I’m blowing my fuse…
QOur narrowboat’s bow thruster fuse blows if we run it for more than a few seconds. The motor has been replaced but we still have the problem. One suggestion is that the pitch of the propeller could be wrong. Any thoughts?
AI suspect you are feeding the bow thruster from batteries at the back of the boat or at least some distance away. First some electrical theory. Cables are sized in two ways: the conductor cross-sectional area and the current rating. Far too many people ignore the first of these. When current flows, some of the voltage gets ‘used up’ in pushing the current through the cable. On long runs with high currents, the loss (called ‘volt-drop’) can be a few volts, leaving insufficient volts to push sufficient current through the motor. The absolute maximum volt-drop I would like to see on a bow thruster supply is 1v, and 0.5v or less would be much better.
The resistance of an electric motor varies with its speed. So at a lowerthan-design voltage, the slower the motor runs, the more current (amps) will flow. This can and sometimes does burn motors out. Fitting a smaller propeller will reduce the load and allow it to turn faster (and so draw less current) but this will only cover up the underlying fault.
I think your cable size is too small. The volt-drop prevents the motor running up to speed so it draws too much current and the fuse blows. To measure volt-drop, connect a long length of cable to one voltmeter lead. Then connect the voltmeter, using the extended cable, between the battery positive and bow thruster positive. Operate the bow thruster and take a reading. Repeat for the negative side. I would like to see no more than 0.25 volt on either side, but up to about 0.5 will do: more than that, and you need larger cables.
The volt-drop for a given cable size can be calculated. Take the total run length in metres times the current flow (amps) times 0.164, and divide by the conductor cross sectional area ( CCSA) in millimetres. This will give you the volt-drop along the cable length. If it is higher than one volt in total ( 0.5 volt in each side) try a larger conductor cross sectional area in the equation until you get an acceptable figure.