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A SIMPLE GUIDE TO SETTING UP AN IOM
BOAT - By Peter Sutton
The following is a simple guide to attaching a rig to
the boat and then making the necessary adjustments to
achieve a good basic set-up which will allow the boat to
sail reasonably well. It isn’t a comprehensive set of
instructions and although they have been researched from
several sources including Graham Bantock of SAILSetc,
Brad Gibson of BG Sails, Lester Gilbert’s Radio Sailing,
the TS3 Instruction Manual and Yacht Design Explained by
Steve Killing and Douglas Hunter, plus my own
experiences you should experiment, read further on the
subject and discuss the various issues with other
skippers in order to further your knowledge and refine
the way you set up and tune your boat for maximum
performance in your hands.
First – attaching the rig to the boat:
1. Keep the jib sail attached
to the mast whilst the sails are stored off the
boat, then provided you don’t slacken the backstay
or forestay you will preserve the settings you have
used for next time.
2. Fit the fin, complete with ballast, to the boat
and place the boat in a stand (be careful it doesn’t
blow over in the breeze) or lay it on the ground.
3. Take the rig from its box or bag holding the mast
and the jib boom to keep control of both sails so
that they don’t flap in the breeze. Allowing the
sails to flap uncontrollably could crease them
badly.
4. Place the mast heel in the mast tube making sure
it is properly located.
5. Locate the mast in the mast gate in the deck of
the boat where there is one and let it rest against
the ram.
6. Check that the headsail leech line and forestay
are not twisted and then attach the head sail swivel
to the deck eye.
7. Attach the backstay to the hook or eye at the
stern of the boat.
8. Attach the side stays to the appropriate deck
eyes- I suggest that the lower part of the bottle
screw is attached permanently to the deck eye and a
‘Nyloc’ self locking nut is used on the top part of
the locking screw to set the tension and position of
the bottle screw. This way once a setting is
achieved the rig can be detached from the boat,
leaving the locking nuts undisturbed, so the same
setting can be used again next time. It is essential
that the side stays are evenly tensioned so that the
mast is upright in the boat and not pulled or bent
to one side or the other; this can be verified by
ensuring the sails have the same amount of twist on
both tacks, or by checking that the main boom has
the same of amount of lift against the kicker/vang
on each tack when in the running or fully out
position. The boom will have more movement on the
side the mast is bent/tilted towards. This can be
rectified by loosening the bottle screw on that side
and/or tightening the other side. The side stays
should usually be looser in light airs and tighter
in strong winds.
9. Attach the sheets to the main and jib booms.
10. Tighten the downhaul on the jib sail and main
sail just enough to remove any wrinkles or bubbles
in the luff of each sail. For very light airs the
downhauls should be left slightly slack allowing the
sails to drop easily into their designed shape.
11. Adjust the mast ram to the correct position,
i.e. so that it supports the mast and pushes the
desired amount of fullness into the bottom of the
sail – you may need to come back to the ram later
when doing the fine tune and adjustment. Usually you
will want less ram in light airs in order to flatten
the mainsail.
12. When dismantling your boat at the end of the day
simply reverse this procedure and remember to take
all of the tension out of the rig by slackening off
both down hauls and most importantly the
kicker/vang. Failure to do this will have the sails
stored under tension with the strong likelihood that
they will become distorted and pulled out of shape.
Now that you’ve got the rig safely on the boat you need
to set the camber in the foot of each sail, the amount
each boom is out from the centre at the close hauled
position and the amount of twist in each sail.
1. The main boom should be
about 10mm (+ or- 5mm) from the centre of the
sheeting post at the close hauled position.
2. The end of the jib boom should be about 55mm away
from the mast at the close hauled position (+ or –
5mm).
3. The gap measured at the centre of the boom to the
foot of the sail should be about 20mm for both the
jib sail and main sail (+ or - 5mm).
4. Set the twist in the sails: Hold the main boom on
the centre line of the boat with the kicker/vang
lightly loaded as it would be with the wind blowing,
and adjust the kicker/vang so the gap between the
mainsail leech and the backstay at the mid batten
position measures between 35mm and 50mm. In flat
water and steady winds the lower measurement (less
twist) can be used (35mm). As the wind becomes more
gusty and/or the water rougher the amount of twist
should be increased. Try setting the gap at 40mm for
a start and see how the boat sails. If in doubt set
more rather than less twist but not so much that the
top of the sail is visibly twisting off so much that
air is spilled, although this may be a useful ploy
if the boat starts to become over powered.
5. Set the gap between the mid point of the jib sail
leech and the leech line to 20mm to 30mm, less in
flat water and steady breeze, more in choppy water
or gusty winds. It is important that when viewing
the sails from behind the boat the leeches of both
sails look even with a similar degree of twist.
