Here a new plan for a M Class model.
Particularities are the Topsides folded inside as to present a full vertical surface to the water once tilted. The basic idea is to reduce drifting and have better pointing !
It can be used with a classic rig as well a swing rig.


Hi, Claudio,
i have been looking around m’s lately, since i am planing to build one.
I like the concept on this one.
but i have a few questions, why don’t u use the full available lwl? or does the boat reach it when heeled?

And another thing i was wondering about, i haven’t found any m’s with a sunken stern. my thoughts are that a relatively low wetted surface design should profit from a sunken stern when sailing close to hullspeed.

Sorry for all the questions.

Cheers, Erik

LWL is increasing proportionally with the wind speed.
All my design for modern sailing models do have static short LWL to exhibit lower wet area with low wind .
Sunk stern why, what could be the rationale from hydrodynamic point of view ?

My dear friend Pierre fully agree with my design choice and suggested to smooth the corners to reduce the turbulences when tilted .

hi, thanks for explaining the lwl thing.
Sunken stern… …if we have a boat that is fast enough we should get a relatively clean separation from a sharp edge on the stern.
Like having a 135cm long hull and cutting of the back few centimeters, the cut hull would still sail at a similar hullspeed. Idon’t possess the knowlage to prove this with numbers, but i’ve read it a few times, and look at some of the modern maxis and open60s.
Ofcourse such a boat would suffer in light air, and its performance advantage would shift to reaching and downwind.
Here in slowenia we have a national 750mm class, with almost no limits(wich results in 75cm boats carrying 5500cm2 of sail), i have a sunken stern boat in the build, will post here wenn she hits the water.


Hi Spade,
I just add some more, in my design the static LWL is always shorter, by purpose, then what could be and this because when the boat start sailing, the heeling produce two phenomena, one due to the hull shape that tend to elongate the dynamic LWL and one, not always considered by the majority, is that under wind pressure on the sail, there is a vertical component force that push down the hull in the water. Some volume in the front could be also useful.
According to my friend Pierre calculations, at 30° of heel, the boat may sink and increase the displacement by about 12.5%.
It is thus very convenient to have shorter LWL at the begin and buoyancy reserve in order to reduce also the wet area with low windy conditions.
The sharp cut transom may offer some advantage at the condition that a high speed is obtained. By high speed I intend a VR above 2/2.5 and more.
Our models, pertaining to the heavy displacement category, very rarely may reach a Relative Speed above 1.3/1.5

Another beautiful design Claudio.

Threre is alot of this current thinking in the M-class (See Brad Gibson’s current world-championship design, or Frank Russel’s). not only does the chine present a lateral surface for better pointing, it also creates a little more volume in the hull at that point.
And, because of that, when the boat heels over the chine causes the boat to lift out of the water a bit more. Therefore reducing displacement at heel, and therefore obviously creating less wetted surface and resistance.

and a faster boat.

Isn’t the M-class cool?
All within the basic guideline of 50" long and 800sq inches of sail. So simple yet so complex.

Thank you Breakwater,
is not entirely similar to the IOM, my chine is developed starting from the center shadows and then progressing to the extreme ends.
In my design there is no intention to get lifting caused by the chine but only a “vertical wall” .
I do not knows how much the vertical down push was considered by Brad when the hull is heeling, but it could compensated by the lift you are referring too that on my M is not considered at all.
The first trial was using a complete round shadow as usual since the intent was only to get a narrow deck in order to gain some weight and bringing away the sheer line from water when heeling and thus reducing the drag. Only after I decided to cut the curved line to obtain a “vertical wall” as depicted below and further getting a narrower deck :

No other thinking about lifting or what else, simply gain of weight to transfer to the bulb and finishing with the idea to apply a vertical cut to reduce further the deck width and getting a “vertical wall”.
Of course the “vertical wall” should help sailing to the wind !
The rounded hull (1st idea) may offer similar behavior although to a lesser degree but with better aesthetic ! I like roundness hehe !!

BTW the M class LOA is a little more then 50" > 50.7" or 1289mm instead of 1270mm

this was the first draw for the ESTEREL M :


Finally the choice is made, the ‘Diamond’ style is the one I will start building because is presenting better the anti drifting feature.
It will be a good contender against AC120 and Esterel 123.

File are posted in PDF format although not to scale, the dimensions are reported for printing enlargement control.


hi again, Claudio thanks for explaining.
Sorry for another question, i am aware of the vertical down force the sails create, but does your friend sugggest a formula for the calculation of the force, or is that a more complicated procedure. Becouse 12% is alot, and footys suffer from this force greatly.

This is a rough calculation that do not take into account the meta-centric variation with the helling. I ignore the formula.
This is the outcome values applied to a class M of 4500g displacement :
heel down force displacement
0° … 0 gr…4500g
15°…151gr…4651g + 3.3%
20°…263gr…4763g + 5.8%
30°…563gr…5063g +12.5%

As can be seen, at 30° the down force is sufficient to produce a sinking of +12.5%
Personally I do not need more to take into consideration these values during my designs.


The Class M is not limited to the Hull Drawing !
Here some other aspects of the Class M (modern Marblehead) starting with the Sail/rig sizing :

The Rigs as function of Wind speed :


Probably the most important part of the yacht model design : The Sails.
Here below the explanation on how to draw the Class M Sails :

To recall that the Rules specify the following Sail Area = 0.5161m²
This area is referred only to the triangles composing the basic form.
The Additional surfaces are calculated according to the parameters : 1/4 - 1/2 - 3/4 as defined by the Rules and expressed above.
The Real surface of a Type A Rig is about 0.72m² against the Rules Specification of 0.5161m² ( 800in²).


Of course opposite to the Sails, the Appendices are needed too.

In the above drawing I used a 6% of the sail area to determine the total Appendices surfaces composed by the Fin and the Rudder.
In this surface calculation the immersed Hull lateral plan is not considered as well the Bulb lateral plan.
With high speed the surface can be reduced to 3.5% of SA for the Fin and 1.8% of SA for the Rudder. IMHO, smaller Rudder surface my produce “stalling conditions” other then maneuvering deficiencies.

It is necessary to note that the sum of the appendices Volumes is 465cm3 and need to be added to the Hull Displacement in order to obtain the total DSPL of the model.


Here two typical sail drawings that could be used with the Esterel “Diamond” model :


Here the possible Servo Winch set-up.
To be confirmed once the hull available.
Not excluded the use of a Servo Arm

The actual servo intended for use at first is an old HS-725BB.
Calculations are added to the above drawing demonstrating that this servo can support a wind speed of 20Kt on 0.72m² of sail and with a margin of safety of 2.
Of course is too slow !!!


I’m thinking to borrow the idea of EC12 to install the servo winch with the use of extended tube to support the front and rear blocs.
This will avoid to cut the deck in case of failure.
Back to the drawing ‘table’

Out of Topic,
all my sympathies to Oklahoma City that once again is suffering !
From a News Paper I extracted these dramatic images and among them the best image of the year for me

Winch setting with carbon tube supporting the pulleys at the extremes ends. Idea borrowed from EC12.
Easy dismount in case of failures without cutting the deck for access.