Boat motors with exhaust over propeller

Above propeller

— in most cases, this term means exhaust into the water through an anti-cavitation plate above the propeller; such systems are simpler and cheaper than exhaust through a screw, but do not improve performance. And in low-cost models of boat engines, exhaust into the air can also be used; such systems have an even simpler design, but are not very convenient due to noise and fumes.