6. Alternatively you can set twist in the following
way; stand behind the boat with the sails in the
close hauled position and a slight breeze in the
sails, and line up the backstay with the centre of
the mast. Set the kicker/vang so that you can just
see; the outside of the bottom sail batten, the end
of the middle batten and the inside of the top
batten. Then set the jib sail twist to match the
main when viewed from behind.
7. Yet another way of setting twist is to have the
top batten on the mainsail parallel to the main
boom, then set the jib to match.
8. Finally you can do it this way; set the twist in
the jib by setting the distance between the middle
of the jib sail leech and the leech line to 20mm to
30mm. Then set the main sail to match.
There are a few other things you should know about which
affect the way a boat sails and handles.
1. Bulb Cant. This is the
amount the nose of the lead ballast is tilted
upwards in order to minimise drag and it is
generally agreed that about 2 degrees is right. If
your boat designer recommends something different
then go with that. The amount of cant can be set by
inserting shims such as flattened crimps between the
bottom of the fin and the bulb.
2. Mast Rake. The position of the mast is usually
determined and fixed by the designer but the amount
the mast is raked back is also very important
because it is the primary factor governing the
‘balance’ of the boat and the amount of weather
helm/lee helm it displays. The more the mast is
raked back then the more the boat will tend to turn
up into the wind, this is called ‘weather helm’. If
the mast is not raked back far enough then the boat
will fall off or bear away from the wind, this is
called ‘lee helm’. A certain amount of weather helm
is usually considered desirable because it means
that when small amounts (no more than 5 degrees) of
corrective rudder are used it produces lift in the
same direction as the fin – to windward. Lee helm
means that corrective rudder increases the load on
the fin and consequently the leeway. Initially set
the mast rake by adjusting the forestay and backstay
without the side stays attached and without bending
the mast until the desired degree of rake is
achieved; as a rough guide you can apply the
following: Rig No.1, – 1degree to 2 degrees of rake;
Rig No.2, – 2 degrees of rake; Rig No.3, - 4 degrees
of rake. Finish rigging the boat, set it on a close
hauled course to windward and observe how it sails;
if she bears off, falling away from the wind, then
the mast needs to be raked further back. If she
turns strongly up into the wind and the sails flap
(luff) then the mast needs less rake, if she sails
steadily to windward without showing any signs of
weather or lee helm then the boat is ‘balanced’ and
will be easy to steer and sail, however she is
probably not travelling to windward at her optimum
speed. For this the boat needs to display a small
amount of weather helm, so the mast will need to be
raked back just a little more. You need to establish
mast rake for each of the 3 suits of sails allowed -
probably one mast position for each (possibly 2 for
the top rig) - once these positions are established
make a note of them, and it will help if you don’t
disturb the forestay and backstay bowsies when
de-rigging, so you can easily reproduce the settings
each time you sail. Minor adjustments to the balance
of the boat can be made to account for stronger or
lighter winds by changing the twist in the sails and
the relative sheeting angle of the main and jib (see
Graham Bantock below).
3. Mast pre-bend and mast bend. Masts usually have a
certain amount of permanent forward bend put in them
so that when the mast is pulled into the correct
shape to set the main sail by using the forestay,
backstay and side stays far greater tension is put
into the backstay and forestay than could be
achieved with a straight mast. High tension in the
forestay prevents it flapping in a breeze and allows
the boat to point higher (pointing high means the
boat will sail very close to the wind when on a beat
to windward). The bend in the mast rigged and fitted
to the boat should match the curve in the luff of
the main sail and allow the sail to set properly,
fall easily into its designed shape and be able to
change from tack to tack without having to ‘climb
over the mast’ unduly. The bend in the mast is
controlled by the forestay, backstay, side stays,
spreaders and the mast ram. The side stays will have
more effect on mast bend if the side stay anchor
points on the boat are set well aft of the mast and
tension here will be transferred to the forestay to
good effect. This also reduces the need for pre-bend
in the mast. The ram is used to control the bend in
the lower part of the mast and to push fullness into
the lower part of the mainsail. Follow the
directions of your sail maker as regards mast
pre-bend; some sail makers cut their sails for
straight masts, some require a small amount of pre-
bend and others specify a quite significant amount
of pre-bend. The part of the mast in which the bend
is set will also vary according to the sails used.
The No.1 rig will require more pre-bend than the
No.2 rig and the No.3 rig requires the least amount
of pre-bend. This is because the shorter masts of
the 2 and 3 rigs are inherently much stiffer than
the mast of the No.1 rig, so sufficient tension can
be achieved in the forestay and backstay with less
or even no pre-bend at all.
The end result of all this setting-up and tuning is a
boat that is well ‘balanced’ and sails the way that you
want it to. When sailing close hauled to windward a well
balanced boat will hold its course without the skipper
constantly having to apply large amounts of rudder to
correct it although most skippers seem to prefer a boat
that exhibits a certain amount of weather helm; i.e. the
boat will tend to turn itself slowly up into the wind.
