Screeching Halt!

This forum is dedicated to the Live Steam Hobbyist Community.

Moderators: Harold_V, WJH, cbrew

User avatar
Posts: 130
Joined: Mon Jan 12, 2009 6:25 am
Location: Derby, UK

Re: Screeching Halt!

Post by baggo » Thu May 18, 2017 9:12 pm

Are you refering to the spreadsheet that I presented on my website? If so, it is merely intended to be a simple to use rendition of the original work by the late Jim Ewins rather than a definitive 'how to design a boiler' reference. I do say in the pages that it is just my interpretation of Jim's formulae which he presented in various papers that he published.

Jim tried to take the figures from various published designs of boilers in order to come up with an average figure for various parameters involved e.g. size of grate compared to the swept volume of the cylinders. The formulae do fit some boilers quite closely but it is way out for others.

I think the biggest failing is in the figure for Kt when working out the efficiency of the boiler. This implies that there is a certain optimum value for the length of a tube compared to it's diameter in order for the maximum amount of heat to be transferred to the water from the hot gases from the combustion process. This ratio was devised by C M Keiller many years ago and was based on the tube sizes for various full sized boilers. It seems to be accepted now that in the small diameter tubes used in model boilers compared to full sized ones, only the first few inches of the tube contribute anything towards steam production. Beyond that, the length of the tube is pretty much irrelevant which means the value for Kt is meaningless.

To me, the important thing is to match the area of the grate to the size of the cylinders to ensure that the boiler can produce enough steam to satisfy the demand and then match the total cross sectional area of the tubes to the area of the grate to ensure sufficient gas flow through the system.

As has been said, the efficiency of the steam locomotive is always going to be very poor compared to other forms of motive power but Jim Ewins did try and come up with something based on data available at the time.

I think it was C M Keiller who said something along the lines of 'it doesn't matter how badly a steam locomotive is designed or how badly built, it will always work!'

Secretary of The National 2½" Gauge Association
Member of North West Liecestershire SME

User avatar
Posts: 4169
Joined: Sun Jan 12, 2003 3:26 pm

Re: Screeching Halt!

Post by Fred_V » Fri May 19, 2017 6:39 am

Is your spreadsheet based on Jim Ewins engine calculations? I messed with those for quite some time and gathered data from a number of engines. None of them seemed to work well by his definitions. His calculations are based on 5" ga. engines and when you go up to 7+ ga. the numbers all go to hell. Don't worry about, just build the engine and have fun. It doesn't matter that you use a half a bag more coal to run it than someone else.
Fred V
Pensacola, Fl.

Posts: 332
Joined: Sat Oct 08, 2016 10:04 pm

Re: Screeching Halt!

Post by BClemens » Fri May 19, 2017 9:49 am

Thanks for the replys and encouragement guys! Yes Fred V, I was working with Jim Ewins calculations as a sort-of check over of this design - which I have agonized over gathering data from different sources to work with. Most of the information for the boiler (other than physical assembly) was as mentioned earlier for ratios of grate area to swept cylinder area and stuff like that. Looking for success and efficiency.... so as it goes along I realize that the machine consists of many compromises.
John, I appreciate you offering the spreadsheet - I just went overboard with it trying to make everything just as it should be. Kt was the main concern and understanding your explanation of it helps reduce the frustration.
I can't quit on this now anyhow even if I was a quitter - I have purchases 1K ft of rail - and talk about discouraging words.... The track is a blind conquest so we'll see..... There's a club nearby.
Thanks again!

Posts: 803
Joined: Thu Apr 26, 2012 10:15 am
Location: Tennessee, USA

Re: Screeching Halt!

Post by Pontiacguy1 » Fri May 19, 2017 10:28 am

I agree with Kcameron above: A good running engine may not show up to be the most efficient on some theoretical chart, but still perform well on the track and prove to be a great workhorse. Main thing that you need to find out is: Will it make enough steam to pull all it is going to pull at the speed and grade that it will be encountering? In other words, will the boiler produce more steam than the cylinders will consume or is it the other way around? When you get right down to it, what most of us in the hobby really want is for our steamers to be able to haul the biggest load possible given the weight/adhesion of the locomotive, and the boiler to be able to keep up with it while we do so. Once it does that, then the design is successful, even if it does take a bit more coal while running.

