What Is a Bow Thruster?
A tunnel thruster that produces sideways thrust through the hull near the bow - the bit of kit that gets the most thanks in tight marinas and the least maintenance.
A bow thruster is an electric, or sometimes hydraulic, auxiliary unit that pushes the boat sideways using a propeller inside a tunnel close to the bow. It is the most practical way to hold the bow up into the wind in tight manoeuvres, but it is only meant to be used in short bursts.
Its current draw is up at starter-motor level, so it needs its own battery or a heavy-duty supply cable. It is also a piece of kit that lives underwater: the anode, propeller and tunnel should all be part of a haul-out check, yet on most boats nobody looks at them for years.
Use and maintenance
Short bursts: use short bursts, not long presses; technique is what determines service life.
Weak thrust: voltage drop (a tired battery, loose connections) or a rope/algae wrap on the propeller; have the battery connections checked first.
Haul-out three-line check: tunnel anode, line around the propeller, and marine growth inside the tunnel.
Intermittent operation means the overcurrent protection has kicked in; that is normal with long presses, but a fault if it happens on short bursts. If thrust has gone weak mid-season, what is often called a 'dead motor' is usually just a loose terminal.