lead back by fairlead or
cheek blocks to the winches. The idea is when the boat is
on a downwind run with the mainsheet let out, the boom
vang holds the boom position when the boat rolls. If the
boom vang is not applied, the boom tends to rise, folding
the mainsail and spilling air. The boom vang has a
secondary application of stretching the mainsail by
bending the boom (this assumes the leech of the sail is
holding the end of the boom in place). If the boom vang
is not released when raising the mainsail, the head of the
main doesn’t reach its proper position on the mast and the
Cunningham doesn’t work properly. For pneumatic
boom vangs, the tension limits how high the boom can
raise with the mainsheets released.
Outhaul
The outhaul is an adjustment for stretching the foot of the mainsail along the boom. When
beating upwind, the outhaul should be pulled tight to help flatten the sail. When running
downwind, the outhaul could be relaxed and should be released in light airs. This contributes to a
fuller sail as it changes the position of the clew thus changing where the leech is attached. The
outhaul is often attached on a cleat along the boom with the line run along the inside.
8
Backstay
The backstay on racing boat with
three-quarters rig masts are adjustable. A
bendable mast will pull the sail below the
forestay attachment forward an inch or
two. It also changes the position of the
head of the mainsail in relation to the tack
and drops the clew downward. The
backstay is applied normally on upwind
beats to flatten the sail. It should move
the draft of the sail forward. Masts on
full-rig sloops (cruising boats) are not
adjustable thus giving the sailor only one
mainsail adjustment for flattening the
main while beating: the mainsheet. Racing
boats with roached mainsails (significant sail
beyond where the leech would normally be
found) have two backstays and when the
main is gibed, the main is paused as it travels
across the boat while the backstays are
changed over. One rule for these boats: one
stay, two stays, but never "no" stays. The
windward backstay is left on as the boat
completes a gibe.
Traveler
The traveler is an adjustment for changing the position of the mainsheet attachment to the
boat. It is not used to adjust the sail shape but to adjust the sail’s angle to the wind. When a wind
gust strikes heeling the boat, the helmsman has two options to ease the weather helm: turn toward
the wind easing the weather helm or dump the traveler spilling air. By utilizing the traveler, the sail
shape is preserved and after the gust passes, the traveler can be restored to its normal position
amidships plus the boat doesn’t change its course.
9
Jib sheets
The adjustment of the jib sheets allows the sailor to change the shape of the jib to set the
sail twist. The fairlead for the jib sheet is what is being adjusted. By positioning the jib fairlead
forward or aft along the track, the jib sheet will tension either the leech or the foot of the jib making
the other edge of the sail fuller. This overall effect is the change the sail twist to match the wind
sheer. On the J-29 are 3 sets of telltales along the leading edge of the jib. When all three lift or
lay-down at the same time or nearly so, you’ve got the jib adjusted perfectly for this wind sheer on
this tack.
Fairlead
Forward
Aft
Leech
Tight
Full
Foot
Full
Tight
Slot
Narrow
Full
By watching the telltales as you tighten the jib sheets you can tell which way to move the
fairleads. If the bottom telltale lies down and streams before the top telltale, the leech needs to be
tighten so move the fairlead forward a notch or two. If the top telltale lays down first, the foot
needs to be tightened so move the fairlead back a notch or two. Do not expect perfection but find a
position that all three telltales lay down and stream when the jib sheet is tighten at some point. The
amount of tension in the jib sheet (in inches or rotations) after the first telltale lays down until all
three lay down reveals the degree of adjustment needed. If the jib overlaps on the spreader and
becomes distorted, laminar airflow is being lost and needs to be balanced against the gain of the lift
at the leading edge.
10
Downwind Sailing with a Spinnaker
Nomenclature
Spinnaker pole—used to hold the spinnaker out to windward.
Topping lift—line on the mast that holds the spinnaker pole up.
Downhaul—line attached to the foredeck that hold the spinnaker pole down.
Guy—line used to adjust the fore and aft position of the outboard end of the spinnaker pole.
Sheet—attached to the spinnaker corner that doesn’t have pole and adjusts the fullness of the sail.
Jaws—attachments at ends of the spinnaker pole to hold the pole to the mast and the guy in place.
Spinnaker halyard—raises the spinnaker up to the top of the mast.
Lazy guy/sheet—line laid in that isn’t utilized on a particular tack.
