A "keyhole" boiler designed and being built by John Davis of Brisbane.

A boiler is just a very efficient kettle where water is boiled. In a kettle the steam escapes through the spout but in a boiler the steam generated is contained at pressure for use in the engine.

The most simple kettle is a container on the fire. In Australia the traditional container is a "billy" More information about "boiling the billy" is available at:

 http://www.gemfields.com/static_html/138.html

In a billy the fire surrounds the water and the heat of the flame boils the water. It's the same in a water tube boiler where the water is contained in lots of tubes surrounding the fire. This means that much more of the water is exposed to the fire and therefore more steam is produced faster from the same amount of fire.

Most steam trains have "Fire Tube" boilers whereby the water is contained within the boiler and the fire is in a firebox that exhausts through tubes to the smokestack. I.e. the fire is in the tubes

The keyhole boiler has a grate area of about 2 square feet, a heating surface of around 24 square feet and will generate in excess of 200lbs of steam per hour on coal or wood.