What is a 'Rhodesian boiler'?

My first thought was, why are these domestic hot water boilers called 'Rhodesian boilers' throughout most of British colonial Africa? And what exactly makes a boiler 'Rhodesian'? Elsewhere these types of boiler are usually referred to as 'donkey boilers' - in lieu of any other leads, I thought I might start my research there.

The Donkey Boiler

There were several suggestions to the effect that donkey boilers get their name from their resemblance to donkeys - and I did find photos of boilers that might be thought to resemble a donkey:

But, hey - most don't look like a donkey at all, and some seem to have a closer resemblance to other animals:

My first lead took me to the 'steam donkey', as used in the lumber industry. This comprises a steam boiler, steam engine and winch, assembled atop a sledge. The name 'donkey', in this case appears to arise due to the use of these powered winches to replace our trusty friends (or more often mules), in pulling lumber out of the forest.













These are, however, steam boilers and most are upright. My next lead took me to the 'maritime donkey boiler'.  This is a small auxiliary boiler used when the ship is in port, to supply steam to deck machinery, such as winches for cargo loading and unloading, when the main boilers are shut down. The name derives from the tradition of using draft animals for this work prior to the invention of steam power.
  
Now, it turns out that these boilers were also used for supplying hot water to the galley whilst the ship was in port - so I guess that's where the name 'donkey boiler' comes from with respect domestic hot water systems.