11
Raising a Spinnaker
While on an upwind run, the boat must fall off upon rounding the windward mark in order
for the following directions to be correct. Let’s assume the boat is on a port tack. Rig the
spinnaker pole first by attaching one end of the spinnaker pole to the D-ring on the mast, attaching
the topping lift so it hold the pole about 2 feet off the port side pulpit rail on the bow. Attach the
downhaul but don’t make it fast. The spinnaker pole is attached with jaws pointed up so that when
the sheet is release on a gybe, it flys clear of the jaws. Bring the spinnaker bag with the folded and
properly stowed spinnaker on deck and attach the bag using a sail tie to the deck cleat. The
starboard guy and sheet, attached to each other, having been previously run outside of all standing
and running rigging is attached “under the sail, over the rail” to the spinnaker (pick one corner, it
becomes the clew). The port guy with sheet attached, and having been previously laid in outside of
all standing rigging, is passed “guy only” thru the spinnaker pole jaw around the bow then “under
the sail and over the rail” to the other spinnaker corner (which becomes the tack). Finally the
spinnaker halyard is removed from the D-ring and passed either around the forestay to the backside
of the jib or around the leech of the jib to the backside of the jib. The halyard is attached to the
head of the spinnaker by passing the first 5 to 10 feet of the spinnaker over the rail and under the
sail. The decision made about the halyard is to check that it is not wrapped around the forestay at
the top of the mast.
As the boat rounds the windward mark, the main can be let out (eventually) but for the first
few trials, the crew should concentrate on getting the spinnaker up and deployed correctly. The
guy needs to be pulled, winched back until the spinnaker pole is at least 45 degrees to the
centerline. The sheet only needs to be pulled tight enough so that the foot of the halyard is not
binding up on the forestay. The halyard should be winched until it is chock-a-block. After the
spinnaker is up, the jib can be furled.
12
Controlling the spinnaker
1. The spinnaker pole is winched in or let out so that the pole is perpendicular to the telltale on
the windward side. The line used to control the spinnaker pole is the guy. If you are having
trouble pulling the spinnaker pole aft, try releasing the downhaul.
2. The sheet controls the fill of the spinnaker. If the luff of a spinnaker starts to fold
(significantly) haul in on the sheet to tighten the foot of the sail. Let out on the sheet until
the luff no longer collapses. The crewmember controlling
the sheet can sit at the foot of the mast or anywhere that the
spinnaker can be kept under observation.
3. The height of the spinnaker pole controls the shape of the sail
in a vertical plane. It should be raised enough so that the
helmsman can see forward under the sail.
4. If the sheet and guy are tied off, the helmsman controls the
spinnaker and should the sail fold, the helm needs only to
turn downwind to fill the sail.
5. If the halyard is let out and the spinnaker doesn’t drop lower,
let out on the guy and sheet an equal amount lets the
spinnaker fly higher and self-adjust for wind shifts. It may
be necessary to tie off the spinnaker pole with a preventer to
let the guy out and still hold the spinnaker out into the wind.
Gibing with the spinnaker
One crewman is all that is necessary to switch the spinnaker pole. The crewman on his way
forward grabs the lazy guy (off the working sheet attached to the clew of the spinnaker) and bring it
over to the mast. At the mast the jaw capture line is operated so that the jaw is opened out on the
far end allowing the tack with guy attached to fly up and out. The spinnaker pole is then removed
from the D-ring and the lazy guy is dropped into the open jaws at the mast. The crewman
announces, “Pole OFF”. The main is sheeted into the center of the boat at this point (if not sooner
when you are running short-handed). The pole, hanging by the topping lift is swung across the boat
so that the other end can be attached to the pole. Here, the crewman may need some help to attach
the spinnaker pole as the pressure increases as the pole is pushed outward. When the pole is
secured, the crewman announces, “Pole ON”. The new sheet and guy are trimmed in. The main is
gybbed and let out on the new tack at this point. All crew members should practice this maneuver
as part of Advanced Sailing.
13
Taking down the spinnaker
Two crewmen are used for this event. Both go forward. The spinnaker pole is eased
forward to the forestay and dropped lower (topping lift) if needed so that the jaw can be accessed
from the foredeck. This maneuver puts the spinnaker behind the main so that air can be dumped
out of the sail. The halyard is dropped only as fast as the crewmen can gather in the foot of the sail.
The foredeck hatch can be opened so that crewmen can drop the sail into below decks. As the tack
and clew become available, the lines are detached and placed on the railings for the next run. The
Halyard is stowed on the D-ring and the spinnaker pole is removed form the mast and stowed away.
For training purposes during Advanced Sailing until the spinnaker pole is stowed, the boat doesn’t
go upwind. When racing, as soon as the boat starts to turn upwind, the spinnaker is dropped and
the jib deployed for the upwind leg of a race.
When wind condition intensify during a spinnaker run, the boat will heel and act with
reverse weather helm. These are indications that you need to drop sail. When you see whitecaps,
it’s too windy to deploy the spinnaker.
