Later Retractable Propeller from S.S. Great Britain, Bristol, England (UK), 2012

The propeller that replaced the original on the S.S. Great Britain had four blades, but some years later this two-blade retractable propeller was fitted. The reason for it was the expectation that the backup sails would have to be used, and leaving the propeller in the water could cause it to windmill, creating massive drag. What I don't know is when this was installed, or how it was preserved given that the Great Britain was sail only from 1881 to 1970.