I have replaced the water pump and rebuilt the carbs on my 1984 50 hp Mercury. I am pumping water with force now while ideling, but when I get up to speed it starts blowing steam and the motor seems to start skipping. I leaned over the back of the boat and it looks like the prop is bearly under the water. I have tried all the manual trim levels but it doesnt seen to make a diffrence. Off hand I would think I need a longer shaft on my engine, but it is a good 10-12 inches below my boat now. The water holes are under water. Any Suggestions?