With what do we measure a ships speed?

-Speed is measured is knots (nautical miles per hour)

That is determined by a few different ways. GPS gives a readout of knots made good

An old school way is to use a time /speed/distance calculator also called nautical slide rule. You can get the distance traveled between 2 points (say a couple of buoys) and the time it took to do it. By using those two figures you get the speed of the vessel. Learning how to do this is a great backup to GPS which can go down or the unit can fail.You have received plenty of replies giving you the units used to indicate a ship's speed, which was not your question. What is actually used is a certified timepiece. In the case of a newly constructed ship built in a Clydeside shipyard as an example, before a new owner would agree to take possession of it, it was necessary for the builder to conduct sea trials, and prove the vessel performed to the specifications laid down in the purchase order. These trials were conducted with the vessel steaming at full power over a 13.666 nautical mile (15.736 statute mile) course between the Cloch and Cumbrae Head lighthouses, in a procedure known as Running the Lights. A carefully timed run was made in each direction in close succession to minimise the effect of wind and current. In the case of warships for the Royal Navy, these days built at Scotstown West, they depart for the trial flying the builder's houseflag and RED ENSIGN, and on successful completion of the trials, return for the handing over ceremony and replacement of the RED ENSIGN with the WHITE ENSIGN. This ceremony would not happen if they failed to achieve the contract speed. The speed unit used is usually the Knot, but can be whatever is written into the contract.
GPS



Your GPS unit will tell you not only how many knots or statute MPH you are making, but also whether or not you are on course.



Um mm....you do have a GPS unit aboard, don't you?



EDIT: sorry, I couldn't resist that one.
Nautical miles per hour,

Knots as it is commonly referred to.
knots. one knot equals 1.15 mile per hour.
Knots I believe
knots while sippin your fav beverage ;)
Knots.