OK, let's start with something I have built in the past. This is a 4mm:1ft scale model of a Clyde Puffer. It's built from a Deans Marine kit which is fitted with 2 channel radio control. While not a very big boat, it's pleasent to sail but needs very smooth water. A change to NiM batteries fron Ni-cads didn't help either. The former are quite a bit heavier and so I need to pull some weight out of the bottom to get the waterline back where it should be and not at deck level !