I can guarantee you that my 2-10-0 isn't going to show up as very efficient if I plug the numbers into your program, but she will make more steam than the cylinders can use up at any speed and load that you'd dare to run her, all the way up to the adhesion limit. Thus, since the locomotive is more than capable of doing what I ask of it on a regular basis, I consider her to be a successful locomotive. If it steams well, but uses a little bit more coal to do it, then who cares!

Posts: 332
Joined: Sat Oct 08, 2016 10:04 pm

Re: Screeching Halt!

Post by BClemens » Fri May 19, 2017 1:08 pm

Pontiacguy1 wrote:I agree with Kcameron above: Thus, since the locomotive is more than capable of doing what I ask of it on a regular basis, I consider her to be a successful locomotive. If it steams well, but uses a little bit more coal to do it, then who cares!
Agreed: the boiler is the key and its ability to make enough steam..... no quibbling over more coal to get that done..
Builder01 wrote:Are you building a published design?
Who designed it?
What exactly is it?
Are there working examples of it?
Are there working examples of it with a steel rather than a copper boiler? (I would consider this a significant modification for a first time build)

I ask these basic questions because I did much research before starting the build of my locomotive. It is a published design by Martin Evan. There are hundreds of examples of it in operation today.....

The locomotive is a Baldwin 8-10-D narrow gauge plantation engine so is quite small. It resembles the Allen Fitchburg Northern but is a bit larger than that. The original engine was 1' 11 1/2" gauge (60cm) but to run on 7 1/2" gauge track the engine was physically too large so I redesigned the running gear for outside drivers and made it a 3' gauge mogul - so 2 1/2" scale on 7 1/2" track. The boiler is basic and firebox is the width of the frame, so the actual FB inside width is ~4 3/4" wide x 9 1/2" long. The boiler dimensions along with the cylinder bore and stroke / driver diameter have been the subject of much research all over the place - books, youtube, here.... The boiler hasn't been started yet except to do some CAD renderings after arriving at some dimensions. The pitfalls of copper are serious business. I built a 1" scale GN pacific boiler (belpaire) years ago out of all copper - that is still around here someplace.... frame and running gear was from the LE pacific... never ran it. This one is going to run.

Posts: 70
Joined: Fri Jul 03, 2015 9:47 am
Location: S-W Ontario

Re: Screeching Halt!

Post by Wolfgang » Fri May 19, 2017 7:57 pm

BClemens wrote:I have and use JFN's book. Books by Martin Evans, Henry Greenly, and other early builders are consulted.... The proportions for the boiler were drawn from data in these books yet the excel program figures it inefficient. A statement in the description relates that the program may not be why present a recipe for slop?
What % of thermal efficiency are you looking for?

Bear in mind that with model steam engine efficiency trials the maximum efficiency achieved was around 2%.

Even for full size hardware the efficiencies were only around 10%.

An exception may have been the SAR, the South African Railway, which used condenser cars to conserve water. These had the potential for higher efficiencies, but I don't know by how much.

At one efficiency competition one wag was overheard as saying that the efficiencies of all model locomotives could be better improved by repairing all the steam leaks rather than tinkering with the hardware.

Methinks that by sticking to well-proven proportions of cylinder volume versus grate area versus fire tube area, and draft arrangements, one cannot go wrong.


User avatar
Posts: 393
Joined: Mon May 18, 2015 5:26 am
Location: Erie, PA

Re: Screeching Halt!

Post by Builder01 » Fri May 19, 2017 8:32 pm

Sounds like a fun project to be sure.

So, this is your own design. You are more brave than I, especially for a first build! Too bad the loco with the copper boiler was not take to completion. There may have been something to learn from that potential experience.

