They have only four possible basic speeds. The earliest (Longboats) are predesigned and can travel 2.5 per turn. As soon as they are available (with the discovery of Map Making), the player can design units with a speed of 3.5. About mid-game, a nation can expect to discover the advance Steam Engine, which increases module power and cost and automatically incorporates Steam Power in future designs, increasing the speed by 1(00).
The only other speed increase available to all nations comes with Nuclear Power, an additional automatic 1(00), unlikely to be available before Steam Power is.
A ship with basic speed of 4.5 will manage only 3.5 if its health is over 50% but under 75% and (as with all naval units) will manage 2.5 at 1% health.
The nation (if any) that owns Magellan's Expedition has all its ships travelling 2(00) faster than their rated speeds.