masts and sails for iacc 120

Dear builders of IACC 120 yachts,

After reviewing the many informative and excellent threads and plans provided, I couldn’t find some of the measurements for the masts and sails.

I was wondering what type of mast are folks using? grooved? wall thickness? diameter? tapered? stiffness? high modulus? how many spreaders and shrouds? how much mast bend is set by the shrouds?

And about the sails: type of attachement for the mainsail? how much luff curve in the mainsail? how much draft at the top of the mainsail and how much at the bottom? How much draft in the jib?



Hi Todd,

Many questions at once, I try to reply to some with the following :

You need first to read the rules about sail surface dimensions.

Please find in attachment some typical sail plans for AC120.

The mast generally is a carbon tube high modulus of 12mm external diameter.

The maximum mast lenght is determined by the main sail luff of 1750mm, and the distance of boom from the deck datum line.
Mast can be also shorter, while keeping the same surfaces. This option has a lower the CE height.

One manufacturer in Italy sell also growed masts for the AC120 see TD Model :


Thanx Claudio.

I am trying to find a similar mast or carbon tubing that is available in the U.S. I assume that some of the builders do not use a grooved mast? There is some benefits to both systems, but, like a windsurfing sail, a sail that rotates about the mast can have better airflow at the luff. I assume that rotating masts (so that a grooved mast/sail can rotate to the outside, like a Hobie Cat) may not be legal, or may not have been tried.

I did find a few sail-making forums and IACC 120 posts with some descriptions of draft and luff curve. I will have to study them a little more.

I am enjoying reading your forums and posts. Thanx for all the work you have done.


Hi Todd,

I build my own grooved masts for the IACC 120.
The first masts I built were two 6mm carbon tubes bonded together back to back… I also am allowing the rigs to rotate and I have a variable third spreader to control the upper section of the rig lateral bend to in turn adjust the mainsail twist automatically.
Since then moved onto building high and ultra high modulus rigs which for me where the gains in performance lie.
I will give you a quick run down:
The basic process for building my rigs is to use a piece of alloy L section, 2 metres long, with a wall thickness of 5mm.
The L section ensures straightness.

I routered a radius down each side of one web so it was uniform along the length. This creates the leading edge of the mast. Release coat the alloy and I then use strips of prepreg uni to create the mast by laying them over the form to create a U section.
This is then cured under vacuum to 120 Celsius.

Once cured it is released and a 4mm OD carbon tube is bonded into the open end of the U to close the mast section.
I then carefully use a diamond wheel to open up the exposed wall of the 4mm tube to create the luff track…
Now you have a mast!

Weight of a 2000mm section is about 90g
I have some developmental rigs in the wings but zero time to put them on the water for testing…

Just some food for thought:
The mast on these yachts needs to be as stiff as you can go fore and aft… This will allow higher back stay tensions to gain decent fore stay tension for fast and high upwind sailing…

All of the above may seem a lot of work, or not (I do not know your skill set!)
But, being totally custom it is far better than a round tube!

Cheers, Jim

Hello Jim,

Thanx for the post. I looked at your build log to get a better perspective. Very clever method for making a mast, similar to d-tubes used for freeflight airplanes. The idea spurred many questions. I tried to make the questions yes/no to save you some time: I assume that your wall thickness on the mast is about 1 mm or so? The high modulus carbon helps minimize the bending in the lateral plane? The bond with the tube hasn’t failed? The ID of the tube is about 2.5 mm to allow for the cord in the sail? Have you made the spreaders so that the mast can rotate?

I also tried to find the cloth type you used on your hull, but could not find a match for RC-300. Is this a twill or satin type weave?

I have also been thinking about how to make a lightweight jib boom that pivots on an axle that can hold against the leech tension, but that is a different topic.



