Ships also began to be fitted with steam engines. Steam engines were first used in paddle steamers. The engine turned two paddle wheels. Paddle steamers were not suited to the open sea because in heavy seas the waves lifted one wheel right out of the water while the other one went right under, and this strained the engines. From the 1840s, screw propellers replaced paddle wheels in steamships.
Propellers work much more efficiently and are still used on most ships today.
Steam ships still had some other problems. A great deal of coal was needed to travel even fairly short distances. On a voyage to a distant part of the world, there might not be anywhere to collect more coal. For this reason, ships continued to be fitted with sails even though they carried engines. Today, diesel or steam-turbine engines use fuel much more efficiently.