this hardwood is maple or birch having the most uniform longitudinal fiber orientation and modulus of elasticity close to 2 000 000 psi 13 000 mega pascals the structural resistance of the front and rear walls of the moldboard is obtained by means of a combination or an orderly superposition of various types of continuous glass fibers tissues having orientation angles varying between 0 u00b0 and 180 u00b0 these tissues are entirely impregnated with thermoset resin such as vinyl ester the low weight of the moldboard is obtained by the balsa wood 32 and by the hardwood 34 which are materials having respective densities which are lower than that of the laminates of glass fibers vinyl ester while maintaining a certain flexibility which is particular to composite materials maximum rigidity is achieved by means of the two structural walls spaced by the core of balsa wood thus providing an important increase of the inertia moment of the cross section of the moldboard without however increasing proportionally the resulting weight maximum rigidity increase for a minimum weight increase the moldboard is made by a molding process which consists in a manual operation of superposing layers of tissues of glass fibers impregnated with vinyl ester resin the possibility of orienting the continuous filaments in one or more principal axes permits optimizing the required thickness and obtaining an optimal specific strength the thrust frame 14 has triangular shape with three longitudinal sides 40 42 and 44 and round corners 76 48 and 50 see fig2 the thrust frame consists of a continuous laminate of glass fibers tissues the fibers being oriented in different predetermined directions which enable to meet the induced constraints of torsion flexion and shearing as well as tension and compression this laminate.