If the spinnaker wraps around the forestay when raising or gibing or any other time, all that
needs to be done to free the sail is grab the taped edge of the luff above the tack and pull with
steady pressure on the sail to get as much of the sail out away from the forestay as possible. The
wind will eventually lift the sail free from the forestay even if it was turned into an hourglass.
14
Stability
Stability is a measure of how tipsy a boat is. At work are two forces in the vertical plane:
the upward push of buoyancy and the downward press of weight. These forces are in equilibrium
whenever a boat stabilizes in one position. Stability is also the solution to a 2 nd order non-linear
equation that is solved continuously by a sailboat angle of heel. You can read the answer off an
inclinometer. On BHSC boats, a number of compasses mounted in the hull have this feature
printed in the trim of the compass.
Let’s examine the two forces. When a boat is tied up to a dock or at a mooring, these forces
are of equal strength, in exactly opposite directions that are in line with each other. The upward
force is the resultant of the buoyant forces of the liquid displaced by the boat. Buoyancy is a single
force applied vertically upward from the center of buoyancy. The center of buoyancy is the center
of the volume of the portion of the ship that is submerged. It always pushes directly up. Opposing
this force is the weight of the boat. Summing all the gravity forces gives a resultant acting
vertically downward from the center of gravity of the mass of the boat. If at rest as indicated
above, the two resultant forces must be equal and opposite and the waterline gives an indication of
the weight of the boat.
The center of buoyancy will be in a vertical line with the center of gravity. A symmetrical
ship will have these forces in a plane along the centerline of the ship. The point representing the
center of gravity doesn’t move off the centerline of the boat but the location for the center of
buoyancy will change (move off the centerline) as the boat heels. Inclination in the athwartship
plane (across) is called heel or list. Inclination in the longitudinal plane is called trim. A force that
causes the boat to heel is the perpendicular component of the wind across the sail.
The main point of this is the resolution of forces of the CE can be a force in the direction of
forward travel and a component of lateral force perpendicular to the forward motion this force
across the boat is opposed by the CLR, under normal conditions, the CLR will effectively oppose
the lateral wind force and the only effect seen is causing the boat to heel. The force in the forward
direction is opposed by the drag of the profile of the boat seen from the front. When the lift force
forward is greater than the drag of the boat profile the boat moves forward.
15
Metacenter
The Metacenter is an index of stability of a ship is the intersection of the forces of buoyancy and
gravity. In the stable position this lines coincide and it is difficult to determine the Metacenter. By
inclining the ship slightly, the location of the center of buoyancy will change causing the line
representing the force of buoyancy to split away from the centerline along which the force of
gravity acted when the ship was upright. The intersection of the new force of buoyancy and the
centerline is an approximation for the initial Metacenter.
The Metacenter (M) and the center of gravity (G) are along the centerline of the boat. The
distance between these two points, GM, is called the Metacenter height. GM is an index of the
stability of a ship. If G is below M, the ship is stable. You have positive stability. The centerline
of the ship when stable coincides with the force of gravity, G, and buoyancy, B. This line remains
in place when the vessel is heeled over. The buoyancy moves and it acts upward. The intersection
of the new force of buoyancy and the centerline is the new Metacenter location. The distance from
the center of gravity, G, perpendicular to the new line of force for the buoyancy is called GZ, Z
being the point of intersection with the new buoyancy line of force. This is the righting arm. The
force at G is pushing down and the force at Z is pushing up. Together they tend to right the ship
bring the two force back into alignment and directly over one another. GZ is called the righting
arm.
16
Heavy Weather
A condition of wind in excess of 20 knots is weather helm. Whitecaps tend to be seen
across the water when the wind reaches 18 knots. If the CE is aft of the CLR, the boat tends to
round up and point into the wind. The helmsman notices this and pulls the helm toward the
windward side of the boat, 15 or 20 degrees of weather helm is excessive and the rudder is only
causing increased drag. The correction for weather helm is to move the CE forward: reduce sail by
reefing the main or easing the traveler to leeward until the helm is corrected or at least eased. In
extreme heel, the rudder loses effectiveness in turning the boat; the solution is to de-power the boat.
In the cases where the vessel doesn’t respond to the helm, air has to be spilled from the sails to
reduce heel first. If the two previous methods for reducing the weather helm don’t resolve the
problem and you find you are dumping air by luffing the main, consider taking the main down and
sailing under jib only. When there is extreme heel, the CLR is reduced and the boat is blown
sideways called leeway. At the same time, the CE is reduced because less sail area is seen by the
wind more of the boat becomes exposed creating a pseudo-sail area that only contributes to leeway.
