Redesigning a boiler from 5 to 7 1/4"

Hi and advice please,

I've been building a 5" guage Sentinal 0-4-0, Sirena which was published in ME in 1961. The chassis is nearly complete, boiler and the D10V are well in hand and I'm hoping to have her running about Easter time. Whilst maching some parts a silly thought entered my mind, I'd really like a 7 1/4" loco, but.... Then it occured to me 5 x 1.5 + 7.5. So perhaps it would be possible to build a 7 1/4" guage Sirena. It wouldn't be exactly to scale, but close enough. The lengthwise plans would work and minor adjustments to the width to meet 7 1/4" standards it should be possible.

Stuart Turner even do a suitable engine, the Twin Launch engine. The one area I am concerned about is the boiler. multiplying everything by 1,5 seems a bit risky to me, especially as my club will want to know the calculations or designs before they even look at testing it. The firebox is 3.5" dia x 4.5" 16g copper.with 12 angled tubes 7/16" x 20g, the shell 4 3/8" dia x 6" 16g with plates from 13 and 10g copper. I've added the plan below.

Advise please on resigning this so it is still vertical feed and aprox 6" dia x 9" tall.

Thanks

Steve

Greetings Steve, you are quite right to respect the views of your club's boiler inspector, and you would do well to consult him (her) about this redesign.

You might also look at the K N Harris book, "model Boilers & Boilermaking", I think there is a design for a 6 inch diameter vertical boiler that might suit.

Regards

Brian B

Talk to your boiler inspector ! If you deviate from the published or recognised design then you will have to show all calculations and materials specs ! There are numerous books on the matter of boiler design, K.N. Harris, Martin Evans, Tubal Cain, Etc. Do the calculations and show them to the inspector, he or she will advise on the way forward ! Bear in mind that it will not always be a case of increaseing dimentions by a simple factor, some will need to be considered on the square. Ask anyone who has made a 3" minnie ! Good luck. Noel.

I see no reason why a boiler inspector would need to see material certs as long as we're talking copper.

Pressed wrong button

Edited By duncan webster on 19/12/2021 21:49:38

Steve

Get a copy of the Australian Boiler Code, AMBSC. Part 1 cover copper boilers. If you work to that in conjunction with your Boiler Inspectors, you should not have a problem. If you want a copy of the code PM me with an email address.

Before you just multiply the (non -boiler) parts by 1.5 check whether the scale of the bodywork is actually the same as the scale of the wheel space relative to full size. 5in models are often scaled as 1in = 1 ft but obviously the wheels aren't.

Good advice from all and many thanks. I've found a photo of the boiler in the Harris book and it seems short for what I;m after also it seems to have a small water capacity. Most other of the larger vertical boilers seem to have side fuel feed which gives footplate problems. But all food for thought.

I'll use the x 1.5 for most of the parts but that only works lengthwise and not for the width as 7 1/4" standards are different to 5" ones. so the chassis will need to be a little narrower. There will be room for improvements i.e. needle roller bearings instead of bronze bushes in the axleboxes. I already have the wheel specs for 7 1/4" and they're not massively bigger than the 5" ones thickmess wise certainly not 1.5 times wider. .

You may also want to look at the Reeves RV6 boiler, good build thread here with a GA so you can see sizes. Might have to register to see the many images as they are posted as attachments.

I wonder if scaling up that boiler will be able to supply enough steam, it is a fairly basic design with a smallish heating area. First thing that jumps out at me is the top flange will end up about 1.5" wide and may well need stays as it's usual to have approx 1" between supports or stays to avoid excess thickness of the plates.

Edited By JasonB on 20/12/2021 07:43:32

Pkease, I have messaged you with my email address

[This posting has been removed]

If one treats the shell and firing tube as girder stays, the span in the original design is pretty much bang on what is allowed by the AMBSC (issue 6 is my latest). I'm sure this is fortuitous rather than by design, but it is at least reassuring. If I then make the shell 6" * 10swg and the firing tube 2.5" * 10 swg, make the top plate 10 swg and drop the operating pressure to 95 psi I get the same happy result, but I'd be happier if someone familiar with AMBSC checked over my ruminations before the OP gets his cheque book out. Any volunteers? pm me your email address and I'll send it across.

Edited By duncan webster on 23/12/2021 22:54:31

Is there a chimney and associated draughting on the 5" boiler - if not then i seriously doubt (infact am certain) that it wont make steam. Scaling it up would just make it worse. No fire tubes, water tubes or cross tubes? I must be missing something - but to answer your original question yes its a simple scale up. As JB has said you might need to look at the flat areas but increasing the thickness will sort that. Ive made lots of 5" and 6" dia verticals and they dont require any stays - but I do work everything out in cad first to ensure I comply with the build codes and safety margins. If you follow the ozzy codes you will comply with the rest of the world. UK codes are fine for UK but lack detail.

Nigel, bottom left of drawing shows the water tubes as hidden detail making up the "triangular shape"

Fizzy, There are cross tubes, twelve of them in three banks of four, all slightly sloping upwards. They are near the top of the firebox and the top row are almost touching the flange of the firebox top plate.

I only posted the main boiler drawing as I feared it might complicate things if I included the lot. But there is a steel smokebox ring that fits above the boiler and has a hole centrally with a lid to drop fuel in and towards the front is a 1" diameter tube that is the chimney base.

Steve, is that a water tank between boiler and the D10?

Yes, 5 1/2" dia x 5 3/4" with the usual manual, mechanical pumps and an injector.

The D10V is modified to slip eccentric valve gear, the mechanical lubricaror is home made and enclosed in a 1" brass tube, the drain cocks are just visable in the side photo and as they wouldn't be accesible with the bodywork in place all 4 come to one valve operated from the cab. The engine is mounted on it's own base so is a complete assembly removable easily.

Over Christmas I'm hoping to get my chassis almost complete with just the pipework to do. Then complete my boiler, then the valve gear on the engine and test it. The final part will be the water tank and with everything else done it will be the bodywork. Hopefully she'll be steaming about Easter time.

After zooming in I can see them -

Looks a very interesting project Steve.

I took over a set of Paddington plans that someone had started to convert down to make a more to scale Speedy.In the end I just drew out masses of bits starting with the frames. Frames being 5/16" slimmer which gives her a more squat apperance. Every so often some more bits get snapped up when they come up for sale.

I have also jumped into 7 1/4" tank loco lovely size.

David.