yamaha 225 4 stroke SHO prop ventilating

[motionman]

Motor and pontoon 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 and the boat slows down, 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. Do the SHO Super High Output 4-strokes have a system to vent the propeller to allow the motor to spin up faster when coming out of the hole? oh, the prop is a cupped stainless steel saltwater series. I can get pitch and diameter info but the max rpms are about right. I just cant figure out why the engine picks up rpms and the boat slows down.

[zenoahphobic]

Look, at first thought you've done all the things. If you even have this problem with 4 bladed prop, It must be your boat (boat/engine combination) and what comes to mind the boat too heavy, horrible shape maybe with to far forward centre of gravity , or even the hull too flat etc. Could even be the combination too powerful, simply engine eating up the water.

Need to know more facts about your boat?

[99yam40]

pontoon boats are a different type of boat and I would think they need different styles of props than normal boats.

Find a pontoon forum to ask your prop questions or a prop forum, maybe they can help you.

Could be the prop is cavitating not ventilating