Open Water Sighting Calculator

Sight Every N Strokes and Sighting Interval are both derived purely from how fast the current is carrying you off your line relative to how wide a drift you can tolerate -- neither formula involves your swim speed at all. Swim Speed only shows up in Distance per Stroke (where it's the entire calculation, under the assumed 60 strokes/minute cadence) and in Extra Distance from Drift, where swimming faster shrinks the drift's share of your total distance.

Distance per Stroke divides your Swim Speed by an assumed stroke rate of 60 strokes per minute, which is exactly one stroke per second. Dividing a speed in meters per second by one stroke per second returns the same number, just relabeled in meters per stroke -- so the output will always equal your Swim Speed input under this fixed-cadence assumption, not because it's a coincidence but because the math collapses to identity at that specific rate.

A stronger lateral Current Speed carries you off your intended line faster, so the time it takes to drift the allowed Course Width Tolerance shrinks -- and this calculator recommends sighting at 70% of that boundary time as a safety margin. That's why Sight Every N Strokes and Sighting Interval both fall as Current Speed rises: less time is available before you'd drift past your tolerance, so you need to check your position sooner.

It's the extra ground you cover from swimming a zigzag instead of a straight line: if a current pushes you sideways while you swim straight ahead between sightings, your actual path is the hypotenuse of a right triangle, longer than the straight-line distance alone. Extra Distance from Drift is that hypotenuse's excess over the straight distance, expressed as a percentage -- it rises with a stronger current and falls as you swim faster relative to that current.

It's a fixed planning assumption built into this calculator, not a measured value -- real stroke rates vary by swimmer, stroke style, and fatigue, commonly ranging from roughly 50 to 80 strokes per minute in open water. If your actual cadence differs meaningfully from 60, treat Sight Every N Strokes and Distance per Stroke as approximate, and rely more on Sighting Interval (in seconds), which doesn't depend on the cadence assumption.