Ventilation

Ventilation

Ventilation usually comes from one of two sources: exhaust gas or surface air. The propeller generates thrust by pushing the water and the higher the density of the water, the more force it can exert on the propeller. If air or exhaust gets mixed with the water where it meets the propeller, the density of the fluid will be reduced and the generated thrust will drop. As this happens the resistance or drag on the prop decreases and it over-revs the engine. Common causes of the problem are having the tilt or mounting of the motor or drive too high, having a broken diffuser ring on the prop, or by having a poor hydrodynamic exhaust seal where the prop fits into the gearcase.