Oliver Lang's 1845 stern redesign, showcasing the propeller and rail system.
On ships like HMS Rattler (launched two years prior to the Franklin Expedition) unshipping the propeller involved a difficult operation involving tackle over the side of the vessel. A significant advancement of HMS Terror and Erebus was a new well system used to raise and lower the propeller directly through the stern of the vessels. The well system was so novel and efficient that a demonstration comparing the unshipping procedures of the Rattler and Erebus was conducted for the Lords of the Admiralty and the press prior to the departure of the vessels (Anonymous 1845:279). The system was subsequently adopted on all screw powered vessels in the Royal Navy of the era.
A significant trait of the new propeller well system was a pair of grooves or rails on the fore and aft sides of the rudderpost and sternpost, respectively. These grooves were used to guide the propeller as it was raised and lowered and to seat it firmly while it was in use. On most ships, these grooves were cut into the wood of the sternpost and rudderpost, but Oliver Lang’s design for HMS Erebus and Terror needed to be different. In his design the grooves were cut into rails which projected into the well and performed two functions: 1) the rails guided and seated the propeller when it was in use and 2) they secured the filling chocks that were to be used to strengthen the stern when the propeller wasn’t shipped.
Unlike many Royal Navy vessels of the era, the 1845 stern plans for the ships show that rails were straight-sided, presumably because they needed to be smooth for the filling chocks to slide along their length (most rail systems of the era bulged laterally at the position where the propeller was seated). Details of the interior of these rails are not shown on the plans, but we can assume that they included a semi-circular seat for the propeller, similar to other rail systems of the era (see here and here). The fore rail would have included an aperture for the propeller shaft, which likely telescoped through this opening to fit in the hub of the propeller (Battersby and Carney 2011:206). The rails needed to be extremely robust to take pressure from pack ice; so they were likely over-fastened – on my version I included bolts at roughly the same interval as those used on Terror’s iron staple knee. The 1845 plans reveal that the rails were made from gunmetal or a similar alloy, but given the ice abuse they would need to withstand, I believe the bolts used to secure them may have been made from iron.
1845 The Arctic Expedition. Literary Gazette: Journal of the Belles Lettres, Arts, Science, &c. for the Year 1845. Pp. 279. Robson, Levey, and Franklyn, London.
Battersby, William, and Carney, Peter
2011 Equipping HM Ships Erebus and Terror, 1845. International Journal for the History of Engineering & Technology 81(2):192-211.
|Sectional Plans for HMS Terror's Auxiliary Propeller System.|
|I used brass tube stock with an interior diameter slightly larger than the propeller hub to make the rails.|
|Tube stock cut in half and filed to the correct dimensions.|
|Cutting inserts for the heel of the rails. The seat for the propeller can be seen on the left.|
|Parts before soldering.|
|The rails after soldering and cleanup. The scribe lines mark the position of the bolt holes.|
|The finished drilled pieces after polishing. The aperture for the telescoping coupling can be seen on the right.|
|Test fit of the propeller.|
|Dry fit of the fore rail.|
|Dry fit of the propeller and rails in proper position. I admit I like the contrast between the brass and Swiss pear.|
|View looking forward. The scribe lines in the rudderpost mark where the rudder groove will be cut.|
|Again, just a dry fit prior to final assembly and finishing.|