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.
References:
Anonymous,
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.
