DEMU's v DMU's

Last weekend I went on the preserved Hastings unit over the Mid Hants.



Which set me thinking, I understand why The Southern Regions produced DEMU's as they were effectively a diesel powered electric train.



But whet were the relative merits of the DEMU against a first generation Diesel mechanical unit

 

The DEMU's used technology closer to electrical multiple units.
Indeed the Hampshire units were designed to be converted to electrical multiple units as and when the South Hants lines were electrified. It never happened, but the driving trailers were similar to 2HAP driver trailers apart from the control wiring.

Cheers, Neil

 

I believe the main advantage was reliability. They were remarkably composed of standard items, basically they were the Southern Railway/Southern Region third rail passenger stock as developed after World War II with the engine and generator from the equivalent standard six coupled shunter. It all just went fine and certainly both are running yet, the shunters I think in the service they were originally built for.

The usual diesel multiple units had mechanical drive, essentially from lorry engines stuck under the floor with clutches, drive shafts and gearboxes as there were at least twice as many engines and all etc in a comparable three car there was inherently more to maintain. Considerable unreliability did not help. By contrast the Hastings/Hampstead/Oxted sets only had four cylinders of a slower speed locomotive engine protected from sudden stresses and strains by the electrical transmission and being inside the body much more accessible especially for inspection as well as protected from direct wet, dust and grit.

There were a downsides to these first generation diesel electric multiple units: they were not highly powered and, though they would get up well into the eighties on a clear suitable road, acceleration was not quick coupled with lower speeds on anything like a stiff gradient, though nice to have the engine under the the carriage roof this cost about 20 seats, they were distinctly noisy, the ride which could turn exciting was not good and the power bogies were hard on the track.

What must have helped when they were ordered was the costs as well as the reliability would have been predictable in advance. The Hastings Diesels were chosen as an alternative to electrifying the line - you accepted the lower capacity and the extra expense of the Diesel engines and generators but saved the costs of electrifying - either way the coaching stock of the trains were remarkably the same. The estimating, projecting and presentation simplified, derisked.

 

It’s the 4 cylinder version in the Thumpers (same as class 73) whereas the 08s etc. have the 6 cylinder version.

I’d also take issue with your comment about the mechanical units being unreliable - some of them clocked up over 50 years of service.


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I believe the main advantage was reliability. They were remarkably composed of standard items, basically they were the bodies of the of the Southern Railway/Southern Region third rail passenger stock as developed after World War II win the engine and generator from the equivalent standard six coupled shunter. It all just went fine and certainly both are running yet, the shunters I think in the service they were originally built for.

The usual diesel multiple units had mechanical drive essentially from lorry engines stuck under the floor with clutches, drive shafts and gearboxes as there were at least twice as many engines and all etc in a comparable three car there was inherently more to maintain. Considerable unreliability did not help. By contrast the Hastings/Hampstead/Oxted sets only had four cylinders of a slower speed locomotive engine protected from sudden stresses and strains by the electrical transmission and being inside the body much more accessible especially for inspection as well as protected from direct wet, dust and grit.

There were downsides to these first generation diesel electric multiple units: they were not highly powered and, though they would get up well into the eighties on a clear suitable road, acceleration was not quick coupled with lower speeds on anything like a stiff gradient, though nice to have the engine under the the carriage roof this cost about 20 seats, they were distinctly noisy, the ride was decidedly poor and the power bogies were hard on the track.

 

IIRC the original Hastings Units were loco-hauled stock that were provided for the electrification of the Hastings line trainsets until the vagaries of the narrower gauge caused a rethink. That rethink resulted in the adoption of an in-unit engine to replace steam traction and the concept was followed for other lines where electrification was seen as too expensive an option to replace steam traction. I well remember bouncing around Hampshire in the mid 1960s as they plied their trade around Eastleigh on services to Portsmouth, Salisbury, basingstoke and Reading.

 

clutches


?

 

The Hastings DEMUs were purpose built, and replaced steam. I’m not aware of any alternative designs prepared for potential electrification, and the 4-CEPs which were built for the Kent coast schemes of the 1950s were used on the Hastings line once electrified with single tracked tunnels.


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The MHR film team have put up a video from last Saturdays visit of 1001.

 

Yes, the DMMU's had manual gear change. Reach maximum revs, power off and let the engine revs reach idle then change gear. Changing down involved letting the engine revs drop back and changing down. Either way it did nothing for performance, particularly on steeply graded lines.

Climbing from a station stop at Upwey towards Bincombe Tunnel was painful and slightly worrying knowing that Maiden Newton to Evershot was much longer....

If my memory serves me correctly at least a couple of units per week were declared failures on arrival at Portsmouth when working Bristol - Portsmouth services. With 5 return workings per day, that is awful! The evening Portsmouth - Bristol Parcel train regularly conveyed failed units.

Cheers, Neil

 

For "clutch" read "fluid flywheel". Does the same job in a slightly different, and automatic, way.

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Yes and No!

The FIRST batch of Hastings DEMUs were built on 59ft underframes because the vehicles were originally envisaged as steam hauled coaching stock to replace the 59ft Maunsel restriction 0 vehicles then in use on the route. They couldn't be any longer (i.e. 64ft like a Mk1) as an allowance had to be made for a locomotive at the likes of Charing Cross station.

But before the coaches could be built the modernisation plan happened and monies were made available to dieselise the Hastings line. Given the desire to procced quickly the existing 59ft coaches (that were on the verge of construction and for which materials were already in stock) went ahead anyway (with mods) and some 59ft DEMU power cars were rapidly drawn up to haul them. This created the "6S" (S standing for short wheelbase units).

However it was also recognised that with a DEMU solution there would be no need for space to be reserved for a loco and that longer vehicles would be possible thus clawing back some of the space lost by the above floor engines in the power cars. This saw a revised design being created for a 2nd batch featuring 64ft coaches (as per the Mk1 length wise) providing more seats. These units were designated as "6L" (L standing for long wheel base vehicles)/

So thats why I said 'yes and no' - quite obviously if the entire fleet had been designed as DEMUs from the outset then the entire fleet would have used a 64ft coach length - the inclusion of the 59ft variant is a dead giveaway that the original plans for upgrading the line started well before the DEMU solution was thought up and loco haulage (steam or diesel) was planned.

To finish up, a final variant of the Hastings DEMU fleet was also made - but as it designed after the DEMU decision had been taken it used the 64ft long coach design to create a set of buffet equipped unit which was classed as the "6B" units.