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

[DCockey]

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.

[jmiele3]

Originally Posted by DCockey

How much thrust for what RPM depends on vessel speed, propeller diameter, pitch and blade area.

First pick a target speed for you boat. Then you need some sort of estimate of resistance for your vessel at that speed. That's the target thrust. Without an estimate of the thrust needed for the desired vessel speed it's somewhat pointless to talk about diameter and shaft speed.

You will also need to estimate some factors relative to flow around the hull.

There are an infinite number of propeller diameter, pitch, blade area and shaft speed combinations which will provide the target thrust at the target vessel speed. With the particular charts and/or software you are using you pick a diameter and a shaft speed and determine the corresponding efficiency and pitch. Also check if loading is excessive. With efficiency, shaft speed, vessel speed and thrust the shaft power and torque can be easily calculated. Repeat the calculations with a different shaft speed and/or diameter and see if you like the results better. Continue to iterate until you are either satisfied with the results or decide to change the target vessel speed and corresponding resistance.

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.

[harry cassin]

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.

[johneck]

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.