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A USV demonstration at Hampton, Virginia; January 2009
Unmanned surface vehicles (USV) or autonomous surface vehicles (ASV) are vehicles that operate on the surface of the water without a crew.
USVs are valuable in oceanography, as they are more capable than moored or drifting weather buoys, but far cheaper than the equivalent weather ships and research vessels, and more flexible than commercial-ship contributions. Wave gliders, in particular, harness wave energy for primary propulsion and, with solar cells to power their electronics, have months of marine persistence for both academic  and naval applications. 
Military applications for USVs include powered seaborne targets. Operational USVs with offensive capability include the Israeli Protector USV.