IACC scaled to a Marblehead?

Has anyone given any thought to scaling and competing these IACC boats in the 50/800 classification?

I’m only familiar with the M-class, and don’t know much of anything about these IACC Boats… in terms of what would be involved in scaling them to fit. And I’m sure the IACC has a huge sailplan that would need to come down.

I ask this, as the M-class is obviously not as vibrant as it once was, due to financial constraints, and superior boats introduced.
Some have gone so far as to claim that the Skapel “ruined” the class… but, alas this is a developmental class, Why couldn’t an AC type boat make the skapel obsolete?

Contributions welcome, as they’ll also help me through hurricane Irene passing overhead!

An AC120 cannot be scaled to a Classe M because :

Class M lenght over all - LOA : 1275 mm min - 1290 mm max

AC120 LOA 1200 mm max

Therefore, as minimum, there are 7.5cm of difference that cannot modified according to Rules.

Instead you can download an AC120 drawing, outside the Italian Rules, called AC120 “Sport” that is lighter (4000g min instead of 4500g) from : http://www.nonsolovele.com/Plans&Models.shtml


I’m watching Irene on NOAA site

I’ve been thinking about the same thing. Stretching the hull 7.5-9 cm shouldn’t be that difficult, and then putting on M class sails (properly adjusted for the hull).

I like the lines of the AC hulls too.

Thanks Claudio.
I understand that there is a difference between the lenght of theIACC120, and the M class… I was referring more toward what Guzz’s train of though is.

In that could the IACC120 lines plans be increased by a certain percentage (not sure of the exact number, but let’s say 3%) in order to achive that 7mm increase. Therefore making a boat that conforms to M-class rules.
If that is done, why couldn’t that boat to “toe to toe” against a skapel?

The goal would be to build a boat that competes in the M, and would not have intentions of competing in the IACC120 (as it will be too long to qualifiy for the IACC120)
The sail-plan would also need modification to fit within the 800 sq/in rule of the M-Class…

In regards to Guzz’s comments, dropping a M-class existing rig you have onto this boat is definetly the way to go to get the boat started after it’s been built. to save time & money…
But, as time & cash permits, why couldn’t the M-class adopt the rig used on these boats.

(I.E. I don’t think I’ve ever seen a M with a square-top mainsail)

The M rules prohibit sq top rigs. My guess is AC style hulls are not as fast as a M hull. They are designed to different rules. A more interesting comparison would be a 10R. then you can sail the sq head rigs and not have to change lengths. M’s make pretty good 10R’s once you add more sail area.

Oh yeah, I’m looking at that now in the M-Rules .

Rule H.7
“Top Width & Stiffening at head”

I also had the same thought in respect to adapting the IACC to the existing 10r class, but didn’t mention it only for selfish reasons (no-one sails 10r around my area!)
The sail area on an IACC could supposedly be increased to reach the 10r calculation, just as some do with the M-Class.

That’s easier than re-designing a IACC lines to fit another class.

I’m not so convinced that a Marblehead spec AC style hull would be so bad. The square tops used on the IACC 120 are because of the max mast length and because we can!
A Marblehead just has a taller mast as allowed in the class rule.
I would defiantly consider making the overhangs as short as possible to gain static LWL, and narrow the max beam also.
With all the M class trimmings, it could be a winner…

At 2010 IACC120 Italian Cup we had one M-Class join in fleet racing, now it was only “one boat” and I don’t know the skipper but it was mid-pack finisher.

Not all IACC 120 use sqaure head sails as you can see in attached pic (ITA 45 & USA 71)

I have scaled an AC 100 plan up to AC 120 specs (+20%) without any problems so 3-4% scaling of AC 120 plan to M-Class should not be any problem at all, mind you I’m not fimiliar with M-Class rules :confused:

Cheers Alan

I’m with you…

As previously mentioned some have gone so far as to claim the “skapel killed the M-class”
But, given that it is a developmental class I have to disagree. At some point along the way when there is interst, and funds someone will design a better wheel.

Are these IACC boats the better wheel in the M-Class? The boats are already so similar.