This means you will have to tack more times to reach an upwind destination. In the photos shown
below, the boat is heeled with noticeable weather helm, when the traveler is on, the helm is restored
to the centerline and the heeling decreases. As a rule of thumb, if the boat is heeled in excess of 20
degrees with weather helm on a continuous basis and the traveler is applied, you should think about
putting a reef in. When the boat heels 30 degrees, you should really think about a 2nd reef.
17
Hull Speed
In principle, anything that creates lift creates drag. When drag of the hull through the water
is equal to the force caused by the drive from the sails, you stop accelerating. Hull speed reached
for a displacement boat when the boat is pushing a wave at its bow whose period is equal to the
length of the boat (the wave at the bow is matched by a wave at the stern). If the boat is a planning
boat, such as a Hobie Cat, it simply mounts the wave at its bow. Theoretically, hull speed is
approximately equal to 1.34 time square root of the boat water line length. Hull speed of a J29
(LOA=29 but LWL=25) is 1.34*(25)^1/2 = 1.34*5 = 6.7 knots (if the boat was being sailed
perfectly flat). Heeling the boat causes the waterline to increase because of the beam thus allowing
speeds of over 7 knots to be achieved when closed hauled and perfectly trimmed.
Miscellaneous data for the J-29
LOA: 29.5
LWL: 25
Mast Height: 47.5 feet above the hull
Beam: 11 feet
Sail area: 453 sq. feet
Draft: 5.6 feet
Displacement: 6000 cubic feet
Last produced in 1987
PHRF:117
The wider beam allows a higher mast because the shrouds can be placed further apart from
the mast for greater sail loads. The also provide a longer water line when the boat is heeled.
Beamy boats with the square sterns simulate the longer keel because the draft of the hull pushes the
water aside as surely as any hull structure. The PHRF is a measurement of racing expectations
against other boats similarly rated. The smaller the number; the faster the boat and the more points
given up in the adjusted finish.
18
Racing techniques
It’s been said that if there are two sailboats on the water, they are racing each other. A well
trim boat properly sailed will be very competitive to a boat whose crew is inattentive to changing
wind conditions or sail trim. To this end, the sailor who has completed Advanced Sailing is well
trained to begin racing. Fine-tuning this knowledge can be obtained by participating in a racing
program where one can observe crews and boats matching wits and skills as they sail alongside
each other.
Polar plot
A polar plot shows theoretical speed attainable by a perfectly tuned boat in ideal conditions.
The angles are the degrees the boat is sailing off the wind with the zero representing the boat
headed directly into the wind, 90 degrees when the boat has the wind abeam and 180 degrees when
the wind is astern. Only one half of a circle is shown as it is assumed the boat can be sailed
identically on either tack. The lines are the speed the boat can attain for various wind speeds. The
actual speed achieved at a particular angle is the intersection of that angle line with the wind speed
plot. The various lines are marked off in knots.
19
Two points are obvious from polar plots: the boat speed falls off if you point too high and
the boat is fastest when sailing on a broad reach. The polar does not include the use of a spinnaker.
When sailing downwind, you cannot only pin the boom to one side of the boat on a broad reach but
with the jib working, you achieve the best boat speed.
Whether or not you would pass a boat sailing directly downwind depends on the angle you
give up as you sail wide of the downwind destination. Sailing upwind has a similar problem,
except to pick up speed; you fall off the wind slightly. In the following table, the various distances
are calculated based on a competitor’s boat holding a steady course for one mile to a mark.
Angle off
5
10
15
Distance you sail
to their 1 mile
1.003 mile
1.0154 mile
1.035 mile
Distance ‘ween
boats at 1 mile
.087 mile
.173 mile
.25 mile
Total distance
sailed extra
.091 mile
1.189 mile
1.294 mile
Approx. % you
must sail faster
10%
20%
30%
The information from the table is used as follows: if while sailing upwind or downwind and
your boat speed picks up appreciably by setting a more favorable course, you will arrive at the
same point sooner provided the increase is 10, 20 or 30 for every 5 degrees change. If the boat
speed is 5 knots, then 10% is 5.5 knots, 20% is 6 knots, and 30 % is 6.5 knots. General rule: every
5 degrees change requires a ½ knot increase in speed to make this adjustment useful. This table
does not take into account current effects.
Another term commonly used in racing that you need to know is lay line. That is the course
that you have to sail after you tack to reach the windward or leeward marks. You tack when you
reach the lay line. For the skipper of a boat to know when they have reached the lay line, they can
use a bearing compass and not tack until the destination is bearing the same as the course you can
sail on a port (or starboard) tack.
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