6
J/105 Tuning Guide
Solutions for today’s sailors
marks on either side and adjust the Caps
until they are equal.
Step Five: Tensioning the Shrouds
Tighten the D2s and D1s to hand tight.
Add 8 full turns to the Caps. Then tighten
the D2s 6 turns. This should be done
incrementally (2-3 turns at a time per side)
sighting the mast to be sure that the mast
is in column. Add turns to one side and
remove turns from the other to bring the
mast in column. This added tension that
you have applied has probably made the
D1s slack, so double check to make sure
that they are hand tight.
Step Six: Check Shroud Tensions with
Loos Gauge
Put marks on the shrouds 2m (6’ 6 3/4”)
up from the deck. Put the top post of
the gauge on this mark when measuring
tension. Be sure to have the backstay
released when measuring tension with
the Loos Gauge. For the base setting for
10-14 knots TWS the shrouds should read
the following:
All Purpose Sails
Heavy Air Sails
Caps:
35
55
D2s:
10
25
D1s: 0
(10 cm play) 10 (hand tight)
You should double check to be sure that
the mast is still in column at the dock
when the rig is at full tension. At the base
setting, the mast should sit with virtually
no pre-bend up to the hounds.
You are now at the “base” setting and ready
to go racing. From here follow the Quick
Tuning Guide to optimize your J/105’s
performance for various wind conditions.
Advanced Tuning – Beyond the Numbers
The North Sails J/105 Tuning Guide is
very refined, and has been used to the
letter to win many events. However,
understanding the fundamental concepts
of how tuning affects the rig and sails is
important to be able to check that the
settings are right for your boat, as well as
customizing your own tuning numbers for
your sailing style. Keeping good records
is the key. Keep a Wet Notes pad on your
boat and record your rig settings, wind
speed, sea state, and your speed and
pointing relative to other boats. This will
help you to see what is working best for
you and fine tune your own numbers.
Changes to shroud tension affect the
rig in two ways: 1) headstay tension & 2)
mast bend. Tighter Cap shrouds generate
more headstay tension by pulling back
against the headstay. A good guide for
Cap (upper) shroud tension is that the
leeward Cap shroud should just go slack
in all but the heaviest of wind conditions.
Tighter D1 (lower) and D2 (intermediate)
shrouds generate more headstay tension
by reducing mast bend and compression.
The important factor of the D1 anD2
adjustment is the relationship to the
amount of backstay used in each wind
condition, which affects mainsail shape.
Tight D1s and D2s will make the mast too
straight (main too full and draft forward)
when no backstay is used, and conversely
loose D1s and D2s will allow the mast
to bend too much (main too flat or even
inverted) when a lot of backstay is used.
The final D1 and D2 rig tension will be
dictated by mainsail shape.
The light air performance of the J/105 can
be dramatically improved with leeward
mast sag in under 12 knots. This is side-to-
side sag from the partners to the hounds.
This can only be seen under sail and will not
happen at the dock. The settings from the
Quick Tuning Guide should generate this
sag, but all masts are a little different, so
you may need to adjust your intermediates
and lowers to achieve it. Loosen the
intermediates and lowers so that there is
2.5cm
(1”) of leeward sag halfway to the