How does a deeper tunnel affect my top speed performance. I have a 225 Mercury outboard on conventional wood tunnel boat?
A tunnel boat is like an airplane wing operating close to the ground – or, in what’s called “ground effect”. Close “ground” proximity increases lift coefficients, making more efficient lift/drag ratios for the craft. For example, reducing the depth of a typical tunnel from 10 inches (at the transom) to 8 inches could improve the aerodynamic lift coefficient by 6% – which means something like 12% more aerodynamic lift at higher speeds. This means less lift required by the sponsons, and an ultimately faster top speed (in your case 2-3 mph in the 85 to 90 mph range). Remember that with the tunnel roof closer to the water, there’s also more risk of water interference, and intermittent splashing and increased drag in heavier water.
