looking for positive suggestions

[David Powell]

We find that coal fired a model traction engine with12 sq inches of grate will pull a heavy driver and passenger continuously without constant firing, total weight of boiler being in the region of 80 lbs. So 4 times that should give enough power to mow with as well. However, by then your boiler weight will likely be up in the region of 300 lbs.
IF you go to a 12 inch by 12 inch grate you are likely looking at a conventional fire tube locomotive style boiler weighing up to 1000 lbs, unless you head into the world of steam cars with wire wound lightweight boilers as per Locomobile. Stanley etc, or flash steam coil boilers.
There is no " Right Answer". But your finances, workshop facilities, the time available for the project, the number of helpers available and other factors will dictate how far you can take the adventure.
Dreaming . planning and eventually building are the most fun you can have while wearing clothes !!! Regards David Powell.

[Harold_V]

Dreaming . planning and eventually building are the most fun you can have while wearing clothes !!! Regards David Powell.

I share that concept, but there's no shortage of folks who don't. I'll spend endless time building something that others would gladly purchase. Buying something others make doesn't do much for me, and I certainly don't learn anything when I do. As I've commented previously, it's the journey that I enjoy, not the arrival.

Different strokes, eh?

H

[James Powell]

However, to get enough hot steam at about 100 lbs pressure( At the engine) to run steadily you need a fairly large free steaming boiler, with a working pressure of 125 to 150 lbs,
IF you fire with wood or coal you will need a big grate area, perhaps 50 sq inches to avoid firing very frequently.

Well dang it, I was planning a boiler with a 12x12 footprint,but I guess I'll go smaller IF I HAVE TO

We was looking at wood pellets to have continuous fueling but that would be a new factor to add math to...but since they push enought BTUs to keep a house warm, a couple 2x2 pistons shouldn't be harder to push

You would be suprised...

1 HP (M)= 2545 BTU at 100%. Now, you will be lucky to see 1% n...so every HP= 250 000 BTU/h. Or more than 20 lb/hr of dry wood pellets, or about 2 gal/hr of oil (or about 20 lb/hr of coal...)

Yep, the math sucks. Each HP is about equal to the rating on my furnace...

5 HP (actual, real HP, not pretend or short term) is going to cost about 1000 lb in weight- the mechanism part of a Stanley Steam Car. There are ways to get it lighter, BUT you will sacrifice a lot of robustness to get that lightness. For example, a Sentinel Steam Waggon was rated at 80 BHP, and weighed in at about 6 long tons. Lots of engineering involved to get there...

10"x10" firebox will give 3-5 HP onto the ground. There are ways to burn more in less space, but n suffers further, and that means more BTU per HP in a fairly rapidly spiraling death spiral. (see Flash Steam Airplanes and Hydroplanes as examples)

James