We propose a semi-automatic inductive process to prove termination of outermost rewriting on ground term algebras. The method is based on an abstraction mechanism, schematizing normalisation of subterms, and on narrowing, schematizing reductions on ground terms. Our method applies in particular to non-terminating systems which are terminating with an outermost strategy. The induction relation, an F-stable ordering having the subterm property, is not given a priori, but its existence is checked along the proof, by testing satisfiability of ordering constraints.