so what we can derive from this is there is no sience to this its almost hit and miss with some experiance to back it up. now i know nothing about this issue hence the original question but if it is so "hit n miss" why do people go on about getting there boat "proped" to achive maximum speed, stopping power and minumal diesel consumption. a prop is by no means in the cheaper section of building a boat and the last thing i want after spending my money is to get it in the water to find that its cr@p for the want of a better term, i have been planning this for some time now and just want to get it rite first time as when its finished i wont have a bean to correct duck ups i hv now spoken to crowthers and they were very helpfull but could not give a final answer as they need to check the engines power band i weill update as to the result (if i ever get one)