In any case, I suppose with the proper research, as you have suggested, there should not be a problem. Also, as Wolfgang has mentioned, that by sticking to well-proven proportions of cylinder volume versus grate area versus fire tube area, and draft arrangements, one cannot go wrong.


Posts: 1372
Joined: Sat Jan 16, 2010 9:29 pm
Location: Florida

Re: Screeching Halt!

Post by WJH » Sun May 21, 2017 5:22 am

Larger driver wheels require fewer piston strokes which means less steam used. I think an Atlantic, spring rigged as an 0-4-0 is probably the ideal live steam engine. Many things do not scale down, and Nelson was filled with a lot of hot air.

User avatar
Posts: 1162
Joined: Tue Jun 24, 2014 2:57 pm
Location: Northern Oregon

Re: Screeching Halt!

Post by NP317 » Sun May 21, 2017 9:40 am

My philosophy for building steam locomotives is to make what you want!
What looks good to your eyes? What feeds your inner child?
The huge effort required to build a model steam locomotive should be applied to what feeds your "steam soul."

Efficiency? As the size decreases, the efficiency results also decrease to the point where is really doesn't make much difference while enjoying steaming on the track. Just make sure the design provides sufficient steam and traction to make you happy. Unless your goal is to win steam efficiency contests, stay focused on what your true desires are.

For me, "efficiency" refers to how well my time was spent building my locomotive!
Fuel and water consumption fades into the background of enjoyment!

My two coal-chunks worth of Steam Philosophy!

Posts: 19
Joined: Mon Aug 31, 2015 12:48 pm

Re: Screeching Halt!

Post by KenG » Sun May 21, 2017 2:53 pm

My experience is that the two items that are most important in the design of our models are the tube cross sectional area and the adhesive weight on the drivers. If the prototype was successful and the rest of the locomotive is anywhere close to scale then the rest should work out fine.

In the past I have seen models with narrow fireboxes having steaming issues, but a few years ago a fellow club member let me run his Allen mogul. Not wanting to run out of steam I overfired it and brought it back with safety valves popping, a firebox full of glowing coal and the boiler water level at the top of the glass. I was a bit embarrassed but very impressed.

I am considering building a 2-1/2" scale Baldwin 8-20-D (Kansas Central 2-6-0, similar to some 8-18-Ds that ran on the Denver and Rio Grande) and looking forward to see if Steve Alley is going to come up with something similar. If the design is anything like the Allen models I will jump at the chance.


Posts: 332
Joined: Sat Oct 08, 2016 10:04 pm

Re: Screeching Halt!

Post by BClemens » Mon May 22, 2017 6:32 am

The 2 1/2" scale 3 footers are pleasing to the eye and not so large and cumbersome to handle. And; proportionally with additional items like a steam water pump, a 'scale' pump can be 1 1/2" to 3 3/4" scale and look just fine.

Most of the major component castings for this 8-10-D came from Allen but 'kit bashing' will require searching. The drivers came from RRSupply due to the diameter needed. (They are also truer to scale since the counter weights differ in size - look good) The only problem with this scale is availability of 'scale' castings. These drivers for example had to have steel tires shrunk onto them due to the width of the 1 1/2" scale castings since the narrow drivers would not look so good on a narrow gauge engine.

A major deviation with this engine too is that the prototype was a wood burner so the boiler had to be totally different. That has lead to some head scratching and much reading and research. I have often kicked myself for not pursuing a 'kit' with everything already designed and tried and true. But then a steam locomotive is a bare, raw machine with nothing hidden behind a sheet metal door or hatch. They're ready and willing to be marveled at and duplicated - with strict authenticity - or not.


User avatar
Posts: 2460
Joined: Wed May 02, 2007 8:33 pm
Location: Chattanooga TN

Re: Screeching Halt!

Post by Fender » Mon May 22, 2017 7:43 am

Will it look something like this? This is based on the Little Engines "old-time" mogul castings.
Dan Watson

Post Reply