Prior to the invention and general acceptance of the screw propeller, ships equipped with steam engines were propelled with paddle wheels. This technology has costs, especially for military vessels, because the huge wheels take space which cannot be used for mounting cannon. 

The lack of space for cannons on the raider is partly redressed by the power of new breech-loading rifled guns rather than the older smoothbore cannon.  However, raiders' firepower and small size will not allow them to stand up to a ship of the line.  

Paddle wheels appeared on many types of merchant ships. The largest was the Great Eastern  which first crossed the Atlantic in 1860. Later this ship laid an Atlantic telegraph cable. No larger ships were built until 1899.

Paddle wheels permit the construction of a large merchant ship and a raider--a fast escort vessel.