Trevithick Dredger Engine at The London Science Museum

by John

I landed at Heathrow at 6am, dropped my bags at the BNB, then caught 2 buses to the Science Museum.  Not jet lagged, but on a high, to see the only intact Trevithick Dredger Engine known to exist.

The room which houses the Trevithick, also contains 4 large beam engines, a Parson’s turbine (of “Turbinia” fame), and a very large 2 cylinder compound.

Disconcertingly, the first atmospheric beam engine, with wooden beam, was partly obscured by a souvenir stall and racks of clothes for sale.  WTF!   Don’t they realise the historical importance and rarity of these engines.  And 3 further moans, to get them out of the way.   The descriptive labels on all items had minimal information.  Nothing like dimensions, power, etc.  The attendants knew virtually nothing about the engines.  And often, items were behind glass or perspex which was reflective, and prevented good visualisation or photography.   To be fair entrance was free, but to get past the entrance desk it seemed pretty clear that a “donation” of 5 pounds was expected, (which I was happy to contribute).   Those complaints aside, I have to say that the collections were fabulous.

I could see the Trevithick at the far end of the room, so to curb my mounting excitement, I forced myself to not rush up to it, but to try to look at every exhibit on the way.

Eventually I was there and it was there in front of me.

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It was one of the smallest engines in the room (The “Energy” hall).  The older beam engines were many times larger, but that was a major reason this engine was so successful..  more power, lighter, smaller, and several times more efficient at converting coal to rotative motion.  No one could tell me why there is a huge divot in the cast end of the boiler.

The con rods, stands, standard cross tie, and chimney are not original, but were added when the engine was restored in ~1875.  But that is now part of its history.

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From that side it was apparent that the main shaft was square over its entire length, something not previously known to me.

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The boiler feed pump was relatively tiny.  I do not know if it is original.

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The firebox has been re-sleeved.  The chimney mount is part of the end plate casting.  And I think that I got most of these items pretty close to right on my model.  Does anyone know what the incomplete flange at 5 0’clock would have been for?

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Note the odd bolt pattern around the inspection hatch.  I got that wrong.  My change to the oblique slide rod stay angle brackets was correct.

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safety valve weight is adjustable.

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Water preheater pipe detail.  Aren’t the square nuts great!

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The throttle restraints are curved, and have fixed position holes for pins.

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Boiler feed tank.  Cast iron.

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Engine supports appear to be cast integrally with the boiler.

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Flywheel hub

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Flywheel spoke detail.  Likely original.

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Chimney mount detail

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Finally (although I do have more photos), a nice view from above.  I do like the crosshead shape.  I wonder if it is original.  Remnants of another Trevithick dredger engine  not currently on display, reveal a wooden crosshead beam.

So there you are.  Fascinating to me.  Interesting enough I hope to you.  I could see no evidence of wooden lagging at all, but i still intend to install some on my model to slightly improve its efficiency.

i have heaps more photos of other exhibits which I may post later.