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My 22 ft 2014Sanpan with a 225 Yamaha SHO seems to have a problem with prop cavitation/ventilation.
Boat was bought new in early summer of 2014. As engine is accelerated pass about 1500 rpm a slight vibration starts with a steady speed increase with higher tach speeds, at 3200 rpm speed is about 15 knots. Accelerating to higher rpms the boat really begins to accelerate to a top speed of about 40 knots at 5700 rpm. Here is my problem, as I slow down back to 3200 rpms the boat will do about 20 knots. As you continue to back off the throttle to about 2800 rpm the boat slows smoothly until the vibration starts and the engine will suddenly increase tach speed about 400 rpm, then your back at 3200 rpm at 15 knots. It seems like at speeds less than 20 knots the prop slips like it is getting air and higher rpms the prop starts to grab better. Sometimes it is subtle and then at others quite pronounced. Once while in a turn the engine was turning 4000 rpm with a speed of only 15 knots. I did not buy the big motor for speed but more so for power to pull skiers etc. I have taken the boat back to the dealer several times where they have lowered the motor on the transom to its lowest position (outboard has a 20 inch shaft) and tried a four bladed prop non of which seemed to help. They have now added a five inch extension to the motor and reset the transom height such that the ventilation plate is about level with the bottom of the pontoon. he problem is still there. I need some help, I love the boat but I feel like at that perfect pontoon boat speed the engine is having to turn to many rpms.