This question of the maximum speed of a narrowboat is a tricky one. It'll depend on a whole host of variables, size of engine, shape of hull, displacement etc. I know, empirically, 5.3 mph is the fastest I can get. That's with a K2 Kelvin and a 70' josher weighing 28 tons. Any attempt to go faster than this simply causes the back of the boat to dig into the water. For example, when opening it up on the Severn, I have had the back deck awash, but there has been no increase in speed – just a waste of fuel. :(When trying to move as efficiently as possible – and that's not the same as moving as fast as possible, I look and see how far the uxter plate is under water. The optimum depth, for me at any rate, is 9”.
Incidentally, some of the fastest craft around on the Tring Summit are those strung-together tin can rafts beloved of the local pub team and the Berkhamsted School sculls.