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The purpose of tunnels remains the same as they did on my old boat: to help reduce the draft. Unfortunately, that goal is at odds with the laws of physics, which hold that a certain amount of propeller blade area is necessary for a certain horsepower, speed, and boat weight. If you don't have enough blade area, cavitation is the usual result. Cavitation manifests itself as noise, vibration, prop and rudder erosion, and degradation of performance, with increased fuel burn and reduced speed and range.

So how do you achieve the necessary propeller blade area and avoid all the problems? Generally, a larger prop diameter is more efficient than a smaller diameter, so bigger props are the obvious answer. Putting physics aside for a moment, that extra diameter has a practical downside: It aggravates the draft problem, since propeller tips are the low point on most boats. There's only so much space for props between the bottom of the boat and the bottom of the sea. We can't move the bottom of the sea, but we can move the bottom of the boat. This is accomplished with prop tunnels.

The sides of the tunnels are parallel to the centerline of the boat, and parallel to each other as well for their entire length. The sides do not converge either forward or aft. Looking closely at the black lines drawn athwartship, you can see that at the forward end of the tunnel, the shape of the tunnel roof begins very flat, and then transforms into a progressively tighter radius as we move aft toward the transom. At the point of the propeller location, very near the end of the keel on this example, the tunnel reaches its smallest radius. Aft of this point, from prop to transom, the shape can be maintained as in this Viking, or might converge just a bit to create a slight "nozzle" effect on some boats.

In addition to having the right shape for the tunnel, it is important to have the cleanest, most undisturbed water flowing into and out of the tunnel. This is done by avoiding discontinuities in the hull bottom and locating "bottom clutter," such as thru-hulls and transducers, out of the path of the water flowing into the tunnels. Shaft strut and rudder palms should be mounted flush, in indents molded into the tunnel, if at all possible.

Finally, there's the question of how close the prop should run to the tunnel walls. On installations without tunnels, the guideline is to allow 15 percent of the prop diameter (4 inches on a 30-inch prop) as clearance between the hull bottom and the prop-blade tips, to avoid excessive vibration. With tunnels, this can easily be tightened up to 5 or 10 percent, and some designers have successfully gone to nearly zero without undue vibration. Obviously, this allows a few more inches of prop diameter, for more blade area and efficiency, with resultant speed, range, and efficiency gains and without added draft.

So there you have it: reduced draft and fuel usage, more speed and range, less cavitation and vibration. Tunnels aren't necessary for all boats, but when draft is a problem, they can be the answer.



This Viking enclosed bridged sport fishing yacht is 18" long.  It was commissioned for a 2017 Christmas present by a group of executives of the world’s leading designer and manufacturer of mobility products.   If you like our work and would like to have a replica of a Viking boat, please let us know at  Thank you.