8
J/105 Tuning Guide
Solutions for today’s sailors
Overall Power – The Backstay
The backstay is the most important sail
control for setting the overall power of
the sail plan. The backstay is the universal
control to modify the shape of the main
primarily, but also the jib. The North Class
Inventory has been carefully designed
so that backstay adjustment changes
the shape of the main and jib in unison.
As backstay is applied the mast bends,
flattening the main. It also pulls aft against
the headstay, reducing headstay sag,
which flattens the jib. By flattening the
sails the boat’s power is reduced as the
boat becomes overpowered and heels too
much. Flatter sails can be trimmed harder
than full sails so when it is windy it is better
to set the sails up flatter and sail with less
twist. This will allow you to point higher.
As stated in the tuning section of this
guide, having the rig set up properly for
the wind conditions will allow you to get
the most benefit of adjusting the main
and jib together with the backstay, but it
is also the control that allows you to set
up the sails as best you can when the
wind changes and you are no longer
tuned correctly.
With a loose rig the main will flatten out
faster than the jib. This is because the
loose D1s and D2s will allow the mast
to bend, which will flatten the main.
However, since the mast is bending, the
main will get to a point where it over
bends before you can bottom out the
backstay. By not being able to bottom
out the backstay, the jib will still be a bit
fuller than is ideal at this point since 1) you
can’t pull on the full range of backstay and
2) the mast will compress more so the
backstay tension will not transfer as well
to the headstay.
Conversely, with a tight rig, the main will
stay fuller when the backstay is eased,
while the overall rig tension will keep the
headstay tighter and thus keep the jib a
bit flatter. It is important to understand
this relationship between how the
backstay and rig tuning affects sail shape
when you may not be tuned correctly. If
your rig is too loose, use more backstay to
set the main up flatter, knowing that your
jib will be fuller than ideal. If your rig is too
tight, use less backstay to set the main up
fuller, knowing that your jib will be flatter
than ideal. In general, it is better to be set
up with a flat main and a full jib so it is
better to err on the loose side of the rig
tuning matrix if you expect the wind to
change. It’s much easier to depower the
J/105 than it is to power it up so
always
tune for the lulls!
As the mast bends it has an immediate
effect on the luff and leech tension,
so other controls must be adjusted in
conjunction with the backstay. When
you pull the backstay on, the mainsail
will twist more so more sheet will have
to pulled on to reset the twist. More
backstay will also loosen the luff, so more
cunningham will be needed. Don’t forget
to ease the sheet and the cunningham
after you ease the backstay.
Other Mainsail Controls
Cunningham
he cunningham controls the luff tension
of the mainsail. Luff tension controls the
draft position of the mainsail. As backstay
tension is applied, the mast compresses
and the main appears to fall down a little
and the draft will move aft. Pull on the
cunningham to move the draft forward
to the desired position. As well as moving
the draft, luff tension will also make the
main somewhat flatter. In light air, you
should see slight wrinkles coming from
the sail slides. In moderate air the luff
should be smooth. In heavy air, the luff
should be tensioned past being smooth to
set the draft position and flatten the sail.
Vang
With the non-overlapping jib, in order for
the mainsail to generate enough power
in light air, a certain amount of depth has
Fig. 9: Light Air Upwind –
Traveler is high, boom is slightly
above centerline. Crew is low and
forward. The boat is balanced
nicely with a good heel angle.
Fig. 10: Nice heavy air upwind
form here. The traveler car is
centered and speed is being
controlled with the mainsheet
fine tune.
Fig. 9
Fig. 10