FAST NUMBERS v2
by Peter Galloway
Proper rig tune is essential for getting the most out of your sails.
Peter Galloway describes how you can set up your boat to get the
winning edge.
One of the finer but least understood aspects in getting the most
speed out of your boat has to do with the set-up and tuning of the
rig. Virtually as important as the set of your sails, rig tune has a
significant affect on the former as the shape your sails will assume
under any given set of conditions is a function of sheeting and the
rig tune. Since setting the sails correctly for any given set of
conditions is a topic covered in many of the excellent books on
sailing, it will not be discussed here. However, we can discuss setup and tuning as it relates to the Sonar. In this article I will attempt
to give the reader some insight as to knowledge amassed by the so
called “Fast Guys" in the class.
As in any competitive one-design class, you can question these
"Speed Demons" as to how they are setting up their rigs and get as
many different opinions. Some might choose fairly tight shrouds
while other like them very sloppy. How much rake and pre-bend?
Just who should you believe and what is the set-up that seems to
work for most, and under what circumstances? What follows are
the numbers and techniques based on observation, common logic,
and results, and appear to work best in most conditions. Let's start
from the beginning...
MEASUREMENTS
In order to accurately reproduce the set-up we are going to suggest
you will need to make some simple measurements.
Since the
amount of rake your mast will assume with a given headstay length
is a function of the position of the headstay fitting on the deck and
the position of the mast step, you are going to have to check these
first. Starting at the mast step, measure the distance from the aft
face of the mast where it meets the step casting to the corner where
the floor meets the cabin opening lip. This is rule limited to
between 717mm and 740mm. Set it close to the minimum. This
places the step relative to the deck mold, which the mast partners
are a part of (what we are really after). Next, measure the distance
from the forward edge of the mast partner opening to the center of
the headstay anchor point. The current class rules limits this
distance to between 2946mm and 2997mm. Get it as close as
possible to the max. It might be wise to be 2mm short to be safe.
BLOCKING TEST
Here's how you can be made a believer in mast blocking and its
affect on headstay tension.
Block your mast as far forward from
the neutral position as can be practically attained, then tension
your backstay firmly to simulate mainsheet tension. Mark the
backstay with a piece of tape where it meets the cleat so that you
can readjust to this position later. Measure the headstay tension
with a LOOS gauge.
Slacken the backstay and now block the mast aft as much as
possible. Retension the backstay to the previous position and
remeasure the headstay tension. It should have changed by as
much as 100 to 200 pounds! By the way, it is legal to adjust mast
blocks while racing. - Ed.
The next two measurements need to be done with the mast out of
the boat. Measure the length of the headstay from the turnbuckle
clevis pin to the bearing point of the upper T-ball fitting where it
meets the mast. Set this to 7925mm (26'-00"). Now check your
spreader sweepback angle. If you have a D.S. Yachts, Ryder or
Seidelman Sonar you will probably find this angle set to 15
degrees on each side. Readjust this angle to 10 degrees, if possible.
Having done all of the above you are now ready to rig the mast and
tune it as follows:
1) Step the mast and attach the head-stay and backstay only. Do
not insert any mast blocks at this time.
2) Remove the slack from the backstay until the headstay just
becomes taught (no mast bend). Place a mark on the deck abeam of
the centerline formed into each side of the mast. This is your
reference point in the relaxed position.
3) Now pre-bend your mast at the deck with mast blocks on the aft
side until you have moved the mast 1" forward of the mark. Make
a new mark and erase the old. This is the new "Neutral" position.
4) Attach the upper shrouds and tighten them to 200-230 pounds
using a 'LOOS-Model A' tension gauge (cable size is 5/32") or
equivalent. Take care to center the mast in the boat and keep it
straight by checking the distance from the masthead to each rail.
Now that you have the upper shrouds attached, you can remove the
backstay and use it for measuring as it will afford the least stretch.
Re-attach the backstay.
