Telltales, tufts, indicators. No matter
what you call them, they are an essential aid
in steering and trimming.
The telltales make the trimming of the sails and
the steering of the boat easier. You shouldn't stick
too many telltales on your sails - when sailing, the
amount of information streaming in exceeds the capacity
of the crew, so keep it as simple as possible. In this
article we explain which telltales are essential and
sufficient at the same time. We recommend not to have
more telltales than that. The telltales in the leech
should be made of a rather strong nylon strip. Normally
they are already installed by your sailmaker. The telltales
in the luff should be of woolen yarn glued onto the
sail with a colorful sticky-back.
In the mainsail telltales are only needed in the leech.
Cut them out of an inch wide nylon strip and sew them
into the ends of the batten pockets. The right length
is from six to twelve inches. On the race course colorful
strips are the best but if you are racing at night,
you are better off using white telltales because they
show clearly against the dark sky. That's why the telltales
on offshore racers often are white.
The telltales near the luff of the sail are used as
a steering aid when sailing upwind. If you are pointing
too high the windward telltales "stall", i.e.
point straight up or stream forwards or twirl around
restlessly. If you are sailing too low the leeward telltales
hang down and die, which is a sign of a serious steering
error - the leeward telltales should always stream steadily
aft. When the leeward telltales stall, the jib trimmer
should ease the sheet until the helmsman is back onto
his proper course again. Stalling the leeward telltales
means that the whole foresail is about to stall. It
then loses most of its drive and only the heeling force
The windward telltales indicate different things depending
of the wind strength. In light winds your heading is
correct when both the windward and leeward telltales
stream steadily aft. The maximum speed is achieved by
steering so high that the windward telltales are just
about to twirl. In medium winds the best speed is achieved
when the windward telltales jump up at steady intervals.
Telltale 1, telltale 2, telltale 3 - jump - telltale
When the wind increases the telltales become less important
as an indicator of the proper course. Steer the boat
according to the heeling angle and the wave conditions
and never mind the (windward) telltales.
Steering for VMG
The steering telltales are most useful when sailing
upwind. Steering according to the telltales in light
and medium conditions normally maximizes VMG, i.e. your
speed towards the mark is the greatest. A high VMG does
not necessarily mean the highest speedometer values,
but indicates the ideal combination of boat speed, pointing
Both the main and the jib should have two to four telltales
in the leech, used to proper sheeting. The leech tails,
as well as the telltales for steering, are of the most
value in light to medium air. The telltale at the end
of the top batten pocket is essential when adjusting
the proper twist for the main sail. When the telltale
disappears behind the sail, the sheet is too tight and
the sail has too little twist. In a light winds (5 to
10 kn) the top telltale should be flying for about half
of the time. In less than about five knots the leech
tails won't work properly. When the wind picks up, the
sail is flattened and all the telltales normally stream
The two telltales below the top one tell us about the
overall shape of the sail. When the top telltale streams
steadily aft but the second one is stalling, your main
is either too full in the middle and /or too flat in
the top. Bending the mid-section of the mast normally
helps. The stalling of the lower telltales may also
indicate that your jib is sheeted too loosely, and causes
the airflow to separate on the leeward side of the main.
Closing the gap by moving the jib lead forward/inward
or changing into a larger genoa will help.
The main sheet is the most important trimming device
on your boat, and the top leech tail is the best indicator
of the main sheet tension. The required tension on the
main sheet depends of the boat you have; use the top
leech tail when you are looking for the right trim in
different condition. In less than 5 knots of wind adjust
the sheet so that the upper tail is visible only sometimes.
In medium air the upper tail should show itself about
50% of the time, and in heavy air all leech tails must
Jib leech telltales
The telltales in the leech of the genoa behave somewhat
differently from the ones in the main. Never sheet your
genoa so tight that all the telltales in the leech stall!
Before stalling, the telltales become slow and lazy;
that indicates your maximum trim. However, if your boat
has a high & narrow jib, with little overlap (like
on some one-designs), you may allow the leech tails
to stall from time to time in a light air and medium
airs. The top telltale is again the most important as
it is usually the first one to react to the wrong trim.
In some boats the spreaders restrict how far in the
genoa can come. In these boats the top telltale may
still stream aft when the middle one already stalls
- ease the sheet a hair. In older jibs the leech may
start to hook in and the telltales become then less
Sticking tails on headsails
Install the telltales at the half and quarter points
down the leech. Two pairs of steering telltales should
be installed in the luff of the sail approximately ten
to twenty inches behind the luff rope *) and four to
eight feet above the tack. In small boats the telltales
should be lower and nearer to the luff than in bigger
boats. Don't set the telltales at the same spot on different
sides of the sail but always fix the starboard telltale
an inch higher. When sailing towards the sun it may
otherwise be difficult to distinguish the windward telltale
from the leeward one. If you happen to forget which
telltale is higher look at the mainsail numbers; they
follow the same rule.
It's a good idea to have two sets of steering telltales.
One set should be installed a bit higher and closer
to the luff than the other. The telltales further away
from the luff are more forgiving and you should steer
according to them in rough seas and in heavy winds.
The telltales closer to the luff are more sensitive
and you should use them in smooth seas and lighter conditions.
The telltales get easily stuck - avoid installing them
too close to the seams. If the lower telltales have
got stuck, or if the crew is obscuring the view, you
can steer by the upper set. The telltales shouldn't
be installed higher than where you can reach from the
foredeck, so that you can easily free them in case they
get stuck. One way to release a telltale is to smack
the clew - this helps to avoid the unnecessary trips
to the foredeck.
The top telltale set is to be installed nearer to the
luff than the steering telltales. They are mainly used
when determining the right amount of twist for the jib
on a reach.
