Exhaust finds its way outward from the engine in three different ways: through the hub, over the hub, and nowhere near the hub. (The hub is the part that connects the propeller to the engine.) While largely invisible to the casual observer, the differences in exhaust systems impact the performance of your boat and the quality of your ride. To answer the question, “Thru-Hub Exhaust: What is It and Why Do I Care?” we offer a summary of boat propeller exhaust systems.
In thru-hub systems, exhaust pours from the engine through a round barrel in the prop and out the back of your boat. This way, the exhaust doesn’t make contact with the blades of the propeller. Without disruption from exhaust, the water makes excellent contact with the blades, delivering good acceleration and a nice hole shot. Thru-hub systems also contribute to a quieter ride. Underwater, thru-hub exhaust looks like a jet stream, or wedding veil, pouring out the back of the prop.
In propellers with over-hub exhaust systems, the blades are attached directly to the smaller tube that fits over the propeller’s drive shaft, eliminating the larger exhaust tube. These props are often used for reaching top speeds, but your hole shot can suffer when the exhaust pours over the propeller blades under acceleration. Lighter boats can run faster with over-hub props than thru-hub props. In heavier boats, however, this can cause damage from cavitation – pock marks – on your propeller from the impact air bubbles.
In non thru-hub propellers, exhaust runs on top of and behind the blades. These boat props are used for inboards using shaft driven propellers, sterndrives using through hull exhaust, and on some outboards that don’t route the exhaust through the lower unit torpedo. (Source: Michigan Wheel)
More About Boat Propeller Exhaust Systems:
Exhaust: Just like the tailpipe on your car, exhaust created by the engine’s combustion needs to escape, or the whole system will overheat and shut down.
The Hub: The hub, as the name implies, is the inner core of the boat propeller. It slides over the propeller shaft and is the point onto which the blades attach. Hubs come in a variety of configurations, including metal, rubber, fixed, removable, and interchangeable. Read more about hubs for boat propellers.
Ventilation: This occurs when exhaust gas or surface air is drawn into the blades of the boat propeller. This causes the boat to lose speed and RPMs to climb rapidly, leading to excessive slippage. Slippage is essentially the difference from the theoretical pitch to the actual pitch. You will want the minimal amount of slippage for top performance. Read more about ventilation here.
Cavitation: Cavitation occurs when there is an area of low pressure caused by the inability to move through the water and then the water around the boat propeller begins to bubble. The bubbles collapse when they reach an area of higher pressure around the blades. Cavitation and changes in pressure can result in erosion on the face of the boat prop blades, ventilation and/or slippage. Read more about cavitation here.
For more helpful tools to understand how your boat prop works, link to SavvyBoater’s Propeller How-To Guide, a simple guidebbok, with diagrams, to walk you through propeller basics.
Illustrations courtesy of Michigan Wheel.