Mechanical Testing: Propellor Research

We know that Robo Goby will need certain props and that props are chosen because of their different capabilities. Naturally we needed to do some research in order to pick these props. After we had an idea of what we wanted for props we would test them with an array of motors chosen for the same reason. We first compiled a list of online sources to research. After reading through multiple websites we would recommend reading this webpage:  modelpowerboat.com. It has basic information on prop pitch/diameter as well as useful boat terminology. Below are the definitions we found most useful:

• Diameter: the diameter of the prop; distance from blade tip to blade tip
• Pitch: a difficult calculation, but it given for most/all props. Pitch is the amount of forward distance for each rotation of the prop (this number should be decreased by 5-30% due to slippage in the water).
• Aspect Ratio: Pitch divided by diameter (P/D). This is an easier way to compare data while keeping both diameter and pitch in mind

Basically a higher pitch or diameter requires a stronger motor because it means more water is moved. We're trying to find the perfect aspect ratio and diameter for our specific motors (check out the Motors... page).

After reading this page we had a pretty good idea of what we wanted for propellors, but it was impossible for us to be 100% positive until we tested our props with our motors. We ended up buying a total of 11 props with a range of 38mm 70mm diameter. We bought relatively cheap molded plexiglass props from OffShoreElectronics.com because of their pricing, but also because they seemed like good testing propellors – our final product will most likely have better, stainless steel propellors. We also chose to use this website because all the shaft diameters were 3/16" which made out lives a whole lot easier. Unfortunately the aspect ratio of our propellors are constant. We used the diameter and blade number for our variable tests.

A chart of the propellors we purchased, their aspect ratio,  diameter, and blade numbers is below:

We will be testing these props with the different motors mentioned on the Motors... page. Stay updated to see our results.