Prospect said:

I checked the no.5 boiler in the Harris book and it is about 5 to 1. Lots and lots of tubes. Doesn't look like it would be fun to build or maintain.

John,

Yes it is a lot of tubes in a small space. If we check the length of the tube to the internal diameter squared I got the length is about 65 times the diameter squared. This is what Martin Evans recommends for a locomotive boiler. He states that the best practice for full size locomotive boilers is 50 to 70 times the inner diameter of the tube squared and uses a figure in the middle of that range.

Now maybe you can see my point of the dangers of using design rules for locomotive boilers for vertical boilers.

The only bit I could find by Jim Ewins on the web is this link. He gives 80 for Kt which is the same ratio of tube length divided by the tube diameter squared. Again this is for a locomotive boiler not a vertical boiler.

http://www.modeleng.org/articles/loco_research_je.pdf
I did find a table of vertical boiler dimensions in

*Boilers Types and Design* ICS 1907.

The table lists 14 boilers ranging from 20" to 48" OD. I calculated the ratios of the grate area to the tube areas given and the values ranged from 3.4 to 5 to one. The average of the 14 boilers listed worked out to exactly 4:1 for the grate area to the tube area.

The grate sizes for the 48" boiler is 42.5" OD and the grate diameter for the 44" boiler is 38" OD in the table so my assumptions for the Shay submerged tube boilers was a good estimate.

I can work out the L/d2 ratios for the table for vertical boilers given in the table to get a ball park figure for vertical boilers as it is a good historical source of information, and that is what was done by other model engineers for locomotive type boilers.

Now why do you not want to simply double the size of the boiler like you did the engine? That seams like the simple approach to me. And speaking of successful vertical boilers it seams to me that the original W. Harris donkey boiler or the steam roller boiler have to fall into that category as I am sure many examples have been made to those plans.

Dan