Beyer Garratt Boilers

Hello, I have a question about Garratt design for anyone who may know.

I've seen examples of Garratts with both round top and belpaire fireboxes, but one thing that remains constant is a parallel boiler design.
Is there a reason (greater heating surface area, weight distribution?) why tapered boilers were never fitted to this design? Was it part of the patent, or just preference?

Many thanks in advance.

C

 

At a guess i would say most Garratts have short fat boilers in order to reduce the length between the bogie pivots. This would be essential to keep the loco within the loading gauge on curves and also help reduce the length of the steam pipes. As well of this there are few restrictions on the size of the firebox. I expect that the shortness of the boilers and large firebox means there is little to be gained from tapering the boiler. It would however add to the cost, which is usually what it all comes down to.
Tim

 

I think you will find that generally speaking short, fat boilers are better steam generators than long, thin ones. Examples:- Gresley K3 (good) , DR Class 45 original (bad).

 

Thanks chaps, both make sense!

 

For anyone interested, I also dug out this thread on taper vs. parallel boilers, now I better understand the weight saving at the front, which would be a disadvantage on a BG as it takes weight off the front engine.
http://www.lner.info/forums/viewtopic.php?t=675

 

My understanding is that tapered boilers are used to reduce the volume of water at the smokebox end of the boiler on account of the cooling of the gasses passing through the tubes. Garratt boilers are generally shorter and fatter, so this isn't really a problem.