Nice pens John, and as you say elm was used to make coffins, keels for boats, bridge supports amongst others due to its durability. The thought was that the coffin would be preserved for longer to help the preservation of the body and the transition to the underworld. In all reality the real reason behind the folklore is that they needed to use elm for something else as you can't use it for heat on an open fire or cook over it as the stench from burning elm is very noticeable and pretty awful.