I do remember from the "old days" that we compared prop load for comparable prop sizing as double the diameter and add the pitch. So a 10 x 6 would = 26. A 10 x 8 =28 so a 11x4 would be a pretty equal replacement prop. So you could go up a few points and make it a 11 x 6 for a 28 and still be in range. Most motor manufacturers suggest a range and you can figure out just would would be approiate from that. The forumula came out of RCM and Clarence Lee I think.