Lee helm, a boat that falls off or away from the wind,
is generally to be avoided. Basic boat balancing is
usually done by raking the mast backward or forward for
each rig as required. See 2 above. However, sail setting
also affects the amount of lee helm/weather helm
experienced by your boat; for example letting the jib
out and bringing the main sail in will cause the boat to
turn into the wind. Sheeting the jib sail in and letting
the mainsail out will cause the boat to bear off more.
These tuning tweaks can be used if the boat is slightly
under or over sailed to correct it without changing the
whole rig or the amount of mast rake.
It is important that the ties securing the luff of the
mainsail to the mast are not too tight. A good way to
achieve this is to ensure that a 2mm drill bit can be
inserted between the luff of the sail and the mast when
the sail is pulled tight against the tie.
The sheeting radius is the distance from the boom pivot
point to the sheet fairlead. The sheeting radius of the
jib boom should be about 10% more than that of the main
boom. This is because at close hauled the jib is further
out than the main, thus to maintain the balance between
jib and main the jib should travel in and out more
slowly than the main. Most IOM’s have a main boom
sheeting radius of about 200mm and in this case the jib
boom sheeting radius would need to be about 220mm.
The jib boom pivot point should be in the region of 20%
- 25% back from the front of the jib boom. This helps to
achieve high forestay tension as the tension from the
back stay and side stays feeds into the jib. More
tension in the forestay also helps to maintain tension
in the leech of the jib sail preventing the jib boom
lifting too much in strong winds, although some people
maintain that a certain amount of lifting is desirable
as the wind gusts up.
Graham Bantock makes the following observations on IOM
boat tuning on Lester Gilberts excellent web site
www.onemetre.net
“For any given hull/rig combination
…… the degree of weather helm is dictated by three basic
factors:
with the minor factors:
He goes on to say:
‘There is a fairly small range of successful pairings of
twist and sheeting angle. The sheeting angle ranges are
covered by the range 8 to 15 degrees for the jib, and 2
to 8 degrees for the main. Keep the twist in the sails
so they look much the same from astern. Use mast
rake/mast position and relative sheet angle to tune the
balance. Use sail camber primarily as a throttle –
fairly flat in very light airs, fuller as wind speed
increases up to the point where you need to keep the
boat more upright for best speed and pointing when you
flatten sharply. Camber can also be used to adjust the
helm of the boat. Experiment.’
Sails can be flattened to some degree by using the
outhaul adjustment at the end of the boom. The mainsail
can also be flattened by increasing the tension in the
backstay. Look at the top seam in the mainsail as you
increase the tension in the backstay and you will see
the point at which it begins to flatten off. Easing the
pressure exerted by the ram on the lower part of the
mast will cause it to bend forward and allow the bottom
of the sail to flatten off slightly, but be careful not
to relax the pressure on the ram so much that it is no
longer controlling the mast so allowing it to bend
forward under pressure from the wind, because this will
change the sail settings.
SHEETING ANGLES
The following extract from Lester Gilbert’s web site is
interesting. These observations were made following wind
tunnel tests on an IOM boat.
“There are big changes in the ratio of drive to heel at
the lowest wind speed, while the ratio flattens out for
the highest wind speed. Nevertheless, the best ratio of
drive to heel is found with a freely sheeted main (8
degrees) and a freely sheeted jib (16 degrees) at low
and high wind speeds, but something interesting has
popped up for medium wind – a second peak in the drive
to heel ratio with the main at 2 degrees and the jib at
12 degrees. Isn’t that curious – these are my ‘normal’
sheeting angles.
What can we conclude? The drive to heel ratio is most
sensitive to sheeting angles at lower wind speed, and
this sensitivity decreases greatly as the wind picks up.
I guess this is good news, since I am not sure that the
drive to heel ratio matters much at lower wind speeds –
the boat isn’t heeled too much, and we want to maximise
drive, not optimise it or balance it against heeling.
So at lower wind speeds, sheet the main tightly, and
have the jib a little free to obtain maximum lift. As
wind picks up, keep the main tight and ease the jib just
a little more.
Remember – we’re just looking at lift and drag here.
These conclusions may not be that helpful if you need to
point, for example, or if you need to maintain drive and
momentum through chop, different sheeting will
undoubtedly be needed. And remember that these values
were obtained with ‘nominal’ twist in jib and main. In
fact for lower wind speeds, main and jib twist was quite
low to my eye, and as higher wind speeds were tested,
jib twist increased some, but main twist increase
dramatically.”
I highlighted the final paragraph because I think it is
important, as Lester says twist wasn’t properly set and
this was in a wind tunnel with no waves to drive the
boat through and shake the wind out of the sails. My own
fairly limited experience suggests that the main may
need to be more than 2 degrees out at close hauled,
especially if twist is moderate.
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