Marty Jones asked me about an IOM rig. I thought it might be useful to
explain some of the basics.
In the following discussion, focus is given to each element of the rig in
turn, and its relationship to the other rig elements explained. This does
lead to fair amount of (intentional) repetition.
Besides holding the sails up, the mast does three important things. The
first is that it handles the rig tensions, in particular taking the backstay and
shroud tensions and transferring them to the jibstay. The other two things
it does is bend: fore and aft, and sideways.
Fore and aft mast bend changes the shape of the mainsail in two ways.
One is that the mast either pushes fullness into the body of the mainsail when
it straightens, or pulls fullness out of the mainsail when it bends more.
Two is that the head of the mainsail either twists off as the mast bends more,
or twists off less as the mast is straightened.
Sideways mast bend also brings two main changes. One is that it changes
the slot, the gap between the jib and the mainsail. If the mid-mast bends
to leeward, the slot closes, and if it bends to windward, the slot opens.
Two is that sideways mast bend changes the effective twist of the
mid-main. If the mast bends to leeward, the twist in the middle of the
mainsail reduces, and vice versa if the mast bends to windward.
Some pre-bend is usual if the mast is a minimum-section
mast. If you have a stiff mast, then pre-bend may not be needed.
Remember that mast pre-bend is "forward"; when you tighten the
backstay you straighten the mast and pull the pre-bend out. When you tighten
the backstay further you then set the
mast into its "normal" bend.
If you don't pre-bend your mast, you'll not be able to crank in much backstay
tension, and that'll generally leave you with too little jibstay tension. I say generally. There are two exceptions. One is, if you are sailing in very light winds, you probably don't want very much jibstay tension, because you will want some jibstay sag to match your jib luff curve. Two is, if you have your shroud base
well aft, then shroud tension will feed into your jibstay, possibly enough so you don't need very much backstay
tension at all. If you have your shroud base well aft of the mast step, then you may not need a pre-bent mast.
An 11.1 mm "A" rig mast with 0.60 mm wall thickness, without
pre-bend, will allow no more than about 0.75 kg of backstay tension before it
bends excessively. With 40 mm pre-bend, you can run up to about 3 kg of
The backstay is tensioned so that, broadly, the mast bend matches the mainsail luff curve. Fine
adjustment of backstay tension (along with spreader "V" and the mast
ram) pushes a little body into the mainsail, or takes some
Having set the broadly correct amount of backstay tension to match the mast
bend to the mainsail luff curve, and having set the mast ram and the spreaders
to be consistent with this mast bend, further backstay adjustment mainly affects
bend in the upper part of the mast, around and above the hounds.
At the top of the wind range for the rig, the backstay can be tensioned further to
increase twist at the head of the mainsail and to flatten the head of the mainsail.
The head will flatten only if the spreaders and mast ram continue to control
their own parts of the mast, the mid- and lower mast. Otherwise increasing
backstay tension will change the whole of the mast bend in probably undesirable
"A" rig with light winds, the backstay can be eased a little to push more body
into the middle of the mainsail for more drive, and remember to ease the kicking
strap a little to compensate for the reduction in twist at the head.
The second purpose of backstay tension is to obtain adequate jibstay tension.
The more tension in your backstay, other things being equal, the more tension in
your jibstay. The major way to change jibstay tension at the pondside and
thus control jibstay sag is to adjust the backstay tension.
Typical values of backstay tension are around 2 kg for a pre-bent mast.
Jibstay tension results from backstay tension and shroud tension (if the
shroud base is aft of the mast step). The higher the backstay tension, the higher the jibstay tension.
In fact, because of the
characteristics of the fractional rig, every gram of backstay tension yields around
1.6 grams of jibstay tension.
Jibstay tension regulates jibstay sag. The tension of the jibstay is
set to match the jib luff allowance, the curve cut in to the jib luff, to the
expected jibstay sag. If there is very little jib luff allowance cut into
your jib, you need a higher jibstay tension to keep the jibstay from sagging as
the wind builds up. If you have considerable jib luff allowance, you need
less jibstay tension. The downside of having much jib luff allowance is
that the jibstay needs to be quite slack in very light airs, otherwise the jib
does not develop sufficient draft.
