HEAT AT­TACK

With reg­u­lar in­spec­tions and main­te­nance of its heat ex­changer, your boat’s aux­il­iary en­gine will run cool and trou­ble-free.

Cruising World - - Contents - Monthly Main­te­nance by Steve D’an­to­nio

Like ev­ery­thing else on a boat, most diesel-en­gine heat ex­chang­ers ben­e­fit from an an­nual 0nce-over.

Un­like 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, rather 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 re­sem­ble drink­ing straws, which are sur­rounded by coolant. Sea­wa­ter passes through the tubes, ab­sorb­ing heat from the coolant. It’s a sim­ple ar­range­ment that al­lows en­gines to be filled with coolant rather than sea­wa­ter, thereby re­duc­ing cor­ro­sion and in­creas­ing ef­fi­ciency. En­gines that are cooled di­rectly by sea­wa­ter must op­er­ate at a lower, and there­fore less ef­fi­cient, 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 sac­ri­fi­cial 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 flow 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 in­spec­tion 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 dis­cov­ered 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 fit 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 flush/ descal­ing will be re­quired.

Steve D’an­to­nio of­fers ser­vices for boat own­ers and buy­ers through Steve D’an­to­nio Ma­rine Con­sult­ing (steved­marinecon­sult­ing.com).

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