Starter boat plans
...
Tug/Anchor-handling vessel:
http://eezebilt.tk/Beaver.html
Barges to go with it:
http://eezebilt.tk/lightplan.html
Cost of these is low - wood about £3, since the tug is only a foot long. Print the plans on A3 rather than A4, and the tug becomes 17" long - and of course you can enlarge it even further...