AC-120 Rating & Rules Guide - International

Dear All,

The AC 120 class has grown significantly since its inception in Italy and France and the class has been blessed with the privilege of having some brilliant RC boat designers such as Claudio Diolaiti & Renato Chiesa that have greatly contributed to the development of this new modern class of RC sailing yachts around the world producing over 30 plans (completely free of charge) of different IACC generation boats that dominated the Americas Cup mono-hull era from 1992 until 2007.

Like anything new, it takes time for a new class to find it’s legs before it is up and running, and recent Italian AC 120 rule changes was classic example of how not to improve class rules.

Without going into details, the Italian AC-120 rules are based only on measures & weights e.g LOA max 120 cm, LWL of 103 cms, Keel/Bulb max 3 kgs, Max sail area of 80 dm2 with minimum total displacement of 4.5 kgs which is fine if all people are sailing single boat designs … but they’re not! And this is what will continue to cause problems there.

For example: If we take ITA-25 Moro di Venezia with beam of 290 mm and water plan of 14,32 dmq and sit it beside USA-98 BMW Oracle that has 165 mm beam (125 mm less) and water plan of 11,96 dmq (2,36 less) …then we add the same sails (78 dm2) same keel/bulb (3 kg) and have same total DSPL (4.5 kg) in the hands of two skippers with same sailing skills …. which boat would win ?

So here we see the extremes of design variations in the same AC-120 class boats between Ver. 1 to Ver. 5 yacht designs and if one wishes to be competitive in Italy, you have no option but to go with Ver 5.0 boat design to be in the hunt, the girls from class of 92, 95, 2000 & to some extent 2003 will die if we don’t help preserve them.

One way this can happen: If yachts of different designs in the same class are given ratings with penalties applied to basic parameters so that all boats have comparable performance when racing together (same as real IACC did) this will provide not only more fun but greater interest in building and sailing boats that people want, while having an equal performance and the focus would be on RC racing strategy, tuning skills and sailing ability…rather than having helping hand with boat design advantages.

The International RC AC-120 rules & rating guide is a reference to help sailing groups that wish to start their own local groups using this document as a guide-line. This is not an officially sanctioned set of hard rules published by any authority, only the accumulation of experience from AC-120 sailors (who learnt the hard way) that would like to see different boat designs, sail competitively in the same class using a simple rating system.

Like to hear your comments & feedback on this and if any group would like to use this guide, can provide Excel spreadsheet for quick rating calculation.

Cheers Alan :zbeer:

Dear Alan

Imagin a match-race between USA-98 and ITA-25… are you sure at 100% that USA-98 will win?
I’m not: if in scale 1:1 we have seen that the max-beam has been reduced edition after edition, increasing boat performance… maybe in scale 1:20 is not the same.
I try to explain me better: during last Italian’s Cup (you was present) I have done very light test between my Azzurra ITA-73 and Moro Venezia ITA-25.
Well… I’m not sure that Moro’s performance are SURELY less then Azzurra’s performance.
I have seen that going Up-Wind Azzurra was little bit fastest, but not down-wind. You have also to know that the Moro we seen on Ravenna had a bulb of 2300gr… so if I imagin a Moro with a 2850gr bulb… I’m sure that a Moro could win against Azzurra.

In the future I’ll build ITA-25 and I’ll do serious test!!!

Have a nice time :slight_smile:


Hi Renato,

“If the only difference” between the two boats was the water plan (wider beam) clearly
from your own plan numbers, Moro II has bigger wetter area (more drag) Yes I’m certain
BMWO would win …why would it not ?

This is the point I trying to make, under Italian rules having both boats with the same
sail area, weight but with different water plans and using this as example where Moro II boat has larger water plan, both boats cannot be equal.

Under rating rule BMWO would have to reduce sail area (penalty) to stay within rating
and both boats performance would be closer.

Further as you have proven with your small test, reduction in bulb weight on Moro II
would make close the gap even further between the two boats because of reduced wetted area.

Hey love to work with you on making further tests, maybe next Ravenna ?

Cheers Alan :zbeer:

Hi Alan,

I’m curious… The Max Draft has always been 420mm… With this interpretation, 450mm is proposed.

Why move another goal-post or is this an oversight?


Hi Jim,
no is not an oversight !

Let’s compare the following :

IOM :… 100cm long, weight 4.0kg, sail area 65dm², Fin/bulb 2.5kg, draught 420mm.
AC120 : 120cm long, weight 4.5kg, sail area 80dm², Fin/bulb 3.0kg, draught 420mm

The AC120 has :

Sail area… +8.1%
Weight… +8.8%
Righting arm… +8%
bulb… +8.3%
weight ratio … +6.4%
Draught … 0%

My question is : why the AC120 that is heavier and has 8000cm² against 6500cm², shall have the same draught as the IOM ?

450mm is not compulsory and this limited degree of freedom contains also a drawback with the increased wet area !
The designer and skipper have the choice.

I remember that the class M, that is only 9cm longher but with similar weights, the draught can go to 660mm (Rules). Probably today nobody use it for various reasons, wet surface and weeds and pond depth, but most are around 520/540 mm draught and with much less sail area !

