Trevithick Dedger Engine -Design Changes?

by John

I imagine that everyone who makes a model reproduction steam engine faces decisions about whether to faithfully stick with the original design, or whether to accept compromises due to accessibility of fasteners, metric dimensions, new materials, safety factors and certification and ease of machining.

The Trevithick dredger engine was a quantum leap in its day.  Trevithick was a brilliant, original, creative, genius.

But his boiler was cast iron, 1.5″ thick, because that is what he had available as the safest method for his revolutionary high pressure boiler.  A 1:8 scale model, if it is to be AMBSC certified, must be made of copper, silver soldered.   And the flat end must be LG2 bronze, not cast iron.  Fair enough.  When painted, the metal type will not look wrong.

And I am working from plans originally drawn in 1985 by Tubal Cain, and redrawn by Julius deWaal in 2016.

Unfortunately, despite manufacturing at least 600 engines of this type, there are NO intact authentic Trevithick high pressure boiler/engines or plans in existence.  The Trevithick dredger engine in the London Science Museum was rebuilt in the late 19th century, in Victorian times, and although it contains some original Trevithick components, some of the components had to be built from scratch, and reflect more of the Victorian style than the 1806 Trevithick style.

There are NO known original Trevithick plans or drawings, except for one concept sketch.

The earliest drawing of the Trevithick dredger engine comes from an encyclopaedia which was published in about 1820.  It is, I consider, the best image for the modeller to work from.  Many of Trevithick’s  engines would have been operating when this image was made.


Note the straight connecting rods, the straight boiler supports, and the absence of decorative mouldings on the flywheel



This is a drawing of the reconstructed Trevithick dredger engine in the London Science Museum.  Some of the features are more Victorian than 1806.

Knowing that the boiler cylinder and vertical cylinder end were all cast in one piece does explain how that complex shape was made.

This is a photo which I recently found, showing the flat end of the LSM reconstruction.   The location and shape of the penetrations is probably accurate, and are possibly original, even though some other details such as the con rods are not.  I am disappointed that the modern plans (Tubal Cain and deWaal) chose to modernise the shapes of the inspection hatch and firebox door.  Unfortunately my 1:8 construction has progressed beyond the point where I could readily make the older, more authentic shapes.

(Note made 12 Sep 2018.  I have actually changed the design of the end plate, and am well into making components which are more in keeping with the 1820 encyclopaedia drawing.  After discussion with the boiler inspector, I have filled the unwanted drilled holes, and made a new, circular inspection plate, and filled the rectangular cut-out in the end plate.  I am sure that the changes will look more authentic.  I will post the changes soon.)



The flat end of my model as per Tubal Cain and Julius deWaal. It is quite different from the Science Museum engine.  The end is bolted to the boiler flange, so it would be a straightforward task to totally remake it.  The newer version has a larger fire door opening which would be an advantage if the model is fueled with coal.  The inspection hatch is the feature which I find most disconcerting.  As an interim measure I will remake it with more rounded corners and no central depression.  I am considering whether to remake the firebox opening, door and surround.  Some drilled holes would need to plugged in the flat bronze.  The shape shown in the LSM engine would make it easier to fire it with propane.

Or, I could just finish the model as per the plans.  Any thoughts from my readers?  And does anyone have photographs of the LSM engine?  I have scoured the Internet but the quality of most published photos is awful.

This post is a bit rambly, because my ideas are changing even as I write.  Clarifying my thoughts is one advantage in putting thoughts to paper.