Boating New Zealand - May 2018

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mechanism. he engines themselves also rest on four large rubber

engine mounts, further isolating them from the hull. here was no

dedicated grounding straps connected to the engine blocks, which

were therefore isolated from other metal parts of the hull and the

sea water outside – except when there is water in the bilge.

he bilge pump has a standard loat switch, which only kicks in

when there is 5cm depth of water, and then pumps until the level

drops to about 2cm. he trouble is that the lowest point of the

sump is about 4cm above the bottom of the bilge. Over time a small

amount of water collects in the bilge and this makes contact with the

sump. We now had an electrical circuit from the battery, through the

engine and sump, through minor imperfections in the paint coating

on the sump which allowed contact with the salty bilge water, to the

hull and out to the ocean.

Because the sump-water connection was imperfect, the resistance

at that point caused current to low and, over a period of under a year,

corroded a small round hole through the lowest point of both sumps.

his was now an expensive repair, since like most boats, the sump can

only be accessed by lifting the entire engine out. And both motors

developed the same problem within a few months of each other.

Repairing the sumps was the easiest part (after lifting the

The question was,

how and why did

this occur, and why

on such relatively

new engines?

OPPOSITE This is a major oil leak, but finding its source

proved tricky – and eventually illuminating.

TOP LEFT The pin-hole in the sump caused by

electrolysis.

TOP RIGHT The welded repair, the stripped sump, and

the coat of fibreglass.