Bow Thruster

There should be a manual on the operation and maintenance of the thruster system...I would locate that manual and proceed with checking any and all possible contributing factors...

http://www.lewmar.com/%5Cassets%5Cimg%5Cdataset%5CManual-200SVT_Thruster.pdf

800kW is a big lump. Contact the ship designers and constructors for advice and a service call out. Do this by telephone after a visual inspection that there is not a lump of rope, a blanket or an old mattress wrapped inadvertently around the propeller.

Changeover timing is incorrect for load, which may involve a mattress.

From what you are saying, it would seem that the bow thruster main motor is running OK but the problems occur when the pitch motor is connected.

If that is the case then that would tend to discount any fouling of the prop or the inlet/outlets, perhaps you need to look at the pitch motor being the culprit. If it is a hydraulic pitch control, then it may also be a jammed piston in the thruster.

Another possibility is that you are attempting to start the main motor with full pitch on the prop, and that may induce too much load.

Have you checked that the pitch control is not going to full pitch as you start the main motor? Manually wind the pitch to zero, set the remote pitch control to zero, connect it up and then attempt to start the motor. Check that the pitch remains at zero. If not, then the problem is with the pitch controller, if so, the problem may be with the pitch motor. This is probably more likely if it is a hydraulic pump rather than an electric winder as the electrics should not be operational with no pitch signal.

Tripping for what reason?. Motor overload? - or is it a fault in the 'alarm' circuit wiring, or a sensor switch adjustment.

Suggestions that it could be a prop seizure due to a mattress would be easy to check, but that could cause problems at zero pitch as well, and in any case at 800kw wouldn't the blanket just get chewed up and spat out?

I speak with little knowledge of bow-thrusters of this size, but rubbish trapped on the drive-propeller of my narrowboat is a common problem - so I carry cutters and hacksaws just in case.