Hi Todd,
The wall thickness is much less than 1mm… The first mast is made from 2plys of 150g (off axis) and 1 ply of 300g which give a nominal thickness of 0.6mm
The latest experiments are going even lighter… But I have serious doubts about the longevity and durability.
My biggest concern is: One knock from a competitor on the water could mean failure.
The ultra high modulus is super brittle and even bending the cloth uncured can break the fibres. It’s very tricky stuff to use. I’d defiantly recommend the high modulus…
There have been no problems with the mast track bond failing but the adhesives I have access too are very, very good.

I do not use a luff cord as this is too heavy, but individual sliders or lugs. Very simple to make too, and easy to put on the main. They consist of a 5mm length of alloy rod with a sticky back strip wrapped around it and tabbed onto the sail.
Initially I had heaps of them, but after experimentation I found that between 5-8 works really well. I double up at the head of the main and also at the end of the main diagonal batten that supports the leech & head as these are very high load areas.

The spreaders are simply bonded to the mast again with a high quality epoxy. The process to make the rig rotate is a concept borrowed from the full sized yachts. Your not going to achieve huge rotation angles, but some it better than none.
I am revealing some more secrets… The whole process simply relies on the relationship between the chainplate, spreader tips and the hounds. It is very hard to explain, but the idea in a vertical plane that the chain plate position needs to be aft of the hounds.( you have no rake on the spreaders)
When you are sailing the windward side is loaded and this makes the whole shroud want to take the straightest line which pulls the loaded spreader tips aft. The mast rotates to follow! I think I had posted an upmast vid which shows this effect.

The RC 300 was a plain weave and was just some cloth we had in the shed at the time, but in all honesty I would not use that again as there we quite a few pinholes. Your far better off using 2x RC 200 or 3x RC 100… As sealing pinholes is a nightmare.

I too have revised the jib boom on my yacht and made an ultra stiff version to maintain higher head stay tension!

I hope this helps!

dear Jim,

I see in the video how the mast twists…clever. It seems though, to my eye, that the mast is not twisting enough to create a nice airfoil entry to the sail, especially since your mast is narrow and long in cross section. I read the rules; I did not see any limitations on rotating masts…yet. US 1 meter does have a rule against rotating masts, and I would assume some of the other classes do too. Windsurfing sails have either “Rotating Asymmetric Foil” tensioned battens, or ‘camber inducers’, but scaled versions of these may need too much force to ‘pop’ to the other side during tacks in low wind. Also may be illegal in model yachting.

Do your sails have a jack line?

Still cannot find any info on your cloth type names…can you translate to gm/sqm, tow used, and weave?

Is your high modulus prepreg a higher modulus fiber than T700? Also find diffiulty in finding higher modulus uni carbon fabrics, although r&G in Germany has some with M 40 J fiber.

Thanx, and happy new year.


Hi Todd,

The rotation is limited as I said earlier… Not by the shrouds, but by the 230mm masthead crane that you need to clear the head of the main!
That is the limiting factor and I am yet to imagine a lightweight rotating solution that will perform well under the stress up there.
I am happy with the rotation I am getting as you can sail very high so the angles upwind are much less than a IOM for example.

The cloth you seek should be on the R & G site…
I think it is listed under the woven fabrics… on there you have the bonus of being able to order the cloth with a 45 deg bias. I would advise doing so as laying up these hulls is nice and easy with the fabric cut in that manner.
Either plain or Twill weave works fine. This is signified by either a ‘P’ or ‘T’ after the fibre type and weight…
“RC 200 T” translates to: “woven carbon 200gsm twill”

I can confirm that “M 40 J” is good quality High Modulus carbon. (M 46 J is Ultra high)
If you can, buy it in 100gsm or 120gsm. This will allow you to orientate the fibre direction as you need without adding too much weight.

Up until now I have not needed a jack line… but adding one would not hurt. I do have a 4:1 purchase on the Cunningham, so perhaps I should be considering it also!

Happy New Year to you also!


Thanx Jim!

