Sleipner, Vetus, and Side-Power bow thruster diagnostics

Marine bow thrusters come in three power classes: DC electric (up to about 25 hp, most common on 30-60 ft), hydraulic (25-100 hp, larger yachts), and proportional variable-speed (Sleipner SEP/SEP proportional, on newer 45 ft plus). DC installation basics: batteries dedicated to the thruster or a hefty parallel bank, thruster inrush current is enormous (300-800 amps for a 6 kW unit at startup). Wire size per manufacturer: 4/0 minimum for most 6-10 kW thrusters, 2/0 for smaller. Duty cycle: DC thrusters have a hard duty limit, typical 3 to 5 percent (about 3 minutes on per 100 minutes). Ignoring this cooks the motor. Modern electronic controllers time out and warn; older units silently overheat. Common failures: (1) Thruster runs one direction only, failed contactor OR broken control cable. Test at the panel with a multimeter. (2) Weak thrust in reverse but strong forward, damaged propeller blade or fouled tunnel (barnacles reduce tunnel efficiency dramatically, clean at haul-out). (3) Loud grinding, gearbox failure or bearing wear, replace the leg. (4) Sudden loss of thrust mid-maneuver, thermal cutout tripped, wait 20 minutes and it resets. Anodes on the leg and tunnel, inspect every haul-out, replace when 50 percent consumed. Hydraulic thrusters share the vessel's HPU (with the steering or crane), check the fluid level and filter with the HPU service schedule.