Well Ray that certainly is a simple question that is going to take a lot to answer!
First of all have a look at my own build thread here to give you an idea of what I have done with my current project:
http://www.scale-models.co.uk/showthread.php?t=1711Now having said that the design of your plant is entirely up to you and depends on what you want out of it, how complex you want it to be and how much money you want to spend.
In it's very basic format the plant will at least require the following:
A boiler with filling plug, pressure gauge, burner arrangement, level glass, relief valve and steam stop valve.
An Engine, prefferably a double acting twin which can be controlled from a servo.
An engine steam control valve connected to your speed control servo.
A fuel supply which could be coal or gas. The gas would usually be found in a seperate tank or can be supplied from the purchased bottle if your boat is designed to accommodate it.
An exhaust seperator. The exhaust from the engine goes through this where some of the moisture is condensed out and most of the oil content is removed.
An oiler
That will get you going but in it's simplest form has limitations, mostly to do with duration on the pond. The biggest concerns with the operation of the plant are usually to do with how long it will last on the water and are related to the capacity of the boiler, the capacity of the exhaust seperator and the gas supply.
The capacity of the boiler is limited to the size of boiler that you can get into your model and it's duration will depend upon the type of engine and how much you use it. If you do not have a means of filling the boiler under pressure you will have to vent it off before filling it and then start off all over again raising steam pressure from cold before you get going again. I have incorporated an on board feed tank, complete with pump which can fill the boiler manually but you can also get engine driven pumps and even an automatic boiler filling system which uses the engine driven pump and a servo controled three way valve.
The biggest single challenge that faces all model boat steam plants is gas cooling. Going back to your very early physics lessons, as a gas evaporates it's average internal energy falls and hence so does the temperature of the remaining liquid. As this temperature falls so does the evaporation rate and eventually the pressure of the gas supplied to the boiler falls and affects the boiler performance significantly. What eventually happens is that the boiler is unable to supply enough steam to supply the engine and the boiler pressure falls off until such point as the boat stops.
I have done a few things to help off set this effect as follows:
1) I have incorporated a 'T' connection in the gas line so that I can raise steam initially on the bank from an external bottle thereby delaying the cooling process until the boat is actually in the water.
2) The on board feed tank and feed pump means that you do not have to fill the boiler with a complete slug of cold water and have to raise steam again from cold again.
3) I have fitted a Cheddar models gas control valve which shuts the gas to the boiler down to a pilot flame when the boiler pressure has reached a predetermined level, thereby dramatically saving gas consumption.
4) I have fitted a heat transfer sink between the seperator tank, which gets very hot, and the gas tank, which gets very cold, to keep the gas tank warm. In conjunction with a gas regulating valve I hope that this will maintain a more regular pressure from ther gas tank.
You can consider all or non of the above depending on how complex you want t make your own plant.
After a few sea trials I have determined that my model will last approximately half an hour of significant use before I need to bring it into the bank. At that time I will top up the boiler from the feed tank and empty the seperator tank. If I want to take it out I can raise steam pressure again from an external gas bottle and then refill the on board gas tank but this is not completely necessary as the gas will last for quite a bit longer than this.
Anyway these are all a few ideas for you to think about as you plan your own model. The problem with most model boat steam plants are related to issues that have not been considered in the planning stage and so lead to dissolusionment and unrelable operation at the pond. Just how far you go to deal with these things is up to you but you must be aware of the limitations of your own plant and then operate it accordingly.
Let us know what ideas you are currently considering and we can advise accordingly.
