Knop

Knop (from the Norwegian word for knots) turns your iPhone or iPad into a speedometer for boats.

The application will show you the current speed in number of knots with one decimal precision, as well as the signal quality (indicated by the circles bottom left on the screen). To function properly as a speedometer, the app will require your location while in the foreground. If you wish to use an iPad or iPod as a speedometer, using one with built in GPS support (cellular) is recommended for a trustworthy result. 

Use of this application as a speedometer is done at your own risk, and I take no responsibility of potential inaccuracies in the measurements.