Richard Stuart's Garratt

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Australian model engineer (in his spare time) and oil industry engineer in 'real life' Richard Stuart has built and operates a 7¼" gauge Garratt of freelance design at the Castledare Miniature Railway, Perth, West Australia.

An useful 'lead-in' to what follows can be found here:

In mid-2006 Richard kindly sent a report on the loco's operation and performance along with an up to date photo:

Richard Stuart's Garratt in Service with his partner Trish driving. 2006. © Richard Stuart
Richard Stuart's Garratt in Service with his partner Trish driving. 2006. © Richard Stuart

7-1/4” GAUGE, 2-4-0 + 0-4-2 GARRATT LOCOMOTIVE

At this point, the Garratt has clocked up over 18 months of operational passenger hauling service at the Castledare Miniature Railway in Perth.  It is a good time to review performance and look at the experiences to date.

The Lempor exhaust has been a great success.  Its performance immediately met expectations without any need for in-service modification.  It was designed in accordance with Porta's published technical paper, with a target maximum smokebox vacuum of 75mm of water.  Measurements with a smokebox manometer showed an actual maximum value of 80mm of water under full load.   No doubt the performance could be improved with fine tuning, but it was very encouraging to find that Porta's theory is a workable design tool, even in the hands of a first time amateur.  The Lempor exhaust is notable for its quietness.  In fact, the steam traditionalists at the railway were quite disappointed by the Garratt's lack of "stack talk".  Under light loads, the exhaust is completely inaudible from inside the driver's cab, although a glance inside the firebox shows a strong steady airflow through the fire.  Under load, the exhaust becomes audible as a hollow thumping sound, perhaps more reminiscent of an internal combustion engine than the harsh "bark" of a traditional locomotive exhaust.

The Lempor exhaust had a very productive consequence, in that it attracted the attention of Michael Guy in Toronto, Canada (see the Lempor Calculator link).  Michael is building a similar sized Garratt, also with a Lempor exhaust, and was interested in the application of Porta's published theory.  As a result, the original haphazard calculations got tidied up into a computer spreadsheet, which Michael suggested should be published for the use of others.  The exercise also led Michael to perform an experimental test programme which is documented on his website.

The boiler has provided some valuable learning.  From the steaming point of view it performs well, to the extent that it can be a battle to control excessive blowing off.  However, it has demonstrated a propensity to prime, which is now believed to be exacerbated by poor design resulting from ignorance.  Appendix B of Martyn Bane's "Porta Treatment" paper describes the technical reasons for priming, and it is likely the Garratt boiler suffers from inadequate steam space.  If starting again, the top row of tubes would be sacrificed in order to lower the crown plate and water level.  The dome is also badly positioned, directly above the front tube plate, where, in the words of E.A. Phillipson (see book review section), “violent ebullition” can be expected.  Experience has shown that traditional boiler water treatment has to be sparingly applied, and care is required to open the regulator smoothly and slowly.  The priming situation was not helped by some kind soul who added washing up liquid (or similar) to the water tank on one occasion.  It was a major effort to fully flush out the water system after this episode.  For the future, the Porta Treatment system described by Martyn Bane is to be trailed, and it will be interesting to see how this performs in practice.

The performance of the direct radiant superheater has not been quantified to date (the benefit is suspected to be marginal due to its small size).  There are plans to measure the steam temperature at the inlet and outlet, but like so many things there has not been time to get this organised.  The blower steam is arranged to flow through the superheater when the regulator is closed, to provide some cooling effect, plus the collateral benefit of superheated blower steam.  So far, the Grade 316 stainless steel superheater elements have sagged slightly but otherwise seem to be surviving the firebox environment.

The axle-driven alternator has worked well, apart from some early trouble with a segmented belt drive parting due to inexpert fitting.  The alternator initially cuts in at 8.7km/hr, and then keeps charging down to very low speed.  On future projects, consideration will be given to increasing the 1:16 drive ratio, and probably replacing some of the belt stages with chain.  The availability of a self contained 12 volt electrical supply is very useful, and something to take advantage of on future projects.

The Garratt makes experimental use of electrical switching for primary controls.  Specifically, the cylinder drain cocks are controlled by an electrical switch in the cab, which operates a remote solenoid valve to apply/release the air supply to the pneumatically actuated drain cocks on the cylinders.  The advantages include the following:

The electro-pneumatic control philosophy is also used to operate the main steam isolation valve next to the dome, and the diverter valve which routes the blower air through the superheater when the regulator is closed.  In this case, the solenoid valve is actuated by a microswitch which detects when the regulator is first cracked open.

There were some misgivings about introducing functionally critical electrical systems into a supposedly "simple" steam locomotive.  However, an analogy can be drawn with motor vehicles, which are totally dependent upon electrical systems for their operation and don't seem to have suffered as a result.  However, two important lessons were learnt:

The next project will certainly incorporate more electro-pneumatics.  This logically points the way towards electronics, but that is another story.

The ride-in cab on 7-1/4" gauge is not unprecedented (see for example the Moors Valley Railway), but it is fairly unusual.  Coming from an oil industry safety culture, the first thing that stood out was the confined space risk.  It seemed a bad idea to be cramped in a metal box with live steam fittings a few inches away from the driver’s face.  As a result, a conscious decision was made to eliminate all live steam pipework and fittings from the cab.  The main change was to locate the water gauges on the firebox sides, instead of the backhead.  They are viewed through windows in the cab front sheet.  All the steam valves and pipes are mounted outside the cab, with operating handles extending through the cab front sheet.  The additional benefit is that the cab is very roomy and comfortable and the locomotive is a pleasure to drive.  Occasionally, looking through the cab window, past the dome and chimney, over the tank to the road ahead, it is possible to forget that this is a 7-1/4" gauge toy and get momentary delusions of grandeur.

Lessons Learnt:

Probably the biggest disappointment has been the performance of commercially obtained components.  A conscious decision was made to use commercial parts where possible, under the (mistaken) impression that professional manufacture with factory quality control would ensure trouble free performance.  In practice, the catalogue of failures included the following:

The Future:

It was always intended to fit a feedwater heater, and the exhaust system was built with an off take to provide the steam.  Somehow, there has not been time so far to design and build the system, but it is still planned and should be a worthwhile improvement.

Gas producer combustion is another possibility, but more remote at present.  The Garratt is quite a good candidate for this, due to its large and easily accessible ash pan.  It also has ready-made penetrations in the firebox walls (for the direct radiant superheater element), and secondary air inlets could quite easily be added.  Gut feel is that the biggest problem is likely to be achieving stable combustion control, given the small size of the system and constantly varying operating conditions in a miniature railway environment.

Conclusion so far:

The Garratt is great fun to drive, and does its job hauling revenue-earning passenger trains on the Castledare Miniature Railway in Perth, Western Australia.  It has been a great source of experience and taught a lot of valuable, if occasionally painful and disappointing, lessons.  Above all, it has been the trigger for discussion and sharing of information with others in the steam movement around the world, which is the most valuable outcome of all.

Richard Stuart
Perth, Western Australia, July 2006

More to follow I am sure!

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