Jibstay sag does two things. First, it pushes fullness into the
jib. Second, it reduces the twist of the jib. Jibstay sag is
probably the hardest thing to control on an IOM rig.
Jibstay tension also feeds in to the topping lift tension, according to the
position of the jib pivot. If the jib pivot offset is closer to the jib luff, less tension feeds in to the topping lift, and vice versa as the pivot
Typical values of jibstay tension are around 4 kg, for a jib with very little
As an aside, the length of your jibstay plus the jib boom pivot length determines your mast rake, if your mast is stepped so that its
heel cannot move and the boat is balanced by raking the mast forward or aft as needed.
The shrouds support the top of the mast. It is my opinion that the
shrouds do not do much to regulate sideways mast bend. I think that the
top of the mast will sag or bend to leeward when the wind gets up, more or less
regardless of the shroud attachment point at the hounds. Much more
significant is the location and length of the spreaders in managing sideways
Shroud tension does three things. The most obvious is that the shrouds
keep the mast upright, and the shrouds need to be tightened as the wind picks up
in order to hold the mast as upright as possible. Less obvious, but more important, is that shroud tension does two other
things. One, it feeds in to the jibstay tension.
Two, shroud tension is what allows the spreaders to be effective.
Shroud tension can vary from about 3 kg to over 15 kg in extreme cases.
It is likely that, when well-heeled, a leeward shroud tensioned to only 3 kg on
the pond-side will become quite slack on the water.
feeds in to the jibstay in proportion to the shroud base. The further aft
the offset of the deck attachment of the shrouds from the mast step, the more
tension is put into the jibstay. Conversely, if the shroud base is abeam
the mast step, no shroud tension finds its way into the jibstay.
It is possible that, if you have your shroud base well aft of the mast step,
you do not need to pre-bend your mast. An aft shroud base means that
backstay tension is less significant in affecting jibstay tension than shroud
tension, and you can use your backstay purely to manage fore-and-aft mast
bend. In this case, your control of jibstay tension and jibstay sag
depends much more upon adjustments to your shroud tension.
The amount of spreader "V" you need simply depends upon your shroud
base offset. The further aft your shroud base, the greater the spreader
"V" angle required.
Between 0% and 30% of
the shroud tension is transferred into the jibstay depending as the shroud base
varies from 0 to 50 mm aft of the mast step in "A" rig.
Broadly, it is my opinion that the shrouds will keep the mast relatively
upright pretty much regardless of where they are attached at the hounds. I
say "relatively", because it is also my
opinion that the mast head will sag to leeward when the wind gets up, pretty
much regardless of shroud tension, shroud attachment, or spreader geometry.
hounds position will have subtle effects upon sideways mast bend. Hounds
which are lower down the mast will allow the mast head to bend to leeward more
when the wind picks up, and vice versa for hounds located higher up the mast.
Perhaps more important is that hounds position controls top mast bend, while the
spreaders control mid-mast bend. Lower hounds allow more of the top part
of the mast to bend to leeward, while higher hounds allow less of the top part
of the mast to bend.
|Backstay and shroud tensions result mainly in mast compression forces.
You can pretty much add up the backstay tension, and the two shrouds tensions
taken separately, to obtain the mast compression. If you use a lot of
tension, you could find 35 kg of mast compression at your mast step. Make
sure you have a good mast box!
Spreaders have three major characteristics. One is their position on
the mast -- located towards the top of the mast, or towards the deck. Two
is their length. And three is their angle or "V".
Angled-back spreaders are needed simply to allow a shroud base to be quite aft of the
mast. If the shroud base is more or less abeam of the mast step, then you won't have angled back spreaders, you'll have "in
The spreaders do two things. One is they provide subtle control over fore-and-aft mast bend, and two, subtle control over
sideways mast bend.
