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I am curious about the exhaust at idle on an F70. I was running my F70 with muffs out of the water and noticed very low noise at idle. According to the F70 hot sheet the exhaust exits above the waterline after passing thru a "labrynth" to cut down on noise. I would say it seemed about 50% out the above water port and about 50% out the center of the prop. I would think this would be an advantage for *****ing. At what RPM does it start to all flow out the prop center?
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Would be nice if Yamaha would provide some theory of operation information but apparently their modus operandi is to not do so.
The upper exhaust passageway is an alternate path for exhaust gases to exit the motor if and when exhaust gas flow out the main exhaust system is restricted by the presence of water. A labyrinth system is used to keep exhaust noise to a minimum. It is for operator comfort, not enhanced *****ing, if by enhanced *****ing the thinking is that a quieter motor will catch more fish.
Think of a motor at idle or just into gear. The water level blocking exhaust flow will be high in the mid-section and the exhaust gas pressure will be low. In this situation more/all of the exhaust gases will exit the alternate exhaust.
Once the motor is in forward gear and the boat and motor are moving forward somewhat then the water level will drop, exhaust gas pressure will be higher, and water flow over the propeller will be helping to extract the exhaust gases out through the gear case and propeller hub.
There is no scale that depicts how much exhaust gas flows out of the two exhaust ports as a function of boat speed and engine RPM.
Here is a different illustration depicting the F70 exhaust system.
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