Compensating backpressure in underwater exhaust systems

Applying an underwater exhaust aids overall noise attenuation and minimises the amount of exhaust gas surrounding a yacht.

Accurate predictions

An underwater outlet is, however, also one of the components of an exhaust system about which it is very difficult to make accurate backpressure predictions. Such predictions are important, as excessive backpressure reduces engine output and increases wear & tear.

Exhaust backpressure can be kept within acceptable limits by fitting an underwater scoop, which is a crucial component of MarQuip’s backpressure control systems. A common misconception is that a scoop creates a vacuum, sucking out the exhaust gas. In reality, a scoop simply reduces backpressure, although it can, at high speeds, create so much suction that exhaust gas is pulled out.


An underwater scoop functions by interrupting the flow of water along the hull, allowing the exhaust gas to be led out. Controlling and predicting the effect of this interruption, and the influence of the water column above it, is a delicate process – fortunately, MarQuip has just the right expertise in house.