*) The correct distance from the luff depends on the
sail size: stick your driving tales at a distance of
7,5-15% of the width of the sail at the level of the
On a reach ...
... telltales are used for trimming the sails. The
helmsman steers by the compass or to a fixed point in
the horizon and it is the crew's duty to keep trimming
the sails so that the telltales stream steadily aft.
On a reach the top telltales in a jib are of good value
when determining the right twist, which is otherwise
very difficult, especially with high aspect ratio jibs.
If the windward top telltales are stalling, move the
jib lead forward. Remember that you can rely on the
telltales only in light and medium conditions. In strong
winds the heel becomes the main trimming indicator and
the excess power has to be released by increasing the
twist. The jib trim on a reach is always a compromise
between the upper and lower sections of the sail. It
might be a good idea to move the jib lead all the way
out to the toe rail and forward so that the windward
top telltale streams steadily aft. Normally it is very
difficult to get both the top and bottom telltales to
work well together.
The top telltales also give an indication of the sag
of the headstay. If the windward telltale wobbles all
the time, the top section of the sail may be too full;
try straightening the headstay. However, it is quite
normal that the windward tail streams right up when
it gets windy and you are overpowered.
Don't use the luff telltales when looking for the right
position for the jib lead - the information you get
is often more harmful than beneficial. You may have
the lead way back but the sheet very tight, and the
telltales be streaming fine, or vice versa. The correct
jib trim is always a combination of lead position and
sheet tension, and there is no single right lead position.
: If the leeward telltales are stalling
or streaming forward, you are sailing too low.
Come up a bit, or ease the jib sheet. bit, or
ease the jib sheet. In light air, steer just on
the verge of collapsing the leeward tails.
If the windward telltales are continuously jumping
up but the boat isn't heeling excessively, you
are sailing too high. In medium air on an ideal
heading the telltales jump up at three to four
In stronger winds (15 kn +) ignore the telltales
and steer by the heel angle .
Telltales make trimming and steering easier. One should
not, however, stare his eyes out at them. If the cut
of your jib is too flat, you will achieve the best speed
by steering the boat so low that the leeward telltales
are partly stalling. You should try to make the jib
fuller by letting the head stay sag a bit. On the other
hand, if your jib is too full (which inevitably happens
to all sails at some stage when the wind picks up) you
should steer the boat so that the windward telltales
are twirling up. In strong winds, let the telltales
live their own lives and steer the boat according to
heel and sea conditions.
Flow separation & boundary
When the wind sweeps past the sail its velocity near
the sail surface decreases because of the friction.
This so called boundary layer can be over
2 inches thick in the middle on the windward side and
in the leech on the leeward side of the sail. When the
velocity of the air stream decreases to a certain point
boundary layer separates from the sail. The telltales
indicate how the boundary layer is behaving at different
moments in different parts of the sail. The separation
of the boundary layer always means a significant loss
in power and is to be avoided at all cost. This is why
the telltales are so useful, and why we try to trim
the sail so that they always stream steadily aft.
The sail is often compared to a wing of an airplane.
A sail, however, has one aerodynamic characteristic
that makes the comparison feeble. In the luff of a sail
the airflow always gets separated from the sail surface
on one side or the other, creating a separation bubble.
The thick and round leading edge of a wing - or a keel
of a sailboat - eliminates the separation of the boundary
layer, which improves the efficiency of the foil considerably.
Windward side bubble
When the airflow meets the luff of the sail in
too steep an angle, it separates and a separation
bubble forms. The bubble is like a small eddy
where air circulates around in the same spot.
The tighter the sail is sheeted the longer the
separation bubble will be. The airflow reattaches
to the sail behind the separation bubble. Depending
on the size of the bubble and the position of
the steering telltales they either stream steadily
aft, twirl in a funny way or in the worst case
even stream forwards against the wind.
If you are pointing too high the bubble forms
on the windward side of the sail. The bubble on
the windward side is less harmful than the one
on the leeward side; in heavy winds there is always
a bubble of some length on the windward side of
The very luff of the sail nearly always has a
separation bubble on one side or the other which
is why the telltales shouldn't be installed too
close to the luff. When the sail is sheeted properly
and the boat is on its ideal course, the telltales
on both sides of the sail stream steadily aft.
In this case, the separation bubble at the luff
is then extremely small and alternates from one
side to the other. The airflow meets the luff
of the sail at an ideal angle and the flow is
smooth at both sides of the sail.
Leeward side bubble
If you are sailing too low, or if the sail is
sheeted in too hard, a separation bubble forms
on the leeward side. This is very harmul to the
performance and should be avoided at all times.
Independently of the luff separation bubble,
the airflow may get detached from the sail just
before the leech on the leeward side. The eddy
formed like this makes the leech telltale disappear
behind the sail. For the sake of clarity, the
size of the separation bubbles is exaggerated
in the drawing.
The graph on the left represents the forward force
or the driving force at different apparent wind angles
for a 40-footer in a light wind (AWS = 12 kn).
The driving force increases steeply until the apparent
wind angle is 21¡. Until then there is a separation
bubble on the windward side of the sail, which gradually
disappears. The windward telltale wobbles or streams
upwards and the luff of the main may backwind. The 22¡
apparent wind angle is optimal for this setting of the
sails; both telltales are streaming steadily aft. This
is when the driving force in relation to the heeling
force is at its maximum. When bearing off further the
driving force still increases, but less steeply than
earlier. The airflow starts to get detached more and
more on the leeward side of the sail, as shown by the
stalling of the leeward telltales.
In this computer simulation only the course the boat
sailed is altered while the sheeting of the sails remain
the same. In a real situation the sheeting of the sails
would be adjusted according to the course.