Bob England wrote, Sorry for my delayed response, been fishing. As it was explained to me by a detroit diesel engine specialist that took care of oilfield pumps, some small some had 16v154's, (pumps are very similar in power useage to full disp. boats) it is the leverage ratio of the rod on the crankshaft multiplied by time that determines load, which is a product of cylinder pressure. Lower rpm means more leverage is excerted on the crankshaft to produce the same amount of power that could be made at a higher rpm. What compounds this is that at lower rpm cylinder pressure remains higher longer, which increases the leverage even more. I think what we usually see in boats, even full disp. boats, is that they are way overpowered for the maximum speed they can make. Turbos compound the problems. Ideally, the cpp and a pyrometer is the best approach. It can be tailored to every situation and not overload an engine. For longest engine life (not neccesarily best fuel economy) the engine needs to be able to get pretty close to rated rpm. Doing so will guarranty that it is not overloaded. It's just a good rule of thumb. Reply, While what Bob says is not generally incorrect, it doesn't apply to the original question. The answer is complex and most don't have the time to fully answer or don't care to suffer others. Just because someone publishes on the web, doesn't mean it's fact. Start with Dave Gerrs book and do your own research. Like everything else, choosing a propeller is a compromise. One answer doesn't work for all. But if you follow Bob's logic, all those users of cpp's and the manufacturers must not have a clue. Clueless in Seattle, John