This section is aimed at answering a few of the many questions we are asked at events around the country.
Britannia is an 8 N.H.P. engine - is that the same as 8 B.H.P. ?
No, N.H.P. stands for Nominal Horse Power and dates back to earliest days (late 1800's) of the manufacture of road going self propelled steam machines. At this time, items that needed to be moved by road were either pulled by man or by horse. The experienced hauliers of the day could look at a laden wagon and with their knowledge determine how many horses it would take to comfortably pull the load, taking into account the distance to be travelled and the terrain likely to be encountered. With the advent of the traction engine, it was logical to compare performance with that of a team of horses. One can readily imagine the builder of a traction engine making a claim such as, "My engine can easily do the work of 6 horses".
The calculation of power in this way was crude to say the least. It really gave no indication of the engines true power, as many vital aspects were not taken into account. No allowance was made for piston stroke, piston speed, steam pressure, gear ratios and above all the capacity of the engines boiler to sustain the work rate. Furthermore the introduction of compound cylinders (where the steam is expanded twice) meant that a different formula was required and exactly what this was has never been established. The various builders followed their own views on this subject.
Whilst today, it would be very easy to disprove the
N.H.P. figures quoted for most traction engines, the correct figure in an historical context must be that appearing on the specification issued by the manufacturer, particularly as these figures were used to determine the price to the customer. In finality, for anyone in doubt let me assure you that an engine rated at 10
N.H.P. is capable of an awesome power delivery and a prodigious work capacity.
This
article has been produced from an edited version of the Knowl Hill Rally 2001
booklet with the kind permission of it's author, Steve Dean.
How Long Does It Take To Get Up Steam ?
To reach a pressure at which Britannia will be capable of moving comfortably down the road (approx 130 psi) will take about an hour and a half although this time depends largely on the amount of air that is being drawn through the fire - if the wind is blowing strongly into the ash pan, the fire will burn better than if there is little or no wind.
The draught of the fire can also be increased by fitting a temporary extension to the chimney or, when a small amount of steam is in the boiler a "blower" may be used to increase the rate at which the air travels up the chimney.
Once on the move, steam will continue to rise towards the maximum of 200psi, this maximum is controlled by a spring loaded safety valve that will release any excess steam above this pressure. The steam pressure can be controlled by opening and closing the ash pan (increase or decrease the draught), adding more coal to the fire or the steam may be reduced by injecting cold water into the boiler (cools the water and also reduces the steam gap)
How Much Coal And Water Does Britannia Use ?
To get up steam to working pressure will use around 1 CWT (approx 50 kg) of coal, with the bunker filled with 5 CWT (approx 250kg) of coal and the tanks with 375 gallons (approx 1700 L) of water, one can expect to travel 25miles - although this depends very much on the terrain, lots of hill climbing means more steam will be required and hence, more coal and water will be used.
What Is The Winch Used For ?
Nearly all traction engines are fitted with a winch that can be run out through a set of rollers on the rear of the engine or through a pulley under the front axle. In the case of showman's road locomotives such as Britannia, it would have been used for pulling trailers in and out of muddy fields etc. where the ground would be to soft to simply drive out.
It is also possible to use the winch if the engine itself gets stuck in the mud, the rope would be run out and attached to either another traction engine standing on hard ground or a large tree before the winch is used to pull the engine out of the mud - However, care must be taken when selecting a tree for this purpose as the driver doesn't really want to be pulling the tree out of the ground rather than pulling the engine out of the mud !