I have a 9.9HP Mercury outboard with a 15" shaft on an 18' pontoon boat (limit on this pond is 10HP, many pontoons with small motors). The transom measures 15", which is why I bought the 15" shaft. But when I accelerate I get gurgling and what someone called cavitation, with little power being transferred to pushing the boat. I know 9.9HP is low, but the other boats don't seem to have this problem. Should I switch to a 20" shaft to help with this? I plan to buy a new motor with power tilt rather than manual as I have now. Thanks.