Re. my previous post. A real narrow boat has a standard size beam of 6ft10ins irrespective of length. So it depends what you are trying to do. If you are building an 18" model of a 70 footer, then the beam will be ten percent of that, just under 2inches, but you are going to have to accept the fact that it will be difficult, to say the least, to get all the RC gear in and will hardly be a sensible size to sail on a lake, a bath tub perhaps. But, if you want a boat that will function as a working model, you need to keep to the fact that the modern, holiday narrow boats are all sorts of lengths up to 70ft with a beam of 6ft10", so your model is just under 7ins. wide with a length whatever you want to make it. Most of the RC model narrow boats I have seen are to those or near dimensions. Working real narrow boats were of course 70ft x 6ft10".