Smaller diameter tunnels have a number of advantages over larger tunnels.

1. The tunnel can be placed further forward in the hull. A tunnel placed further forward increases the distance between the thruster and the pivot point of the vessel. The resulting increase in leverage produces greater turning forces to move larger, heavier boats. In planing hulls, placing the tunnel further forward, in combination with the tunnel being as much as one quarter the size of others, is sometimes enough to raise the tunnel out of the water completely when the vessel is in motion, resulting in an increase in hydrodynamic efficiency and a decrease in fuel costs.

2. In displacement hulls, hydrodynamic efficiency is increased due to a smaller aperture reducing direct obstruction to water flow caused by the aft wall of the tunnel. A smaller tunnel opening in the hull increases efficiency by reducing disturbance of laminar flow. These advantages result in reduced fuel costs for the vessel during voyages.

3.  A smaller overall system reduces fuel costs for the vessel. At about 8 lbs per gallon, water is heavy. Less water filling a smaller, shorter tunnel is a major advantage due to reduced weight and greater bouyancy. For instance, an 8" tunnel 3 ft long will hold less than 8 gallons weighing about 63 lbs. A 16" tunnel 6 ft long will hold about 63 gallons weighing about 501 lbs. A 20" tunnel 7 ft long will hold about 114 gallons weighing about 914 lbs. A 24" tunnel 8 ft long will hold about 188 gallons weighing about 1504 lbs. Pipe also tends to get heavy as the diameters increase. For instance, in schedule 40 steel pipe, an 8" tunnel 3 ft long weighs about 85 lbs; A 16" tunnel 6 ft long weighs about 500 lbs; A 20" tunnel 7 ft long weighs about 860 lbs; A 24" tunnel 8 ft long weighs about 1370 lbs.

4. Smaller tunnel sizes allow the installation of more than one tunnel in larger vessels. This can dramatically reduce the cross-sectional area of a thruster system resulting in reduced drag and reduced weight and increased bouyancy.

5. Smaller tunnels improve the structural integrity of the hull by reducing the amount of hull that needs to be removed during installation. A specific instance where this becomes a critical factor is in a vessel that is built with structural frames placed close together. In most cases, Shipwrights' tunnels are the only tunnels small enough to fit between these frames maintaining the original structural integrity and yet providing sufficient HP to provide adequate side thrust. This occurs frequently in vessels of aluminum, steel, and wood construction.

6. Smaller tunnels allow vessels of shallow draft to benefit from bow and stern thruster installations. Furthermore, many lightweight, modern, motor and sail vessels have minimal submerged hull in the bow section, allowing only enough space for a small tunnel. Catamaran motor and sail vessels are severely disadvantaged by having to install a large tunnel in order to have enough power to provide functional side thrust. Many commercial river vessels and house boats are severely limited in draft and can only use small, high-powered tunnels of the type Shipwrights manufactures.