I read some place that the most efficient prop was one which had a pitch of about half its diameter. I also have the general idea that large high pich props turned slower are more efficient than smaller low pitch props turned fast. I am presently flying an old style long-stroke 15 diesel on a control line airplane. I have tried 8 x 6, 9 x 6, 9 x 8 and 10 x 5 props. So far the 9 x 8 is giving the best performance. with the 9 x 6 not quite as good. Hopefully someone who actually knows something will answer your post.