I have a 25hp s/shaft yamaha[with a larger anti-cavitation plate installed] on a 3.6m inflatable [inflatable keel] with a 15" transom .this engine unfortunately ventilates @ 1/4 throttle.The prop I have is alum 9 7/8 x 10 ! I also have a 9.9 x 12 prop but neither work without ventilation.I need...