There are two really good articles in the Bing Maps documentation around the tiling system and scale/resolution:
Assuming you have or can get the latitude and longitude of the center of the map, or the center of the ship for more accuracy, you can then calculate the ground resolution in meters/pixel for that level of latitude. At zoom level 19, a single pixel is approximately 30cm in size, or 1 foot, at the equator. The formula for this is:
ground_resolution = (cos(Latitude * pi/180) * 2 * pi * 6378137) / (256 * 2 ^ zoomLevel)
Once you have this, you can get approximate distances by converting your pixel distances to meters.