Boating New Zealand - May 2018

86 Boating New Zealand

T

he last time we took our big twin-engine diesel

powercat out for an extended run, we noticed the

oil pressure on the port engine dropping slowly

on the run home. Checking the bilge, we noticed

a large quantity of oil in there and realised we had

a serious leak. We made it home safely, cleaned up the oil and

unsuccessfully tried to locate the source of the leak.

Since we could not ind the cause of the problem, we booked

the boat for a haulout. For a variety of reasons, it was nearly three

months before we were allocated a space on the hardstand. When

we started up the starboard engine to motor in we were horriied

to discover the same problem on that side – low oil pressure and

a bilge full of oil. Luckily it was a very short trip to the boat lift, so

we topped up the oil then quickly motored over.

Once we had cleaned the mess and lifted the engines out we

discovered an almost perfect pinhole had been eaten through

both sumps in almost the same location. hese Volvo D4-260

engines are only a few years old, and the sump pans were in

other respects largely free of corrosion. When we showed them

to Lachlan Trembath of Volvo agents Ovlov Marine, he said it

was the clearest case of electrolysis he had seen in a long time.

he question was, how and why did this occur, and why on such

relatively new engines?

Looking at the engine installations, at irst glance

everything looked as it should be. A stainless grounding strap

made a circuit between the drive shaft and the aluminium

part of the hull, and it looked like there was a good path from

the battery negative (which is connected to the engine block)

through the grounding strap to the hull, where the sacriicial

anode on the outside makes contact with the surrounding sea

water. his is referred to as bonding, and according to Warren

Tait, General Manager of marine electrical specialists EAL

Electric, in most cases all the major metal components of the

boat should be bonded together. Anywhere there is a poor

connection, especially between diferent metals like the cast-

iron engine block and an aluminium hull, a current can occur

which will rapidly result in corrosion at that point.

A closer examination revealed that the grounding straps

were installed on the hull side of the lexible couplings on the

drive shafts, and there was nothing on the engine sides. Since

these couplings are made of a strong polymer plastic, it forms

an insulator between the engine blocks and the rest of the drive

with NORMAN HOLTZHAUSEN

DIYBOATING

An investigation into an oil leak led to a surprising culprit.

Stumped by Electrolysis