My 1991 MKII calls for a 13x10. I couldn't pass 2800rpm and under the best conditions, was barely able to get 6 kts. I also had more prop walk than I was happy with. I changed to a 13x12 and am now able to get up to 3200rpm, with the ability to reach 6.7 kts under good conditions. There is now controllable prop walk that I am able to utilize while docking. But, no two boats are identical as no conditions are ever the same. Good Luck. I hope you find the right combination for your boat. Happy New Year.