A tunnel hull is a type of boat hull design that uses two separate sponsons or hulls narrowly positioned parallel to each other with a solid center section that traps air between the two hulls. This tunnel hull design is widely used in powerboat racing due to the amount of aerodynamic lift and speed associated with the hull design. The catamaran is often mistakenly identified as a tunnel hull, however, the catamaran has widely-spaced hulls with a commonly open center section. The tunnel hull design is also used in the construction of some kayaks, creating a much more stable platform for beginners.
Placing two hulls parallel to each other and spaced slightly apart, the tunnel hull creates a stable design that rides in a lessor amount of water than a comparably sized V-hull design. In powerboat racing, the tunnel hull is used to create a lifting force by trapping air under the center section of the boat's hull. This allows the boat to move through the water more easily than other hull designs, thus resulting in a faster boat in most cases. By enclosing the cockpits on each hull, the craft becomes much more aerodynamic than an open cockpit style of boat.
Commonly using an engine in each hull, the tunnel hull is capable of reaching top speeds that can create dangerous blow-over accidents. A blow-over is when the front of the boat begins to ride up and out of the water on a cushion of air trapped between the twin hulls. If the throttle is not eased off, allowing the hull to settle back into the water, it can eventually continue up past the point of no return. When this condition occurs, the boat will flip completely over and typically land upside down in the water.
Many racing teams that use a tunnel hull rely on an on-board oxygen system to provide breathing air to the crew when the boat suffers a blow-over. Oxygen tanks pumped into the crew's helmets can help sustain life until a rescue boat arrives on the scene of the crash. Most tunnel hull teams position a driver on one side of the boat and a throttle man on the other. The driver steers the boat and is responsible for locating turn buoys and remaining on course. The throttle man is responsible for accelerating and decelerating the motors, as well as monitoring critical engine function such as oil pressure and temperature.