Since the introduction of its first diesel-powered tender, the 515, Williams has rapidly rolled out this range.

The benefit of a diesel-fuelled tender being that it runs off the same fuel as the mothership. This all-new Williams 415, at a smidge over 4m long and 850kg dry, is the smallest model in the Dieseljet range, targeted at 82ft (25m) boats and above.
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https://www.mby.com/news/williams-dieseljet-415-first-look-entry-level-tender-112675