Looking at iacc rules I would expect them to be slower if built to the same sail area. The M has 30% less measured area 20% less if the iacc area is the whole sail area. They also have less righting moment than an M. Could be changed if you purpose built one as a M. Not saying it isn’t an interesting question.

I don’t completly understand your statement… Sorry.
I kind of get it, but do you mind re-writing it… Something about the way I’m reading it makes it not click with me.

I understand about the righting moment… The max depth on the IACC120 is 420mm (about 16.5 inches) whereas the M is 660mm (almost 26inches)
So that is a significant difference between classes.

Locally boats in my pond can’t sail with really more than a 18" keel (including lead) because of ledge and rocks, and that’s if the pond is full of water.

My point was that a M can be built with a significantly higher righting moment as you say, more depth, mine are all between 22 and 25" deep. In the US, there is no depth limit at all other than local venues. Red’s pond is one example where the older M’s do well since the new ones run aground. I know this is an issue in England as well, one of our local 10R’s went to sail a big race and was only able to sail in certain area’s of the pond. My personal opinion is no venue should be used for a high level event that can not handle boats build to the class rules. With the extra depth a designer can build a narrower boat since the righting moment of the hull is not as significant. IMHO most of the reason a IOM is so wide in relation to the M’s. Also the IACC has a big sail area advantage, so if you sail the IACC as a M it will have less HP. The us M nats will be at race week in San Diego next year, might be a good chance to see which is faster.

On the paper the M class may profit of longer Waterline compared to AC120, almost 29cm difference, therefore should be faster
The M class can carry max 7300 cm² of real Sail area against 8000cm² for the AC120 therefore the M is penalised in this respect.
The righting moment is in favour of the Class M but need to be considered with the CE height.
The bulb of the class M is free and pratically can vary from 2.8kg up to 3.5kg, some time more .

Sail Area/Displacement may play an important role between the two models,
therefore it may be possible that a light M could become competitive with low winds mainly because of the longer LWL .

Wiith the same Sail Area of 73dm², the M class will be ahead for the same reasons as above and better righting moment.

This drawing will shows the two models and relative dimensions :

Stretching an existing AC120 up to 1290mm and using the M rig and appendages, is not sure that this boat could be competitive since the hull body may be not well adapted and the LWL will be shorter any way.
The Sui100 or the Etnz are the ones that could be used for that purpose, in spite of the wetted surface area and the LWL. Certainly will not be a performer M model.

Definitively are two rather different models nevertheless, under certain meteo conditions, could be competitive each other, but a real test should be done.
I think that above 8-10 knots the AC120 may become a faster boat while the M shall reduce the SA.


I understand now…
I also agree that a high-level event should not be held at any pond that can’t accomodate current class rules.

I.E. Redd’s, where the “international M-Class rules” allow for a 660mm draft. Redd’s can’t handle that, Therefore It shouldn’t be held there. It’s somewhat counter-intuitive since that’s the birthplace of the fleet, but “it is what it is”

I didnt’ realize that the USA rules don’t limit M-class draft… Is that just another thing us American’s need to do different from the rest of the universe… Argh.

Here’s what I’m thinking at this stage…
What If I (or someone who beats me to it) makes a bullet-point list of advantages & disadvantages the IACC has on the M-Class.
I know the differences have been explained, but maybe we should clean them up minus the explanations.

-Righting moment goes to M

There we’ll have a clear list to see which boat is superior in some respects, and which is deficient. Maybe then it will give us a clearer understanding of the differences between the two boats, and maybe begin to draw conclusions as to if we think it is worthwhile to begin to extrapolate and modify an IACC to M-class rules to be a high-level competitor, or if the idea just won’t create a competitive boat.


With the M rules the sail area for “smaller” suites of sails are not really much smaller. The B rig on a M is still 800sqin measured, with just a lower CE. The actual area is slightly lower since there is less roach on the main. This gives a very large overlap of wind speeds of when to change unlike say the IOM class where the area drops significantly.

Pitty but, until now no comparaison has been made on purpose in the water.

It is my opinion that the lighter version, never agreed in Italy, for an AC120 “sport” it can become the faster of the two models AC120 - M in spite of the LWL lenght.

