Rob Roy boiler tubes are shown to slope slightly upwards towards the smokebox, but there are no real dimensions shown as to the extent of this, apart from roughly scaling it off the drawing. The lowest two tubes are shown just missing the throatplate, but no dimension is shown. Linked to this there also doesn't seem to be a definitive measurement shown on the drawing for the distance between the top of the firebox and the underside of the boiler tube, which would be the height of the girder stay. Can anyone who's built one give me a nudge in the right direction for this? Many thanks.
I don't know any answers to your questions - I suspect you may have to draw out the boiler for yourself and measure off what you need.
I find the archive section of the Station Road Steam website a valuable resource. There are several part-completed examples of Rob Roy, with the boiler innards on display. No measurements of course - but might be useful:
Again, not having built a Rob Roy, these are suggestions. At the smokebox end the tubes need to clear the tubeplate flange and really, the radius between the flange and the flat part of the plate, so this would probably make the clearance between tubes and bottom of wrapper on the inside about 3/16". I don't think the tubes at the firebox end should come closer to the inside of the wrapper than 1/8" so this gives you an upwards slope of tubes of 1/16" which is probably the right order of things for a boiler this size. As Wilf has said, draw it out, maybe twice full size. Paper is cheaper than copper!
The Rob Roy book does suggest a rise of 3/32" i.e. the tubes at the front tubeplate are 3/32" higher than at the firebox tubeplate. However, on the drawing, the tubes at the firebox end look as if they only just clear the boiler barrel and are much too close in my opinion for satisfactory circulation. Personally, I would raise the firebox crown and the tubes at the firebox tubeplate by the 3/32" and have the tubes horizontal to help circulation at this point.
There are numerous Rob Roy boilers built to the published design. Mine included, which is now over 25 years old, and still performs well. As organiser of 10 annual Rob Roy rallies, I've encountered quite a few. Never heard of any complaints re the position of the tubes.
I agree with John B, and the Rob Roy firebox crown is quite low anyway. Worth adding that if you raise the firebox crown you may also have to raise the position of the longitudinal stays.
If you raise the crown this will raise the minimum water level to keep it covered this will reduce the steam space and make it difficult to drive with a narrow operating window for adding water and running low on steam available not something you would want on a small boiler. Regards Jason
As I said in my post above, that is what I would do 'personally' to correct what I consider to be poor design. The firebox tubeplate area is one of the hottest parts of the boiler and needs good circulation. In my opinion, raising the crown by 3/32" is not going to cause any issues as there is plenty of water space above the crown. If people don't agree then that's fine. Stick to the original design
Just because a lot of boilers have been built to this design and steam ok does not mean that the design is perfect and cannot be improved. It reminds of what C M Keiller once said - Words to the effect that 'The problem with steam locos is that no matter how badly designed or badly built they are, they will still run'.