Thinking about small (15-20’) sailboats with solar electric motor drives today. Demands for such motors for small fishing boats and general improvements in EV tech mean that electric drive for this size boat might be fairly practical, provided you aren’t motoring for hours as the primary locomotion.
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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