This simple Vic Smeed design was originally for the 'Unit Steam Powerplant' but the plan does say suitable for electric...!😊
I've had this plan for decades, finally using it to give myself a break from building RC planes and re-kindling my interest in boat-building.
Mine was blown up to 33.75ins from the original's 27ins, giving a beam of around 4ins and a weight of 3.8 - 3.9kg. I used a re-purposed RS370 motor gearbox from an old foamie RC trainer plane, with the gear ratio around 3:1. This allowed this otherwise high-revving motor to easily drive a 3-bladed 40mm prop off a 6volt 4.5ah Lead gel battery, drawing very little current. Only drawback is the gearbox is a little noisy, but the boat sails very well after a slight adjustment to the positioning of the lead weighted keel. I've sailed it around for up to 20 minutes at reasonably scale speeds so far, and only had to put around 450mah back into the battery, so I guess it would probably sail for well over an hour if necessary.
A great build for having a go at making your own scratch-built fittings eg, railings, vents, windlass, derricks, rigging ...

Nick