Thanks guys.

Dont worry jason i regard failures as learning but will not start until ready.

Thanks Sandy for excellent calcs and explanations. Now looking at loco or marine style boilers. May move to bulld pyrte first as just got hold of decent piece of 6" tube. I intend building 50% bigger so it becomes 3" instead of 2" scale model. Still intend loco but more needs planning and sourcing before starting. Will leave loco on this topic and start new for traction engine when ready.

Julian

Hi Julian,

I am afraid 'Jason' and 'Gas Mantle' are correct, you will need to consider a much better boiler design than the one offered by the PYRTE, especially if you go 50% larger.

Firstly you would need to increase the thickness of the barrel due to the additional dia + you will also need to allow for the fact that the boiler is also the chassis on a traction engine and since you propose adding approx 6" to it's length this would also need extra thickness to be added.

At 1/4 scale (3"/ft) you are close to the maximum for a copper boiler.

A few quick calculations on a 6" boiler x 18" long at 75psi ( the higher pressure specified for PYRTE) would require the thickness to be increased to 0.108" minimum... the nearest easily available standard size would be 10swg @ 0.128".

Secondly... a 50% increase would imply a firebox length of 9" which, with a 6" dia. barrel, would give you roughly 85sq in heated surface without any studs... perhaps 160 sq in if you managed to fit enough studs.

Cylinder size would increase to 1 1/2" bore x 3" stroke giving a total swept volume per revolution of 10.6 cu in.

Rear wheels would become 15" dia. and assuming a scale speed of 1 1/2 miles per hour (6MPH full size) this would equate to approx 11.2 revolutions of the rear wheels per minute.

Assuming also that the gearing remained the same i.e. 16:1 as per PYRTE then the engine speed would become 179.2 RPM.

179.2 x 10.6 = approx 1900 cu ins steam per minute required.

At 75psi 1 cu in water gives 306 cu in steam.

You would need to evaporate 1900/306 = 6.21 cu in water per minute.

Assuming the basic x 1.5 times conversion factor this would require a heated surface area of: -

6.21 x 100 / 1.5 = 414.0 sq in H.S.

You only have 85 (or 160 at best with studs)

You would therefore be wasting both your time and money going down this route with such a boiler.

Unfortunately there is no easy solution and a proper locomotive style boiler with a fully wet firebox and multiple fire tubes will be required if such a model is to be successful and almost certainly a coal fired version will be the best option.

Sorry and all that... but there just are no free rides in this game.

Keep Happy.

Have a great Christmas and New Year.

Sandy.