Ok, so maybe it's not so much a myth as a disagreement. There's been some discussion in another thread about the advisability of using a fine pitch prop when running in as a means of reducing the load instead of/as well as reducing the diameter. One school of thought says that the fine pitch won't give enough airflow to cool the engine sufficiently and the other (OK, my) school of thought says it won't make any difference. The first school of thought says to use at least a 6" pitch while I've been saying a 4" pitch is more than sufficient. Both have actually been just guessing, one based on things that have been read and the other (me ) based mainly on having felt the airflow from a fine pitch prop.

So which is correct? Well, after much thought and many sleepless nights I figured why not just try it and get some actual real world figures? So I did.

I chose to use my old ST GS45 (because it was handy) and I had a couple of props that suited both it and the test. First up I fitted an APC 10x6, fired it up and got it close to peak at about 13,800 then backed it off to 12,000 to more closely simulate where an engine might be run in (actually a lot faster than I'd do it myself but I had to pick a figure that was well off peak). So 12K revs gave a head temp of 220F. I pulled the fuel line and shut it down.

Then I fitted an MA 11x4 and without touching the needle setting I started it back up and went to full throttle. I didn't touch the needle because I was interested in seeing how much difference in load there was. This gave right on 13,000 revs and the head temp was.....wait for it....220F!

But because it was now running 1000 revs faster I decided on one final check by reducing the revs to the original 12,000 on the APC. That gave a head temp of 200F but was right on the verge of 4 stroking rich. Which would have been perfect for running in anyway.

So a fine pitch prop cools just as well as any other.