I've completed work on a handheld Merchant Speed Calculator that is in the .jpeg format. The Calculator can be printed from the following web site.
http://mysite.verizon.net/ress1z18
The Calculator has a range in speed from 4 to 24 Knots, based on the given distance of 1 Nautical Mile. To use, a ship must be "Timed" on the Navigation Map for a distance of one nautical mile. Take the time, and match the Inner Dial corresponding time to the black arrow. Read the Knots per Hour at the left.
The calculator only measures to 24 Knots, due to the degree of accuracy needed at the higher speeds, and the constraint of fitting the range in speeds on a printable, handheld device. I will soon work on a "Warship Speed Calculator" that has a range from 20 to 40 Knots.