Wednesday 30 April 2014


Having completed the Royal Navy’s last great journey of exploration under sail power, HMS Terror’s next commission was destined to be the first major voyage of discovery to use auxiliary screw propulsion.  

The 1845 stern plans show that Oliver Lang chose a two-bladed Smith type propeller for the voyage (Battersby and Carney 2011:204). This choice makes perfect sense; in January of 1845 the Admiralty had just concluded extensive tests of screw propellers on HMS Rattler, finally settling on a two-bladed design by Smith (Carlton 2012:6; see also Bourne 1855:136).The propeller was likely made of gunmetal, similar to those used in subsequent Franklin search vessels (Dickens 1850:8) and other Royal Navy ships of the era. According to Lang’s plans, the screw was ca. 6 feet 11 inches in diameter (from tip to tip). HMS Erebus and Terror also carried a spare four-bladed propeller of the Woodcroft type (Battersby and Carney 2011:204). Roughly the same diameter as the Smith propeller, its shape and size would have necessitated that it be unshipped over the side of the vessel rather than through the propeller well.

Plans for HMS Terror's propeller. The filling chocks used when the
propeller was unshipped are on the right. 

Following others (Battersby and Carney 2011:204), I originally believed that the propeller was set in a frame that would sit flush inside the fore and aft rails/grooves in the propeller well (the rail system will be described in my next post). This was a prudent assumption, as a frame-mounted propeller was standard in screw-assisted vessels of the Royal Navy in the latter half of the 19th century. The frame was an important feature because it was used to raise and lower the propeller along the rails in the well, while providing stability when it was in use.

However, despite their common use in the era, I was puzzled by the fact that no frame is visible in the 1845 stern plan. I recently purchased high resolution scans of the plan, which permitted me to read Lang’s thorough annotations.  Needless to say, the last annotation in the list, labeled “P”, describes the reason that Lang didn’t include a frame in the plan:

“P: Propeller in place with a hole in the end of the fan to take it up by and to lower it down in place when required.“

The annotation exposes the highly expedient and experimental nature of the design, which represented a great simplification of the complex propeller lifting system used for the Rattler (view it here). While very rare, the hole-in-blade lifting system was subsequently employed on RRS Discovery by Robert Falcon Scott during his 1901 Antarctic Expedition (pictures of the hole in the blade can be found here and here). In fact, RRS Discovery appears to have borrowed many design elements from the Erebus and Terror, a testament to the advanced and efficient nature of their systems.  

Because Lang’s plans don’t show precisely how the propeller articulated with the rails in the well (the rails on the plan obscure those details), I was forced to speculate that the propeller included two retaining ferrules which would both seat the propeller and guide it as it was raised and lowered into position along the rails. In modeling this, I took inspiration from the ferrules used in contemporary Admiralty models and the RSS Discovery, though I admit they are highly generalized and speculative.


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.

Bourne, John
1855       A Treatise on the Screw Propeller with Various Suggestions for Improvement. Longman, Brown, Green, and Longmans, London.

Carlton, John
2012       Marine Propellers and Propulsion. Butterworth-Heinemann, Oxford.

Dickens, Charles
1850       A Visit to the Arctic Discovery Ships. Household Words: A Weekly Journal 1:8.

Propeller blades cut from brass strip stock. 

A guide for bending the blades to the proper angle.

Bending the blade by hand. 

Checking the angle. 

Blades after bending.

Rough filed to shape, with posts for attachment to the hub.  

The hub is made from brass tube stock. 

Parts prior to assembly. 

The finished propeller, including the hole in the blade. The blades were silver -soldered to the hub,
then the entire piece was sanded with 400 grit sandpaper and buffed with superfine steel wool. 

Compared to the plans. 

Assessing the profile shape. 

Comparing the prop to the model of HMS Rattler in the NMM. 

Checking fit in the well. 

Marking brass stock to fabricate the ferrules. 
Ferrules roughed out. 

Prior to final assembly. The ferrules are glued in place with metal epoxy,
rather than soldered.

The finished part - yes, it spins!

Wednesday 2 April 2014


Oliver Lang's 1845 stern redesign. Note the iron strapping. 

Oliver Lang’s 1845 modifications to Terror’s stern involved a significant amount of iron reinforcement. In my last post, I documented the design of the iron staple knee, which was central to the construction of the propeller well and new rudderpost. However, Lang also added four large iron straps to reinforce the joints between the keel, the rudderpost and the sternpost. These straps were u-shaped and passed between the false keel and the keel, permitting the former to break free in the case of the stern running aground.
According to Lang’s notes, the straps appear to have been bolted entirely through the width of the stern to provide extra rigidity. This created an exceptionally strong iron-laced structure, a design which exposed Lang's worry about the ice pressure to be exerted on the Terror’s stern.

The 1845 stern modification plan reveals that the straps were 4 and 1/2 inches wide, with bolts approximately 1 and 1/8th inches in diameter. I estimate, based on bracing and strapping shown on cross section plans of HMS Erebus/Terror and HMS Investigator, that they were ca. 1 to 1.5 inches thick.

All of the parts shown below are made from larger brass strips, which were cut, filed, and drilled by hand. The bolts are made from brass straight pins; the heads were filed by hand to scale (2 inches) and soldered into place. The brass was then blackened to simulate iron. 

The foremost strap prior to adding the bolts. 

The staple knee and remaining straps with bolts soldered in place. The three aft straps would normally look similar to the one on the right, but I've decided only to model the visible portions of the straps (i.e. the portions not buried between the keel and false keel). Each is composed of two parts to be glued on opposite sides of the model. 

Detail of the newly added bolts. 

Brass parts soaking in the blackening agent. 

The finished parts.

The iron reinforcements in place (dry fit only until the propeller rails are completed).

Again, just a test to see how they will look when finally installed.