As you change the angle (the "V") of the spreaders, you are using shroud tension to pull the middle of the mast
aft and thus straighten it, or you are allowing the middle of the mast to move
forward and thus bend more. You are fine-tuning the mast bend, having set it grossly with the backstay.
Keep in mind that the mast ram will also affect mast bend lower down, and your
spreader "V" will have to be consistent with your mast ram setting.
As you change the length of the spreaders, you are using shroud tension to pull or push the middle of the mast to windward or leeward when beating and heeled, subtly opening or closing the slot.
Shorter spreaders, under 50% of beam, have the effect of pulling the mast to
windward and opening the slot, while longer spreaders have the reverse effect,
pushing the mast to leeward and closing the slot. To see how this works,
remember that the windward shroud tightens as the boat heels under wind
pressure, and the leeward shroud slackens. The windward spreader then gets
to work, pushing or pulling, while the leeward spreader loses its effectiveness
and is much more of a passenger.
Keep in mind that, as
the boat heels when the wind gets up, the top of the mast will sag to leeward
regardless, and so it makes little sense to use very short spreaders in the hope
of keeping the mast relatively straight. In my view the mid-mast must be
allowed to sag to leeward a little, so that the whole of the mast maintains a
consistent bend, and the ideal spreader length is therefore
around 55% or 60% of beam.
The effect of the spreaders depends on shroud tension. Their control of the middle of the mast works off shroud tension, and if you don't have much shroud tension then your spreader effects will either be less, or you
will have to exaggerate your spreaders (length, "V") in order to get the effects you need.
The position of the spreaders, higher or lower on the mast, is not critical in my opinion.
Nevertheless, if your boat has a high mast ram and effective mast partners, the
spreaders can be about 60% up the mast. If your boat does not have much of
a mast ram, then your spreaders would do their best work about 45% to 50% (halfway) up
The effect of the mast ram is to control
fore-and-aft mast bend in the lower part of the
mast, complementing the spreaders in their control of mast bend in the middle
part of the mast. To be effective, a mast ram needs a secure mast step,
and needs to be as high above the mast step as possible to ensure sufficient
leverage and control.
Some hull designs also have mast partners at or near the mast ram
position. These partners give sideways support to the mast, and are very
useful, but their effect is quite independent of and different from the mast
Mast bend in the lower part of the mast transfers to the gooseneck axis, and
as the mast ram is wound in or out, changes will have to be made to the kicking
The position of the jib pivot, the amount of the jib pivot offset, has two
major effects. One is it regulates the amount of tension in the topping
lift, and two is it exposes more or less of the jib to the wind when on the
run. There is some discussion that it has a third, minor effect, in that
it displaces the jib luff from the boat centreline and thereby reduces the drive
of the jib when close-hauled. It is my opinion that in fact the
displacement of the jib luff to weather is very slightly beneficial, not
detrimental, but that this is not an important consideration in any case.
A rule of thumb is that the pivot offset should be about 20% to 25% of the
jib foot. If you have a shroud base set well aft of the mast step
and have higher shroud tension, the offset could reduce to about 15% since
otherwise excessive tension may remain in the topping lift when the wind gets
The length of the pivot, if adjustable, affects mast rake and hence boat balance.
The topping lift has one purpose, to regulate the amount of twist in the
jib. The more tension the topping lift has, the longer it will keep its
setting. At some point, when the wind reaches the top of the rig's range,
there is insufficient tension in the topping lift, and the jib leech opens,
increasing relative jib twist.
It is interesting to recall that, as the wind rises, jibstay sag increases,
decreasing relative jib twist. It is my opinion that there is therefore
little need to have excessive topping lift tension, since you do want the jib
leech to open (ie the topping lift to release) in order to counteract the
effects of jibstay sag.
Typically, a topping lift will have about 0.75 kg static tension.
||The kicking strap regulates the amount of twist in the mainsail. When
attached to a mast fitting that includes the gooseneck, it is affected by mast
bend, and will need adjusting as mast bend is adjusted.