The CrawlStroke app helps you to monitor and improve your freestyle/front crawl swimming. It renders sensor values of your watch, taken 10 times a second. The records are displayed on your phone. You find detailed lists of the individual strokes and sample summaries.

The app exploits the attitude values pitch, roll and yaw combined with time and acceleration measurements provided by the watch on your wrist.

Crawl strokes are identified from steep drops of the roll value - anatomically speaking from the shoulder joint turning the arm forward after pulling backwards through the water. Inside a stroke the recovery ends / the pull phase starts with the pitch value passing the horizontal line - the hand crossing the water surface. Both events are derived by following a short sequence of sensor shots.

Yaw values show the outreach of your hands. As well known, the arms of freestyle swimmers better remain near the body line.

The swimming speed can be seen from the velocity recorded during the last move of recovery and pull phases. The swimmer’s speed multiplied by time gives the distance made in a unit / stroke / sample.

Sensor values are transmitted as they come in, with some aggregation to min/max values, averages and ranges.

The app includes handling instructions. For swimmers who want to know more it hints at good freestyle advice on the web.

CrawlStroke does not depend on pool / lap size. Version 1 has been tested on a watch 3 and an iPhone 6 in Hannover and nearby, pool size 25 meters. Version 2.0 was tested on watch 3 and iPhoneXS in Hannover, pool size 33 m.

CrawlStroke has no privacy problems. It records sensor data at your command. They are stored on the watch as long as they are not yet sent to the phone. On the phone they persist as long as you do not delete them.

CrawlStroke integrates into HealthKit. It uses HealthKit samples with their sample-long observation times. Neither in HealthKit nor elsewhere any personal data of yours is required, used or stored. Nothing is shared with any party outside.

If you cannot agree with the treatment of your data as described above using the app makes no sense for you.

Have fun!!