AC 120 scale 1: 20 type "sport"

This Model is not conforming to the present Class Rules

This a lighter version of the previous design discussed in others treads.


Hi Claudio,

Looking very nice!
Maybe if there is increased interest in these more thoroughbred designs perhaps the rule might be modified.
When I get the chance I will post some pics of my interpretation of the rule. I experimented with casting a female tool using “white cement and chopped up fibreglass”.
The results where a fantastic success and the materials cost no more than 35€ !

Speak soon,


How about some winglets on the bulb as well? ;-)…

Hi Jim,

The Rules changes ? I din’t manage to convince the responsibles that argued to be a treason against the few modelers equiped already with certified AC120 .

Formula 1 change model every year, car manufacturer introduce new model every 6 months, computer are renewed every 3 months !

In this New design I included the reduction of minimum displacement from 4.5kg down to 4.0 kg and longer Fin for 3cm up to 45cm draugh.
The displacement coul be further reduced, but will be very difficul to to build it and probably loosing overall performances, this is why I presented the New formula “AC130” of 80cm lying between the RG65 and IOM. It could be also named differently. The decorations recalling the AC Cup are free, any one could create his own one.

Winglets ? why not but I beliewe that are presenting more problems than solutions. I my opinion the choice remains with the modeler.

I hope with this design, as well with the AC130, to promote new interests in creating new racing fleets around the countries.

I will be interested to reads your comments on the Rules.



Hi Claudio,

Change is inevitable, isn’t it?.. In the big boat world the guardians of the rules use athe term"Grandfathering". This keeps the value of the previous generation whilst enabling modifications to the existing rule to keep the class from stagnating or all the designs looking more or less identical. VOR 70’s and Open 60’s spring to mind. I agree,but also disagree with the custodians of the rule and that change would be a bad move. I am currently modifying TP52’ as fast as I can to take in the rule changes for 2010. Bowsprits, new chainplates, hatches, bigger bulbs, fin mods, rudder mods, new backstay chainplates, are all being done to optimize the yachts s they would no longer be competitive if they didn’t keep up with the new ways of thinking. The down side is that it all cost someone time and money which is normally payed for with sponsorship or a man with a very deep wallet. As modellers and not “campaigners” I guess we might have to wait a while for some change!

I’d say that if you could drop the bulb weight and lengthen the fin on the new boats why not try it on a existing yacht? the rules are for a max LWL 1000 mm & no minimum as far as I am aware… These boats will always sink back once heeled. Just an idea.

Off to work,

Ciao for now,


Hi Jim,
the actual rules fixes the Fin/bulb to a max of 3000g but also the max deept of 420mm from the bottom of the hull.
My proposal would be to let go up to 450mm just to increase the righting moment for better performances.
The righting moment actually is 2.85kg X 0.42m = 1.19 kgm -
with my proposal would be 2.85kg x 0.45m = 1.28kgm
equivalent to an increase of 9.2%.
Further, reducing the low limit to 4000g, also the ratio Bulb/Boat will increase.
Actually is 3000/4500 = 0.666 ,
instead my proposal would allow to increase
up to 2850/4000 = 0.712.
The two mentioned factors will drastically increase the stability.
All that , including a better Prismatic Coeficient, is translated in my new design presented above.
Obviously these changes requires some skill in manufacturing, but in my opinion all boats, heavy and light, could race togheter knowing that the heavy ones could perform very well in light winds if properly steared.

I remember that the race for very light displacements in the class M is not anymore a must !
In my thinking as well for many others, is not escluded that some racers will make two boats for “all seasons”


Here the Appendix for the AC120 " Sport" " FIN, RUDDER, BULB" and the extimated sub assembly weights :

The actual calculated displacement is 3572 cm3 that added to the 430cm3 of the appendix, the total displacement will be just 4002cm3 including a bulb of 2850g that can become 2750g and still keeping a good righting moment and providing an extra margin of 100g.


This plan is very similar to the one already shown, just 1mm deeper and 2 mm narrower with some additional 76cm3 for displacement as extra margin.

For those interested , there is only the dilemma of choice !!! hehehe !


The attached revised file contains all the drawings needed for the construction of an AC120 “Sport”

The frames are in scale 1:1 and can be printed directly on a A4 paper via a PDF Reader like Foxit Reader adjusted for 100% magnificaton.

The Sail Plan is already available on previous post

Enjoy eventual construction.