amorce du filet d'écrou

10:05 Jul 22, 2020 

the nut (and the associated thread on the rod) is mentioned just to indicate the location - what "cracked" must be the axis i.e. "la tige de commande du pas d'hélice", and the weak point where it happened is "where starts the thread on which the nut moves around" - more precisely the end of this thread that is somewhere along "la tige", not the end of the thread that is at the end of "la tige".

09:10 Jul 22, 2020 

Although cracking may not have been mentioned explicitly, the failure is "au droit de l'épaulement", an absolutely classic place for a stress fracture to initiate. Highly stressed bolts should always be radiused to reduce this stress concentration. I had a motorcycle whose crankshaft had been incompetently reground, and which subsequently cracked at the sharp corner. I think "where the thread starts" is just fine, but I'm pretty certain the failure is in the shaft, not the nut. I suspect "écrou" may have been added for emphasis, or just possibly to highlight that it had nothing to do with the other sort of "filet" (being a trawler).