Have you tried a rig with similar shrouds to a dinghy, where the spreaders are raked back, and the forestay is attached below the shroud hounds to develop bend without a backstay? A similar rearward position of the chainplate could also encourage twist, without the crane resistance. This would limit the boom angle downwind a little.

Did you mount the mast with a rod and tube at the base, or is it attached firmly along its long axis? Deck mounted or through the deck to a thwart?


Hi Todd,
I personally have not used swept back spreaders… I do recall someone else had them, but you could never achive enough tension in the forstay without the back stay fitted, swept spreaders or not.
If the mast is nice and stiff fore and aft, there is no additional benefit from raking the spreaders aft. You will just have longer spreaders, that weigh more. have more windage and prevent the boom from being eased all the way out.
My mast step is a closely guarded secret, but the rig is stepped inside the hull and the mast rotates all the way to the butt. have a The partners (deck collar) limit the rotation to 5 or so degrees at deck level. The rest is from the mast twisting itself.

New Year Greetings from holidays down under :snorkel:

I liked the profile mast until I did deflection test:

200 cm x 12 mm round mast, (weight 105 grams) 1,250 grams weight @ 50% of length = 30 mm deflection 200 cm profile mast (weight 115 grams) 1,250 grams weight @ 50% of length = 30 mm deflection, sideways deflection = 95 mm :scared:

Hence I ditched the profile groovy mast in preference for round

Having not experienced rotating mast can’t really comment, but thinking having round with mast loops virtually does the same thing as having rotating groovy.

I know Jim uses pretty high shroud tension, over time I had been increasing my tension to the point of being concerned about pulling the chain plates out, but rig control improved dramatically, hence on my next build I shall be increasing reinforcement around this area.

When I stiffened the rig up with higher back-stay tension even using aero alloy I found distortion of the crane was pretty bad until I sleeved it with carbon tube, another area for further improvement on my next build which is fine for top 2/3 of mast control, but I could not live without mast ram for bottom 1/3 of mast control.

Agree with Jim that swept back spreaders are waste of time as you lose too much boom especially when running.

Cheers Alan

Hi Alan,
I hope the weather is treating you well down there!

Nice to have another side to the story with pics and Alan has surpassed everyone in the build department!

It is true, I have very high tension especially in the first diagonals. 6.5 kg is now my base setting for wind of 12knts and more if the wind is fresher than that. With the boat heeled to 90 deg the static load doubles.
Which leads into the discussion of how to support the mast which is as important as the basic tube design.

You will notice from looking at the videos on You Tube, that there are several differing concepts in how people have chosen to lace up their rigs.
Mine is conventional using 2 spreaders with diagonals to provide sufficient lateral support.
Others have chosen to just have shrouds and do not mind if the rig sags to leeward.
This encourages a better sail shape, but in my opinion is a bit of a quick fix and is a setup I would use for light airs only.
There is quite a bit of adjustment that can be done in this area, the relationship of the mast tune and how it affects the mainsail shape is a key area to test and learn from.
Ultimately the decision is yours, but knowing how you want to set up the rig will assist in placing the chainplates.

Cheers, Jim

Thanx Alan. I saw the diagrams you drew in a different post for the mast ram.

Jim, if you have two spreaders, I assume you have three shrouds coming down to the deck? I saw in you thread of building your boat some carbon posts coming up from the deck to attach to the shrouds…not sure how this works…

Do you put bend in the mast, and then create a luff curve to fit the mast bend and desired draft on the sail?



Hi Todd,

The rigging is split into the Cap shrouds,D2’s and D1’s. All are adjustable individually and the upper are also adjustable in tandem.

The carbon uprights you described are actually anodised alloy turnbuckles for the M class.

As for the luff curve, I put almost max back stay on and take measurements at set intervals down the rig to a straight edge. This is the curve I use as you can later almost totally flatten the main for high wind sailing. You have a little more back stay in reserve for the 15 - 18knt days and that allows the top third of the main to be fully depowered upwind, and you have your hands full downwind! Makes for some spectacular sailing…