Eng Cap vs Prop Dia Imp

[PCA]

Eng Cap vs Prop Dia Imp



http://www.paddleducks.co.uk/smf/index.php?action=downloads;sa=view;down=347

An imperial version of Malcolm Beak's engine size versus propellor diameter.  I have had this graph in the book Scale Model Steamboats (page 68) for a couple of years and never understood the formula.  It has no power shown at all.  In Malcolm's August 1992 Model Boats article in the download section of this site the power is unreadable.  I had been trying squares when my partner finally figured it out that it was the cube of the prop dia.

[malcolmbeak]

PCA
You are of course quite right. I never had the chance to proof read entry in Scale Model Steamboats, and the same goes for the Model Boats article. However there is a clue. In the notes just below the graph, the first line states "Power needed to drive a propeller goes up as the cube of it's diameter"
Also (as you probably realise) as long as the units for the capacity and diameter match, the graph still holds true.

Malcolm

[PCA]

Hi Malcolm,

Thanks for your work.
I'm interested in (very) small steam units and your work has been the only information I have found that helped with sizing.  Now that you point out the text it should have been obvious and I probably read that text twenty times in the last couple of years.  I guess I'm a formula type of person and kept going straight to it and just skimming over the text.

Regards
Peter

[malcolmbeak]

Hi Peter

Glad it's of use to you. I've just realised that that aticle was written over 18 years ago! At least I've had no complaints during that time, and even a few nice comments.
When you say "very small" what size are you aiming for?

Malcolm

[PCA]

Hi Malcolm,

Very is a very subjective word
I have seen some incredibly (another subjective term) small engines on the web.  Some that will sit on a coin.  This isn't what I mean.
Basically I'm making boats with a hull length between 1 to 2 foot long.  A few years back I got rid of all of my standard sized RC gear and now only have mini to micro gear which makes it possible to make small models.
I have only made one successful steam model thus far (three engines in total, one that works).  It is 21 inches long and has a 1/4" Bore X 1/2" Stroke single acting single cylinder engine.  This gives a capacity of 0.02 cu inch or 0.4ml which is at the low end of your graphs.  The vertical boiler with 55ml of water gives 20 minutes run time using 20 ml of metho.

See http://www.paddleducks.co.uk/smf/gallery/3/2103-010910100604.jpeg

Presently I'm working through your August 1992 Model Boats article and trying to make a spreadsheet calculator to help predict the requirements for my next project so I don't have to make three engines and two boilers to make it work!
The current version gives what appears to be a low steam usage which doesn't correlate to the usage in my current model.  It could be just that some of my assumptions like steam wastage/leaks is much more than I think.
I will post a copy of the spreadsheet soon and perhaps you might see where any errors are.

Regards
Peter