As an example, here’s a small sailboat that has been converted for rowing and solar electric drive: http://forum.woodenboat.com/showthread.php?261007-Electric-boost-for-a-camping-rowboat/page6
(It is a John Welsford-designed 16’ Walkabout which is a very nice little open cockpit sailboat. http://jwboatdesigns.co.nz/plans/walkabout/index.htm )
On that thread, this earlier post provides a graph of speeds rowing, augmented rowing, and electric motoring: http://forum.woodenboat.com/showthread.php?261007-Electric-boost-for-a-camping-rowboat&p=6175091#post6175091
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