Fit­ting air horns

Canal Boat - - BACKCABIN -

QI have a set of air horns I want to fit on my boat, they don’t have a com­pres­sor so do I need to use a re­lay.

Gra­ham Beal­ing

ATONY REPLIES: If they have no com­pres­sor I sus­pect they are sim­ply “long trum­pet” con­ven­tional horns. Their vol­ume will be in­dica­tive of the cur­rent they draw with an ini­tial surge per­haps 20% higher than their rated am­per­age. How­ever a switch may be de­scribed it will still have a cur­rent rat­ing but it might be dif­fi­cult to ob­tain. If it came as a kit with the horns and no re­lay was sup­plied then one hopes it will be ro­bust enough for the job. The prob­lem comes with the length of ca­ble runs. For any given cur­rent the longer the ca­ble run the thicker the ca­ble needs to be. Oth­er­wise in­suf­fi­cient cur­rent will reach the horns so they may just give a quack when you push the but­ton or not work at all. If the horns are close to the bat­ter­ies then a re­lay may be ap­pro­pri­ate but if they are a long way away then a re­lay would de­mand two thin­ner ca­bles to sig­nal the re­lay and two thick ones to feed the horns. You might just as well use two thick ca­bles feed­ing the horn di­rectly. I can’t ad­vise on ca­ble size with­out the to­tal length of the pos­i­tive and neg­a­tive ca­bles plus the horn cur­rent. Once you know those two things you can find ta­bles on the in­ter­net to ad­vise the con­duc­tor cross sec­tional area you need. Try to keep the to­tal volt drop on the ca­bles to less than 0.5 volt.

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