Plan to build....steam!!

[Jasonb]

I generally don't ream cylinders I bore them.

Unlikely to get a brake hone to fit this engine so turn down a bit of scrap aluminium or even hardwood and use that as a lap, charge with 600g silicon carbide powder and oil mix

 

[Maxjon]

I generally don't ream cylinders I bore them.

Unlikely to get a brake hone to fit this engine so turn down a bit of scrap aluminium or even hardwood and use that as a lap, charge with 600g silicon carbide powder and oil mix

View attachment 156265

Yes I'd be confident of a nice finish in the CNC, not so much on my lathe in the shed at home.....I will leave a heap of stock for practice feeds....

 

[Maxjon]

https://miniaturesteammodels.com/products/copy-of-4059-3-inch-vertical-marine-boiler-1

This would be a good thing! 45psi

 

[Steamchick]

See my comments above.
45psi is more than adequate for free running small engines on the bench. - Say less than 1 in bore x 1 in stroke? But good for a performance large boat - say 5ft long. - depending on the engine and speed - IMHO.
But if you want to run a 1in bore by 1 1/2in stroke engine developing some decent power - say for a dynamo - for bench display, then the "fire in the single boiler tube" won't develop adequate quantity of steam, so the pressure will drop rapidly when you try and run the load.
Problem - the Boiler fire tube is just 1 large tube to capture the heat. The burner is only maybe 30mm diameter, and may well be restricted to a gas jet that gives 1.5kW of gas heating power, as lot of which will come out of the chimney as hot exhaust.
Try and find a boiler that takes a much larger burner - positioned beneath the boiler shell and allowing the gases to pass through flue tubes before going up the chimney. That way yo can have maybe 3 times the size of gas burner and much more surface area to take all the heat into the water to make lots of steam for a larger engine.
K2

 

[Jasonb]

Do we actuially know if the flue is just a plain bore or more likely has a series of cross tubes which will greatly increase the heating area.

 

[Steamchick]

Assume the worst... I have experienced (scrapped) boilers (made by others) that had cross-tubes and had failed due to stress concentrations versus over-pressuring the boiler. If fitted and the job made properly, including calculations - and certified, then all should be OK and safe and durable. But cross tubes also severely restrict the burner you can put in the fire-tube, as they usually take-up about half the cross sectional area of the flue, and cause the burner to be limited to around half of the size you can put in a fully open flue.
My Marine boilers take the flue gases straight thorough a fire-tube, down and beneath the boiler shell, then up the sides of the boiler to the funnel at the top of the casing. Some with exhaust passing through flue tubes going the whole length of the boiler... (depends on "size and simplicity" demand). But my point is that these boilers look OK, and have a pressure rating, but no clues as to BURNER kW rating or how MUCH steam can be delivered. Usually they are the simplest design to "Grab your money" with the nice external appearance.
"Look, No Lagging" = waste of heat and shortage of steam.
At least:
Lagging Kit:
Each boiler is delivered with a wood lagging kit as shown plus fitting instructions...
My lagging is about 1/2" thick (balsa over a composite radiant and thermal barrier) so you can hold the boiler with a bare hand when in steam (but I don't). It really makes a difference to the QUANTITY of steam delivered.
No Superheater - something that really makes a difference to the engine's performance without needing a bigger burner.
But why do they not mention cross-tubes? - probably there are none.
My 3 in dia boiler with a 1.2kW gas blowlamp can power a 5/8" x 5/8" (aluminium body) Stuart Star engine adequately to drive my 48in naval picket boat at a reasonable scale speed (10kn). But cannot realistically do the same with the heavier (cast iron) 3/4" bore and stroke Stuart Sun engine. And it is well lagged, passes exhaust around and through the boiler, has a superheater coil, and re-cycles warm condensate into the boiler. It really needs over 2kW of burner - not in a fire-tube - so it can accelerate to scale max speed for the class of picket boat.
Public Domain, https://commons.wikimedia.org/w/index.php?curid=807204

Shown with Sun engine (low at stern from heavy engine). The photographer wanted it "posed" so was not running...
So "Buyer beware" when buying off the web... ASK the seller about the cross-tubes & flue tube surface area, power of burner, (gas, pressure and jet can be converted to kW), lagging details, superheater, etc. before paying any cash.
K2

 

[Steamchick]

AVI file (plays with Microsoft Media Player for me) of the picket boat mentioned above in action. Uncontrolled as the crew have jumped overboard I think?
M'sft isn't good at advising how to convert the AVI file into something this site can accept...(MP4, MOV, etc.). Must go now but will try again later.
K2

 

[Jasonb]

If you are extracting more of the heat from the burner due to having cross tubes rather than it going straight up the chimney then the burner doe snot need to be so large as the greater efficiency means you get more from the smaller one. A spiral arrangement of small tubes will take up a lot les sthan half the flue area. Take a look at this boiler for example.

https://www.modelenginemaker.com/index.php/topic,2851.msg56214.html#msg56214

Easy enough for max to contact the maker and ask what the exact build is. They say it will run a boat with twin 19mmdia cylinders so that is something around Stuart D10 size running reasonably fast. Example above also powers a modified D10.