I’m blow­ing my fuse…

Canal Boat - - Back Cabin - TONY REPLIES… GLYN WHITE, via email

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.

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