This iPhone app is designed to make sure that when you change boats you do not need to change your gearing, often with poor results.
Hence simply enter your current oar measurements and one measure from the new boat and it will calculate how you need to change the rig to maintain your current gearing.
Simple and quick to use.
See the App at apple here:
If you want to see the formulas demonstrated, please see
.3.1.4 Gearing Formulae and Calculation 5:27mins