Is there a simple relationship between prop rpm and boat speed?

Is this the JavaProp which has been suggested? http://www.mh-aerotools.de/airfoils/javaprop.htm It's a blade element code, and wouldn't be my choice for trying to size a propeller for a boat. I'd use the conventional propeller chart methods.

 

DC: Yes! Exactly how DCockey stated it... This is the procedure we use (several more measurements / calculations) when quoting our projects. It takes a lot of practice to visualize the forces involved, but keep experimenting and you'll start to see the relationship between the variables develop.

 

One of my friends owns a 47 crowther and he had been running the wrong size props for years, and the speed suffered because of this, he eventually went with larger engines..But at the time he could of increased the size of the props and the rpm would of drop and he would of gained more speed, The props he had were way to small, who did the original fit out i do not know.

 

In theory, larger diameters at lower RPM provide higher efficiency. The problem is that there are limits based on lift coefficient and cavitation that prevent you from actually reaching an optimum. You need to look at the max dimeter that you can fit, then adjust RPM to find a workable design point. With a fixed pitch prop, once you have selected a design point, any other operating point can be calculated if you know the curve for vessel resistance. A propeller has a thrust production and torque requirement that follows RPM squared for a given j. The vessel resistance curve never (except for a submarine) does due to wavemaking. That is the reason you don't have a simple relationship between RPM and V.