Larry,
Earlier in this thread I calculated the 61,250BTUH like you did but then took out 10% for shale, rock, bone and 80% boiler efficiency. Total, a Whopping 44,100BTUH!
The 5 pounds per hour has been pretty much verified by weighing the ash and calculating the amount of coal it would take at 18% ash. In cold weather it comes out just about perfect. In warmer weather 20% ash is closer, and that makes sense because more idle time makes the ash heavier making the ash a higher percentage of the coal used.
The amazing thing is that lowering the BTUH of one of these boilers does NOT effect the way the coal is burnt. It just burns less coal but the fire stays the same size.
I've tried this with other stokers and it just doesn't work. I think this has a lot to do with the "hand fired" nature of this type of boiler.
-Don