I am often asked if a particular boiler will be suitable for such and such an engine. The answer is always the same, "it depends what you want to do with it".

You will see that each of my marine boilers has an evaporation rate shown in cubic inches of steam per minute. Note that this figure is calculated at 50psi. If you operate your boiler at a lower pressure it will make more steam and if you operate at a higher pressure, less steam.

As an example, my 3 1/2" " boiler will evaporate approximately 1.5 cubic inches of water per minute.
1.5 cubic inches of water will give 600 cubic inches of steam at 50psi
531 cubic inches of steam at 60psi
711 cubic inches of steam at 40psi
1089 cubic inches of steam at 20psi
1554 cubic inches of steam at 10 psi

To calculate the size of boiler required for an engine simply divide the steam output of the boiler by the swept volume of the engine. This will give you the maximum speed in revolutions per minute that you could expect with that boiler/engine combination. I know some people will argue that this doesn't take valve cut off into account but it simplifies the sums and any cut off will only improve performance.

The swept volume of an engine (that is the volume of steam used per revolution) is 3.142B x S x C x 2 where B = the bore radius squared, S = the stroke, C = number of cylinders and x2 for a double acting engine (leave out the 2 for single acting engines).
So for example, if you have a twin cylinder double acting engine of 1/2" bore and 5/8" stroke the swept volume per revolution is:

3.142(1/4x1/4)x5/8x2x2 = 0.49 cu ins/rev

If you wanted to operate this engine at 40psi, then the 3 1/2" boiler will make 711cu ins of steam at 40psi

so 711/0.49 = 1451rpm This is the maximum continuous speed at which your engine will operate at 40psi.

The above figures are approximate but should give you a good idea of the performance you could expect. You can increase the performance very slightly by lagging the boiler, engine and steam pipes properly.

If you are going to run your engines light for display only i.e. not driving anything like a generator, boat etc. then, when they are properly run in, they should operate at very low pressures, typically 10 psi or less and so a given boiler will drive a correspondingly larger engine at these low pressures..

A couple of final points.

1. Just because a boiler is rated at 60 or 100psi it doesn’t mean it has to be operated at this pressure.

2. It is easy to get a boiler to make less steam, you just turn the gas valve down a bit, but almost impossible to get it to make more. So always err on the cautious side and if you're not sure if a particular boiler will be big enough for your engine then it probably isn't, so go for the next size up.