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→Bow Thrusters
Tunnel diameters run 5-12 inches. In general smaller tunnels are better because they accelerate the water more. They can be placed further forward. They have greater structural integrity. A smaller opening also reduces drag from the aft wall of the tunnel, and disrupts laminar flow less.
Consult your designer about putting an eyebrow fairing in front of the tube, or a scallop behind the tube, to reduce drag. Don’t put fine grids over the openings, as they clog up. Paint the inside of the tunnel with [https://en.wikipedia.org/wiki/Anti-fouling_paint antifouling paint].
Bow thrusters come with either single or twin propellers. Each has its advocates. The efficiency of one versus the other is unclear. Twin propeller designs contra-rotate and are more complex. Propellers are usually square-bladed, made of Dupont Delrin® or Zytel® plastic, and designed to be efficient in both turning directions.