Due to a design flaw on the Westerbeke 55B with non stock larger
alternator, the crankshaft pulleys are miss matched and can cause failures
of belts, crankshaft damper pulley, coolant water pump bearing and
alternator bearing.

Problem.

Estrella’s Westerbeke 55B has three fan belts. The forward belt runs the
raw water pump, The inner two is a dual belt setup that runs the 150 amp
alternator and the cooling water pump. If not properly tensioned, the
alternator fan belts will slip and screech especially when cold.

Estrella has had ongoing issues with tensioning the alternator fan belts.
At one point, an over tensioned belt ripped apart and due to my diligent
engine condition checking I found that belt missing before losing the
second belt.

Bottom line: There was something very wrong with the alternator belt
tensioning.

What I found.

This Westerbeke 55b has three pulleys on the crankshaft. Two are one unit
and that unit is bolted to the third which is the ‘dampered’ crankshaft
pulley keyed and bolted to the crankshaft. Upon inspection, I found that
the ‘pitch’ of the pulley (portion the fan belt fits into) was different
between the crankshaft damper pulley and the bolt-on pulley set. A rough
measurement showed that it was 1/8 of an inch off between where the inner
belt and the middle belt sat in the pulley (alternator fan belt matched
set).

1/8 of an inch does not sound like much, however, it increases the
circumference on the fan belt wrap by at least 1/3 of the increased
circumference or 0.5 inches.

Westerbeke is very clear that the ‘deflection’ of the fan belts should be
no less than 3/8 inch and no more than 1/2 inch. Adjusting the inner belt
to 3/8 inch allows the outer belt to deflect at least 7/8 of an inch ( my
measurements were closer to 1 1/4 inch)

Clearly the middle belt was not tight enough and the inner belt would do
all the work. Not a good design.

As I further inspected the pulleys I discovered that the crankshaft damper
pulley had failed. The rubber damper had ripped away from the pulley and
the pulley freely turned on the crankshaft damper pulley hub. This failure
was due to the crankshaft damper pulley doing all the work when the 150 amp
alternator was working hard. It simply ripped the damper appart.

The result was a screeching middle fan belt as it could not be properly
tensioned, As the load increased on the alternator, the fan belt would slip
and screech. (often just after starting the motor when the starter battery
had been partially discharged)

Solution.

The crankshaft damper pulley had to be replaced. The outer two pulley set
was in good shape but did not match the damper pulley pitch. Replacing the
damper pulley without modification for pitch would result in the same
problem occurring some time in the future.

I chose to purchase a new damper pulley and have a machine shop trim the
pulley pitch to match the outer two pulleys. This seemed the most
straightforward way to solve this problem.

Chatting with the machinist, we decided that the stock crankshaft damper
pulley was spec’d for a metric belt and the outer two pulleys were spec’d
for inch belts. The physical diameter of the pulleys were the same. The
pitch was different.

Either Westerbeke or the OEM Isuzu miss-engineered the pulley set and
could never work. If the manufacturer spec’d the outer pulleys for metric
and specified metric belts, this would not have been a problem. The
crankshaft pulleys on this engine were never correct and were shipped this
way from Westerbeke.