Hangdown Movies

Sunken Line Simulator

Fly Depth Results

Rule Numbers

Fly Depth Formula

Sink Tip Length Formula

Estimating Water Velocity

Fluid Dynamics Principles

Rule Number Lookup Tables

FAQs

Fly Depth Formula

While wading (and waiting for that steelhead take!), have you ever wondered how deep your fly is at hangdown?  Here's a simple way to calculate it based on sink tip length, water velocity, and rule number.
Fly Depth (ft) = 3 x Sink Tip Length (ft) /  (Water Velocity (mph) x  Rule Number)
For example, suppose you are using 12-feet of rule 6 sink tip in 2 mph water. Then at hangdown, your fly is 3 ft deep, calculated as 3 x 12 / (2 x 6).

For 3 mph water, this formula is virtually exact.  Over a wide range other practical fishing conditions (e.g. water velocities from 1 to 5 mph), this formula yields estimates that are accurate to within about 10% of our more-precise simulator-based calculations.