Rating parameters tend to prove that cannot converge in one single direction and a compromise is needed as shown above by Alan.



I may have overlooked this, but is there any provision in the rating to compensate for the change in righting moment?

It looks like I’m building a longer ultra high modulus keel the Autoclave then!

Hi Jim,

The righting moment is determined by the maximum Fin length (max 450 mm) combined max Keel Fin/Bulb weight (max 3 Kg) together with sail area (max 80 dm2) by equalising these parameters to have the yacht within its rating.

Mother nature then plays her hand for provisions of righting moment penalties (namely the weather conditions) and skippers are free to experiment here with the Fin/Bulb length, without affecting the yachts rating.

Imagine an adjustable length Fin with max length setting for heavy winds would create better righting moment but incur penalty of having more wetted area (increased drag) whereas a shorter Fin setting for lighter wind conditions could be better for having less wetted area (less drag) where righting moment is not as critical.

Makes one think little more about set-up for the days weather conditions a little more interesting & if the fin is adjustable (not being exchanged) you can change settings as weather conditions change throughout the day, without changing the yachts rating.

Cheers Alan

The rating aspect, tend to push toward the lower rating values, but if on one side there is an advantage to do that, on the otherside there is probably the loss of power coming from the forced reduced sail area.

The righting moment has some limitations too, since one can shorten the fin lenght to reduce wet area , but it cannot increase the bulb weight over 3000g, fin included. Good solution for calm days !

I invite the interested modelers to play with the “Rating Formula” aiming to find the ‘ideal’ solution including the capability to drive the model under different windy conditions : strong wind = heavy model and low sail area or light wind = lighter model* and higher sail area.

  • under certain aspects with light airs is often interesting to have medium displacement and eventually a ballast weight on top of the mast to force and produce a tilt. Similarly as done on real sailing boats to ‘inflate’ the sails and playing with rudder to create its own wind…

I’m a bit worried about the ability to alter the draft as its a costly unit to experiment with…

Also bemused by the fact you can build an IACC 120 to be in rule, but penalised in this rating system.
IE: boat #3… No mention if it has high/low or medium righting moment… But still out of rating…??

Hi Jim,

Most people overlook the AC-120 class is culmination of five generations of IACC boats designs from 92-07 so it is impossible to have comparable sailing performance if everyone uses min weight & maximum sail area.

Where people wish to have option of all boat designs represented from IACC mono-hull era, there are going have to be compromises in order to make it as fair as possible and IMHO rating is fairest way.

So yes no one yacht can have optimum use of class rules, if we wish to have more equitable racing.

In the attachment above I have shown extreme examples of Ver 1 designed hull vs Ver 5 RC design under the rules with only difference being the beam width. The Ver 1 hull can go to min DSPL with max SA but she will have larger wetted surface (higher drag) design disadvantage against Ver 1.0 yacht.

The Ver 1 hull advantage is then penalised with SA reduction so that both yachts will have closer performance, that’s the theory anyway :rolleyes:

As Claudio has asked, play with the “Rating Formula” aiming to find the ‘ideal’ solution !

I have done this using extreme examples of boat plans available, and found the max rating of 34 looks pretty good for light wind conditions, however for heavy winds I think we need to adjust the min rating down from 31 to 29 to allow use of min SA with increased ballast.

Cheers Alan

For those that use Excel Spreadsheets I have attached snap shot of how the formula is written for the rating for you to play with.

As there are so many virus going around, thought it better not to post .xls file

Afternoon gentlemen,

Reading through these posts one thing occurs to me which could have an adverse effect on fleet/class growth. If as is proposed a rating system is introduced, the skippers at the top will just take it in their stride. However, those coming in to the hobby (sport?), could potentially be put off by an additional complication potentially choking off the class in its infancy. In terms of which versions are faster, surely the builder/skipper will just lean towards building the faster designs so that all will then be sailing on a ‘level playing field’ (?!!).

It’s also worth noting that even rating systems have inequalities built into them. I remember a ‘Round the Island’ race (big boats) about the time laminate sails were becoming affordable. Two identical boats (among 1600 entries), one with brand new dacron sails, the other with laminate. They rated differently and yet it’s arguable that the laminate sails gave no advantage over the dacron (being new).

Food for thought if nothing else…



Agree with you Row … the class could very well evolve into roughly one design (Ver 5.0) without a rating system, but with over 30 designs currently available the option is there to try and put all design variations onto roughly a level playing field, without going over board, simplicity is the key “if” people wish to preserve all IACC designs into the AC-120 class.

Rating sail “materials” as you describe as an example is paramount to throwing the baby out the window with the bath water, depends on opinion and if it goes one design for competitive racing, so be it !

Only a proposal at this stage :slight_smile:

Cheers Alan

For those calculating ratings I have compiled list of known plans available, there could be more but these are the basic numbers …under current rules all these of boats can use the same sail area & with same rule of max keel fin/bulb weight of 3 kg and min total displacement of 4.5 kg

Red highlight is max beam and green is minimum which are the extremes I’ve been using.

