Cruising World - June 2018

cruisingworld.com

82

june/july 2018

cruisingworld.com

82

MONTHLY MAINTENANCE

U

nlike that of an

automobile, via a closed

cooling system and a heat

exchanger, your boat’s diesel

engine transfers its waste heat

from coolant to seawater, rath-

er than air. The heat exchanger

is the radiator of your auxiliary.

While there are several

different designs, the most

common on cruising-boat

auxiliaries are the tubular

variety. The external shell looks

a bit like a pipe; housed within

it is a series of tubes that re-

semble drinking straws, which

are surrounded by coolant.

Seawater passes through the

tubes, absorbing heat from the

coolant. It’s a simple arrange-

ment that allows engines to be

fi lled with coolant rather than

seawater, thereby reducing cor-

rosion and increasing effi ciency.

Engines that are cooled directly

by seawater must operate at

a lower, and therefore less

effi cient, temperature to

prevent salt from precipitating

and accumulating within the

cooling passages.

As simple as heat exchangers

are, they are far from foolproof.

The mixture of hot coolant

and cold seawater makes for

a stressful life where metal is

concerned. Add to that the

potential for corrosion, and

the likelihood for failure goes

up considerably, particularly if

they are ill-maintained.

Many, but not all, heat

exchangers are equipped

with sacrifi cial anodes, which

must be replaced regularly.

Their consumption rate varies

with conditions, use, water

temperature and salinity. While

many users simply replace them

seasonally, it’s best to initially

remove them monthly for

inspection, to determine their

rate of consumption for your

use patterns. In some cases,

they may need to be replaced

annually; in others, more often.

One of the more common

maladies where anodes are

concerned is the accumulation

of depleted pencil zincs in

the chamber at the end of the

heat exchanger. Contrary to

popular belief, these anodes

are not consumed, because

their electrical contact with

the heat exchanger housing

is tenuous at best. Instead,

they simply pile up, blocking

water fl ow in the process. If

you remove the anode plug and

there’s no remnant of a pencil

zinc attached, it likely means

two things: Your replacement

interval is too long, and some

portion of the anode has been

left behind (they rarely erode

away completely without

separating from the plug).

For this and other reasons,

heat exchanger end caps should

be removed annually for an in-

spection and clean-out. Doing

so is typically straightforward

and well within the capabilities

of a do-it-yourselfer with

moderate technical skills.

Replacement gaskets should

be on hand (see “Gasket Case,”

March 2018) because gaskets

often tear or are not reusable.

Be sure not to overtighten the

end caps because many are

conical and will crack if their

fasteners are over-tensioned.

Look carefully at the detritus

you remove. Dead zincs and

a bit of sea grass are typical,

but if you see impeller parts,

be sure to check the raw-water

pump. Virtually every heat

exchanger I open does contain

some impeller parts, which

often are years old and from

previous impellers, so don’t

be surprised if the impeller

inspection yields no damage.

Regardless, it should still be

checked if pieces are discov-

ered in the heat exchanger.

If both ends of the heat

exchanger can be opened, do

so and place a bright light at

the far end. Look through the

tubes to determine if there

are any obstructions. If so, a

shot of water from a garden

hose might blast them out;

otherwise, you might need

to be more aggressive. Soft,

solid copper wire (insulated

is best) that offers a loose fi t in

the tube can be used to gently

push through obstructions.

Alternatively, a wooden dowel

may be used. But never use

steel wire because it might

damage tubes. The watch word

here is caution. Go easy, and

never force anything into the

tubes. If there is evidence of

scale buildup, a chemical fl ush/

descaling will be required.

Steve D’Antonio offers services

for boat owners and buyers

through Steve D’Antonio Marine

Consulting (stevedmarine

consulting.com).

With regular inspections and maintenance of its heat exchanger, your boat’s auxiliary engine

will run cool and trouble-free. BY STEVE D’ANTONIO

5 HEAT ATTACK

Anode replacement is perhaps the most critical (and most neglected) aspect of heat exchanger maintenance (left). At least

50 percent of this heat exchanger’s tubes are occluded by debris, marine growth and depleted zinc pencils (middle). The

“bundle” of a heat exchanger (right): the assembly is bathed in coolant, while seawater fl ows through its tubes.

STEVE D’ANTONIO