Wednesday, 30 April 2014

HMS TERROR’S SCREW PROPELLER

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.

References:

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!

8 comments:

  1. Very smart work - I don't know how I missed that note P. It all makes complete sense.

    ReplyDelete
  2. Hello William, thank you for your kind comment. I almost missed the annotation myself - after pouring over the images at the NMM, and reading the various treatises on propellers, I was initially convinced that the arctic ships used the frame system like every other vessel after HMS Rattler.

    I'm not certain why Lang used a hole in the propeller. It is very rare. I can only find two instances of it ever being used; on HMS Erebus and Terror, and some 55 years later on the RRS Discovery. The obvious link is that they are all polar vessels so there must have been some advantage specific to arctic exploration.

    Perhaps it has to do with the potential for damage to the frame. If the frame was warped by ice pressure, it might have become jammed in the rails making it impossible to remove the propeller. The hole in the propeller system was certainly simpler; perhaps easier to repair and remove if the rails were damaged.

    ReplyDelete
  3. So beautiful. You do beautiful work.

    ReplyDelete
  4. Thank you, Grace! Perhaps we'll actually get to see the real thing soon!

    ReplyDelete
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