This table shows some combinations of the various parameters contained in the Rating :

Boat 14 is almost inside the actual rules and 16 could enter the Rating by increasing the bulb weight !

It is understood that there will be always the first and and a last at the race arrival.

Already today there is a serie of boats that are escluded from the final race, but most of them are based upon the capability to trim the package and the use of good thumbs.

The publication of the “Document” is supposed to offer matter for discussions

The Rating is a more democratic approach, since others boats may enter in the final fight just by choosing the right set up , that means, the Sail Area, the Bulb weight, the fin lenght, the beam, the construction weight and trim ballast.
From my calculations based on a empiric formula, I got the confirmation that the bulb weight coud be increased at least by 150/200g with few mm increased LWL or by using the appropriate Fin lenght only and paying with some wet area !.

It is not escluded to widen the Rating formula to 30<>34 and authorising the Bulb/Fin weight to 3150g/3200g.
In that case the skipper will have plenty of room to stay inside the Rating .

I would also suggest to split the Sail Area range in two parts: from 60dm² - 70dm² for Rig 2 and from 70dm² - 80dm for Rig 1.
60dm² is already the 75% of 80dm² almost equivalent to rig C1/C2 of class M, the 4th out of 6 rigs.

Under 60dm² the RATING should not be used since the bad weather would be already a selective tool !!


What was the reason for choosing this route, rather than a corrected course time using a handicap scheme?
Numbers are quicker to crunch and a heck of a lot cheaper than fins, bulbs and new sails…

This was never going to be easy, reigning in a class that is hell bent on out performing other rivals… The hats off for trying!


Good question Jim, maybe I’m too much of a purest being avid AC mono-hull guy with a passion for sailing that is deeply embedded in my Kiwi genes :stuck_out_tongue:

My AC-120 class inspiration comes from the original ACC rule (see attached Ver 5.0) that states in the opening pages, which in my eyes I read as “the spirit of the class” both big & small:


  1. This vision statement does not form part of the ACC Rule.
  2. The America’ s Cup Class is intended:

(a) to produce wholesome, fast and manoeuvrable day sailing monohulls of similar performance intended for spectacular match racing in a wide wind range while fostering design developments that will flow through to the mainstream of yachting.[/b]

This threat is merely an open discussion forum to exchange ideas on AC-120 class for those who are interested to establish their own local rules to avoid potential issues.

Already I’m seeing that the status quo of AC-120 class rules maybe “under-dimensioned” as Claudio is suggesting and fostering class development in the right direction.

I wish to be clear that I do not want to discriminate against the Italian version rules, it’s their local rules after-all :rolleyes: but now I know the “loop-holes” I can happily sail with them, but I know it’s not a “level playing field” and that’s the reason for choosing this route to try and help others avoid potential issues in starting their own local groups that would like to have different boat designs, sail more competitively together.

Very interested in your idea of “corrected course time using a handicap scheme” can you give us some idea of how you see it working ?

Cheers Alan

P.S Note original ACC rule Ratings (page 43) and Penalties (page 44) :scared: we wish to keep AC-120 simple don’t we ?

Not having any experience in devising a rule of this kind, I will offer what I can…

The ideal and aim of this would be to even out the design differences that exist between the v1.0 and v5.0.

The biggest difference everyone is agreeing on is in the wetted surface.
So in the interests of promoting the class I’d suggest removing as many other variables as possible, and this will help simplify the calculation for the Handicap and keep the budget in check.

The handicap is then used to recalculate the final winning order in a race.
Instead of first across the line, wins. The time to complete the course is used.

Depending on your handicap-
Faster yachts have a time penalty, or a smaller percentage added to their time.
Slower yachts have either a small or no time penalty, or a bigger percentage added to their time to assist their performance.
This gives a Corrected course time, and the quickest after calculation is the winner.

With this any yacht built to the IACC 120 rule can race at the same time, and stands an equal chance at winning I guess…

This type of system is used for big boat racing… Can it work in this instance as well?

Cheers, Jim

I just thought of this and wanted to add:
It would work" right out of the box".
No messing around with fins, bulbs or building new sails… Hours and hours of testing and development saved and no real 2 boat testing advantage…

Hi Jim,
unfortunately there is no way to classify wich boat is fast and wich boat is slow, unless as occurs on real world, where the rating are based essentially on statistical analysis of speed.
This approach take several years of racing results in order to certify that this boat has a rating speed of 7.23kn.
Further all boats of the same design, being monotype, have the same rating.

Since our boats do not exit from the same mould, a variety of design and construction are presents, the only way to produce a “rating” is the one based on “dimensions” and not on “speed of a monotype”.

In this application we obtain the same result since each boat has a certified “rating” that will be added to the ranking arrival order. Is the same process as adding or subtracting “speed rating” and the system you suggest.

In the presented Guide, there is only a added value therefore much simpler.

The certified “rating” is applicable to that boat “only” and to a “Class” and therefore each boat has is own Rating figure that will be comprised from 31.0 to 34.0.

So, the use of “Speed Handicaps” presents no difference in calculation with the “Dimensional rating”. in both cases a simple arithmetical operation is needed !