Despite what you may have read by people who claim to know better, I have no doubt that horizontal boilers are far more efficient than vertical boilers and produce far more steam for a given size. The main reasons for this are:
1.The heating surface is smaller in a vertical boiler and decreases as the water level falls and more of the flue and cross tubes are exposed. In a horizontal boiler the heating surface remains constant, providing the main flue remains covered at all times (which it should be).
2.The hot flue gases tend to leave a vertical boiler more quickly, more heat disappears up the chimney instead of being transferred into the water. If you don’t believe this, try holding your hand over the chimneys of vertical and horizontal boilers of a similar size and compare the temperatures of each. You will find that the exhaust gases are much hotter in a vertical boiler than a horizontal one.
3.In a horizontal hot water boiler, the furnace (flue) is exactly where it should be, at the bottom of the water space, in full contact with the water at all times, allowing for maximum heat transfer and maximum water capacity.
I have performed considerable testing on my boilers, I think that I am the only manufacturer to publish steam production figures for my boilers, and the results consistently show that horizontal boilers make more steam than vertical ones. Horizontal steam boilers also have the advantage that the centre of gravity is lower, making your boat more stable. So unless you are building a model of an open steam launch, the prototype of which had a vertical boiler, I would recommend a horizontal boiler every time.