Even small boats can be seaworthy ;-) http://youtu.be/zP-G-ovutto
Actually, this is my first motor boat, but I had as yet not such a small receiver. That's why it comes at last.
Additionally now with a keel a stern tube a motor mount and a rudder unit to make it ready for RC and of course a propeller.
The boat which is shown on the video has no additional stern tube. The shaft runs directly in the plastic.
It must be well greased and you have to look out not to overheat it. Otherwise you will get holes very soon.
The second version (motor_boat_stern_tube_small) has a bigger stern tube outside contour. This is destined to insert a brass stern tube (inside diameter 1 mm, outside 1.5 mm), like my bigger boats have. It should fit, but I haven't tested it.
The coupling simply consists of a heat shrink tube
The best is, if you print the hull with spiral vase methode. Then you will get a neat surface without any noses from layer change.
The hull is 1.5 mm thick.
I printed with layer hight: 0.3 mm and width: 0.6 mm.
If you need to scale down you maybe need to reduce the width also because there must be a gap between the two perimeters. Otherwise it may not print continuous without stopping - even in spira vase methode.
I added a small base on aft to get more brim. I printed without heatbed and used 6 mm brim.
And yes, be very careful, if you reach the nose.
Reduce the temperature and speed by and by in small steps.
Propeller: layer height = 0.1 mm, width = 0.3 mm
All other parts: layer height = 0.2 mm, width = 0.3 mm
Propeller shaft: diameter = 1 mm, length = 47 mm
Instead of printing the rudder with shaft you can print only the rudder and use for the shaft 3 mm filament. Then drill a 0.8mm hole in the filament, connect both parts with a 0.8 mm wire and glue them together.