Hi Allen I believe the first number is the diameter and the 2nd is the pitch. I think the 2.388:1 is the gear ratio for the command thrust lower unit. We were out yesterday also and rechecked my speeds. I had 13.8 mph at 3300 rpms and 21 mph at 5200 rpm which was wot. I thought that the rpm and speed were both higher in the past. The trim did not seem to affect performance much.Just back from a nice cruise on the ottawa river - found these #s on the prop - 13x31 / 2.38 8:1 it has 4 propellers -Could anyone explain what if anything these #s mean? Also had these results - at 3500 rpm 14.7 mph - WOT 5200 rpm 27.3 mph with my wife and Dog with me / 1/2 tank gas and no wind -as a new member will post few pictures of boat as soon as I figure out how to do it