The AC120 can also reduce the Sail Area as well and reducing further the Mast height to compensate for the lateral stability.

The idea to let “free” the draft and bulb weight was also considered in my old propositions without success.

Actually, since over ocean people may search for competitive models compared to the well established M class, the idea to modify the Rules of an AC120 outside Italy is not forbidden.

I know also that in Italy, for the time being, will be never accepted !
To my knoledge, non one has until now constructed and tested an AC120"Sport"


O.K. So here is what we have on paper as a side by side comparison between the two classes.

This assumes that the IACC120 is increased by scale 3 or 4 % so that the Lenght overall of the hull fits within M class length rules.
Along with that expansion in length comes the 3-4% increase in everything else on the boat in order to keep the same aspect ratios.

No Advantage to either class category:
-Overall legnth (Assuming IACC is scaled 3-4% the length should be equal)
-Sail shape (allowance for square-top mainsails and other ideas on IACC, but I will argue this is equal due to the M-class able to counter this with eliptical mainsails such as those on the Sterne Viper) The resulting effect in both circumstances is to move the CE higher than it normally would be.
See here for details:

Marblehead “winning” categories:
-LWL (IACC has a bow knuckle, and transom overhang that takes away some LWL)
-Draft (Max draft on M 660mm, Max on IACC 420mm)
-Maximum Rig height. (Maximum 85 inches or 2160mm)
-Overall aspect ratio. (a combination between Draft & Rig height. Higher ratio than IACC)

  • Overall boat weight. As M-class bulb may be modified to any weight desirable in consideration of the aspect ratio and depth of keel utilized.
    -Beam. Given a higher aspect ratio, a narrower beamed boat may be used. May be desirable under certain circumstances.

IACC120 “Winning” categories
-Sail area Maximum 80 decimeters (1240 sq inches) compare to 800 sq inches of M-class

First off, does everyone agree, or disagree… so we’re on the same page.

Unless I am missing something, that would make the M class the clear winner on Paper.
The only advantage the IACC has over the M is that the sail area is larger. Given these assumptions, a scaled IACC120 to M-class length would not be as competitive.

And, you would not be able to take this category and adopt it into a M-class as it is against class rules (Maximum of 800 sq inches)

Therefore, the next and only remaining question would be, what would happen if you esentially took the IACC120 class hull, and put on it all the M-class advantages (keel, rig, etc)
therefore having the shape of the IACC hull being the only thing transferred over.

There’s a difference between these IACC boats, and most M-boats as the IACC boats typicallly have a U shape to thier hulls, and the M boats typically have a very sharp V bow sections, following into wider U sterns.

I know some this may sound repetative, but i’m hoping the bullet points take verbage and explinations from the contributions that have been made, and put it into a more clear statement.

And by the way, Claudio.
from what I see in your designs they are top-notch. Your contributions on this site are amazing, and I don’t know how you are coming up with these drafts.

Regardless, I hope you take this thread as a bit of respect from someone who thinks your designs are so good that they are worthy of a comparison and possibly a transfer into another class that I am involved in.

I don’t think I would look at the IACC class if I didn’t think your designs had a chance against competing against some of the best, and possibly even taking a run at a boat that, as previously mentioned some have said has “killed a class”

The traditional and current M class boats don’t have the same ashetics as these IACC boats, so I’m curious if you can knock of what some have notched as the “end of the line” in M-class development.


If you look at British M championships, the skalpel has not been the top boat there. The starkers line, crazy tube too and others have been successful since its introduction. It has really only been in the US that the class has died off due to the skalpel.
Personally I dont like the knuckle that the IACC designs all seem to have. Just doesnt look good to me when scaled down. No reason that you could not design a M with U shaped sections. I know Claudio has shared some of his M designs which have a definite IACC look to them. I think a lot of the skalpels success is not due to the hull shape, but due to the highly optimized rig. the rotating wing masts are a work of art. the booms with on shore adjustable outhauls are well designed and the butterfly makes the conventional rig the equal to the swing rig. I think you could put that same rig setup on another hull and have an equally fast boat.