Strath Steam manufacture HMTU boilers horizontal multi tubular underfired for power generation and vertical firetube boilers VFT for marine applications so if your heading toward owning one, this covers what they are about. Firetube boilers can fall into two categories which is whether they have a wet back fire box or a dry back. This refers to their design if the fire box has water surrounding it or if it has a refractory lined and insulated firebox. We have only ever made dry back VFT boilers as they are the cheapest to build, seem to work very well and are quite economical to run.

Our VFTs consist of the main outer shell which has 3 large plugs arranged around the bottom just above the tubeplate so one can remove them to aid cleaning  and inspection. Also right on the level of the bottom tubeplate is the blowdown outlet which can be a socket or a flange mount. This is were the blowdown valve is attached which must be a full bore quick acting valve so one can rapidly dump a heap of water fast while the boiler is under pressure to scavenge any sediment off the bottom tubeplate.
Be warned it makes a rather loud noise. There is one other socket at this level which is the water inlet point.

The next 2 sockets we should mention are for the gauge glass mounts and close to the top tubeplate are the rest of the sockets which comprise of the steam outlet for the engine, a dedicated pressure gauge outlet, a dedicated safety valve outlet and then a couple of others for your whistle and water heating device.
In either end of the shell is welded the 2 tubeplates which can have varying number of tubes fitted into them depending on the size of the boiler and if the tubes are copper they are expanded in or if they are steel tubes then they are more than likely to be welded.
The firebox is attached after the pressure vessel is finished and is lined with a castable refractory cement while on the outside has a layer of insulating material  covered with a sheet of stainless steel.
The firebox as one can imagine has 2 openings in it the larger being for stoking while the smaller one at the bottom is for air control and ash removal.
The other important component of this section is the fire grate which although primarily serves the purpose of supporting the fire has many considerations. In regard to supporting the fire it is quite important that, depending on the type of fuel which is being burnt that the grate is positoned so there is sufficient room for combustion when there is a respectable load of fuel dumped on it and the flames just lick the under side of the bottom tube plate.
The slots and bars of the fire grate also serve multi purpose jobs the slots allowing air to get up into the fire to support combustion and subsequently carry the hot gases up the  tubes, while at the same time allow the ash from the burnt fuel to drop through into the ashpan.
The bars however support the fuel and firebed but it is important that they extend deep enough into the ash pan area so the incoming air has a continous cooling effect upon them so they don't burn out or melt .
Which brings us to the importantance of them being fitted properly so the widest section of the bar is at the top and then they taper away to the bottom so as to be self clearing although coal can still clinker and block the slots , leading to a lousy fire and an over stressed grate.
We supply a comprehensive set of instructions and drawings on the correct method of intallation so you don't set your boat on fire.

The Horizontal multitubular underfired boiler used in conjunction with our power generating engine is of a similar construction but lies flat with the fire being positioned under the boiler and all the tubes are submerged thus giving them a longer life as they are not subjected to differences in temperature. The fire impinges on the bottom of the shell and then the smoke, flames and hot gases pass to the back of the firebox via a hood, then through the tubes in the boiler and finally out through the smokebox and up the stack.
The construction differes considerably when it comes to the fire box though as that must be bricked up on sight and the boiler is nested into it after the grates have been laid and the blow down pipe fitted.
The boiler comes with the front firebox plate attached. incorparating the fire door and ashpan/aircontrol door. This front plate must have castable refractory poured into it before it can be installed.
The package comes with a detailed set of intallation intructions.