5) Attach the lower shrouds and tighten until the slack has just
been removed. There should be no tension and you should be able
to move the shrouds about 4-5" from side to side about 5 feet
above the deck.
There, you're done! This is the neutral position for wind strengths
of 0-12 knots. Adjustments will be made from and relative to this
position. Here are the adjustments:
Wind Speed
(knots)
Sea Conditions
Adjustment
Mast Position
(+)= fwd of mark/(-) aft
0-12
Smooth
Neutral
(Medium outhaul on mainsail)
0"
0-6
Choppy
Add 1” blocking behind the mast
(Loose outhaul on mainsail)
+1"
7-12
Choppy
Add 1/2” blocking behind the mast
(Medium outhaul on mainsail)
+1/2"
13-15
Choppy
Neutral
(Medium outhaul on mainsail)
0"
16-UP
All conditions
Add 1” blocking in front of the mast
(Max outhaul on mainsail)
-1”
BLOCKING
Mast blocking has two profound effects. First, it has a significant
affect on the headstay tension for any given mainsheet tension. The
more mast blocking behind the mast the less headstay tension you
will have and the more it will sag. (If you don't believe it, do the
test described in the box) This translates into a deeper, more
powerful jib - just what you want in choppy conditions with light
air. Obviously, the opposite is true. The more blocking in front, the
more headstay tension for a flatter jib. Just what you want in strong
winds to keep from overpowering the boat.
The second is the
relative draft position of the lower part of the main. A straight mast
in light conditions will tend to add draft forward down low. Prebend removes this forward draft and decreases the depth of the sail.
This also decreases any backwinding you may have. In heavy air
conditions where you will be blocking the mast aft for more
headstay tension, this draft movement is remedied by increasing
tension on the outhaul thereby flattening the sail further.
SHROUD TENSION
The Sonar has a relatively large main, considerably larger than that
of a Soling for example. It's slender mast section means the mast
tends to 'tip-off' above the hounds in breezy conditions. Just sight
up your mast some day when sailing to windward in a stiff breeze
and you'll see what I mean. This cannot be countered by adding
upper shroud tension as it merely tends to bend the mast more due
to the swept back shrouds turning the mainsail inside-out instead.
Interestingly enough, the solution is less lower shroud tension!
The key here is to sight up the mast while sheeted in and sailing to
windward. Then adjust the windward lower shroud so as to achieve
about 10-15mm of sag to leeward at the spreaders. This will force
the mast back into column above the hounds. You will find that
there will be considerable slop in the leeward shrouds when sailing
to windward because, as the mast bends, the distance from the
chain plates to the shroud anchor points on the mast become
shorter and all of the tension is taken by the windward shrouds. In
his book "Sailing Smart" Melges calls this "slopping the rig." In
fact, blocking the mast aft in stronger winds has the added benefit
of slackening the lowers, further helping reduce “tip-off”.
SPREADERS
Because of the flexibility of the mast, the Sonar virtually never
needs backstay tension. The mainsheet provides all the mast bend
you need in breezy conditions—and then some! More mast bend
forces the center of the mast forward tightening the lowers and
causing the uppers to become more slack. This results in “tip-off”
as we discussed earlier. By reducing the spreader angle, upper
shroud tension supports the center of the mast better thereby
controlling overbend due to sheet tension.
BACKSTAY
Most of the time when sailing upwind keep the backstay firm. This
will control the headstay sag as well as the bounce. In breezy
conditions you will need to added increased tension, particularly
when combined with flat water. In these conditions you can use the
backstay like a POWER CONTROL. Tensioning it slightly
reduces the power in the mainsail up high by opening the leech
thereby reducing heel and weather helm. Use it to advantage by
tensioning it just as the puff hits then slacken it after you regain
control.
Some of these numbers may vary based on the brand of sails you
use, the type of prevalent conditions you sail in, and personal
preference. However, the theories hold regardless of these
variables so you should benefit from them.
Try putting these "FAST NUMBERS" to the test this summer and
move into the fast lane!