i was wondering if there is any way to approx the loaded rpm based on motor size, unloaded rpm, and prop size/pitch</p>

Typically around 70-85% of the unloaded RPM using nominal voltage for LiPo's under a moderate load. It does vary quite a bit, even outside of this range depending on several factors.

Ryan

thx that does help alont, btw jus curious what are some of the other factors

List of factors - some minor, some major

- prop pitch
- prop diamter
- prop efficiency,
- Driveline efficiency
- Strut Depth
- hull Drag
- Hull Type
- motor efficiency
- Amount of Load placed on motor
- LiPo C rating
- LiPo Capacity
- LiPo Voltage Drop
- ESC/Wiring efficiency
- Water Conditions

and several more

Ryan