We report on an experiment in combining the theorem prover Isabelle with automatic first-order arithmetic provers to increase automation on the verification of distributed protocols. As a case study for the experiment we verify several averaging clock synchronization algorithms. We present a formalization of Schneider's generalized clock synchronization protocol in Isabelle/HOL. Then, we verify that the convergence functions used in two clock synchronization algorithms, namely, the Interactive Convergence Algorithm (ICA) of Lamport and Melliar-Smith~\cite{lamport_cs} and the Fault-tolerant Midpoint algorithm of Lundelius-Lynch, satisfy Schneider's general conditions for correctness. The proofs are completely formalized in Isabelle/HOL. We identify the parts of the proofs which are not fully automatically proven by Isabelle built-in tactics and show that these proofs can be handled by automatic first-order provers with support for arithmetic like ICS and CVC Lite.