Interesting topic, however, I must disagree with Greg on one point: -
The thickness of the piston is always subtracted for swept volume, and as Derek correctly says the piston rod volume is also taken into account on engines where it matters.
The piston thickness is NOT
subtracted for swept volume.
The overall cylinder length takes the piston thickness in to account as follows: -
Cyl length = Stroke + piston thickness + top steam clearance + bottom steam clearance.(see note below)
Therefore swept volume is: -
= Stroke x Piston area in one direction... and
= Stroke x (Piston area - Piston rod cross section area) in the other direction (taking account of the piston rod area).
Total swept volume = sum of both the above.
Note... in practice the O/A cylinder length would also take account of the length of any location stubs on the inside faces of the cylinder covers, however these would not have any bearing on the Steam Swept Volume.
In most small model steam engines the area of the piston rod is generally ignored since the small resulting error is usually outweighed by other losses, however it must certainly be considered if it is a significant value, (eg. in a trunk piston design).
On the subject of boilers: -
The Cheddar boiler has a flu dia of 28mm OD (26mm ID) and uses a 22mm dia burner.
It has 6 x 1/4" dia cross tubes.
Rough calculations give Flu Heated surface area of 19.294 sq ins.
With an additional heated surface area for the 6 tubes = to 4.823 sq ins.
The Maccsteam boiler has a flue dia of 35mm ID and uses a 28mm OD burner
and 6 x 5/16" dia cross tubes.
Rough calcs give Flu Heated surface of 24.489 sq ins (which is approx 27% more than the Cheddar).
With an additional 7.6525 sq ins heated surface area for the cross tubes (which is approx 59% more than the Cheddar)
I think you will agree with me that this large difference in available heated surface area coupled with the much larger burner results in the MaccSteam boiler being an altogether different animal.
My ACS 3 1/2" horizontal has 5 rows of 2 x 5/16" OD cross tubes ( 10 tubes total) in a 35mm flu and also uses a 28mm burner.
I think the answer is HEATED SURFACE AREA... no point firing a lot of heat into the flu if the available surface area can not transfer it to the water, the extra heat just passes straight through and does not do any work and is just wasted ... From Ian's description of his set up... Funnel exhaust is very hot... it would seem that this is the case.
Quite what Ian can do to improve his set up is not a simple question.
Slowing the combustion gas flow (up to a point) can help but taken too far and he will end up with a flu full of unburnt gasses (with no oxygen available) which would reduce the efficiency of, or even extinguish his burner.
I do not think MORE heat is the answer.
So... how does he increase the available heated surface? is really the question.
Hope this is of some use.