How Long Will My En­gine Last?

Passage Maker - - Troubleshooter - BY STEVE ZIM­MER­MAN

At our diesel en­gine classes at TrawlerFest, the ques­tion of en­gine longevity al­ways comes up. This ques­tion also plays a key part in most con­ver­sa­tions about buy­ing a used boat. And while there is never a de­fin­i­tive an­swer, there are sev­eral fac­tors that con­trib­ute to how long an en­gine runs and sev­eral things you can do to pro­long the life of your en­gine.

En­gine life can be de­fined as “time be­tween over­haul” (TBO). Ev­ery en­gine man­u­fac­turer cal­cu­lates a de­signed en­gine life, but only some­times is this in­for­ma­tion avail­able to the pub­lic. The life of an en­gine can be pro­jected in three ways: gal­lons of fuel burned, hours op­er­ated, or years of oper­a­tion. A given en­gine, for ex­am­ple, might be ex­pected to run for 50,000 gal­lons of fuel, 10,000 hours, or 20 years. Years mat­ter only be­cause of the avail­abil­ity of re­place­ment parts and gen­eral de­te­ri­o­ra­tion due to con­stant ex­po­sure to mois­ture.


If you want to take a stab at the life ex­pectancy of your en­gine, you can start by com­par­ing your sit­u­a­tion to the con­di­tions an- tic­i­pated by the man­u­fac­turer. En­gi­neers must de­sign an en­gine with a par­tic­u­lar ser­vice life in mind. A tug­boat might run 24 hours a day at full load and can ex­ceed 5,000 hours of run­ning time in a year. A cruis­ing power­boat might run closer to six hours a day at 30% load and ac­cu­mu­late fewer than 500 hours of time an­nu­ally. To pre­dict en­gine life, builders weigh three ba­sic vari­ables: how hard the en­gine will work (load fac­tor), how many hours it will be at full power com­pared to at nor­mal cruis­ing load, and how many hours per year of oper­a­tion it sees.

Load fac­tor pro­vides a use­ful met­ric for how hard you run your en­gine and is cal­cu­lated by di­vid­ing the amount of fuel burned by the full-power fuel burn for your en­gine. Let’s look at an ex­am­ple: 6-hour run burn­ing 20 gal­lons per hour: 120 gal­lons Max rpm fuel burn per hour: 50 gph x 6 hours = 300 gal­lons To cal­cu­late load fac­tor: 120/300 = 40%

With re­spect to en­gine longevity, we are talk­ing about the load fac­tor over the life of the en­gine, not a sin­gle day’s run. If the num­bers above re­flect typ­i­cal oper­a­tion, then we can es­ti­mate that on av­er­age your en­gine has a 40% load fac­tor. For recre­ational engines, most man­u­fac­tur­ers want to see a load fac­tor less than 35%, and 10-30% would be op­ti­mal. You can de­ter­mine your load fac­tor by look­ing up the fuel curve for your en­gine and com­par­ing the rated full rpm fuel burn to the rate at your cruis­ing rpm. Boats cruis­ing at full dis­place­ment speeds (1.2 x the square root of the wa­ter­line length) will usu­ally cruise at load fac­tors of 25% or less. At plan­ing or semidis place­ment speeds, that num­ber will be closer to 35% or above.

The higher the load fac­tor, the shorter the en­gine life. In fact, at least one en­gine man­u­fac­turer uses the load fac­tor to pro­rate their pre­dic­tion of en­gine life. They start with a pre­dic­tion based on gal­lons of fuel pro­jected be­tween over­hauls. Here is the for­mula: Pro­jected Gal­lons of Fuel Con­sumed be­tween Over­hauls = Load Fac­tor x Fuel Burn (at full rpm)

Let’s use 100,000 gal­lons as their longevity base­line. On a boat with a load fac­tor of 30% and a wide-open-throt­tle fuel burn of 50 gph, the pro­jec­tion works out as fol­lows: 100,000 gal­lons ∕ (.30 load fac­tor * 50 gph) = 6,667 hours

How does that com­pare to a boat run­ning slower at lower load fac­tors? Let’s use the same en­gine, but with a 10% load fac­tor: 100,000 gal­lons / (.10 load fac­tor * 50 gph) = 20,000 hours

Now let’s look at an ex­am­ple of how man­u­fac­tur­ers de­scribe their ex­pec­ta­tions of how hard you will work your en­gine. John Deere uses a rat­ing sys­tem of M1 to M5:

In this sys­tem, M5 points to engines for recre­ational cruis­ing boats. The man­u­fac­turer is telling us that the M5-rated engines will max­i­mize longevity when those three pa­ram­e­ters are met. Other man­u­fac­tur­ers use dif­fer­ent nomen­cla­ture but sim­i­lar pa­ram­e­ters. M1 for John Deere cor­re­lates to “Con­tin­u­ous Duty” for Cum­mins and “Con­tin­u­ous Duty Com­mer­cial” for Volvo.

A given en­gine model can usu­ally be pur­chased with dif­fer­ent rat­ings. A Cum­mins QSB6.7, for ex­am­ple, can be pur­chased with the recre­ational rat­ing (“High Out­put” in Cum­mins nomen­cla­ture) or in a “Medium Con­tin­u­ous Duty” con­fig­u­ra­tion (which would fall be­tween M2 and M3 on the John Deere chart).

As you can see from these rat­ing sys­tems, the recre­ational ver­sion of each en­gine rep­re­sents the weak­est ver­sion of each en­gine model.

At this point you might be won­der­ing why we de­fault to the weak­est? Most plea­sure boats re­quire the high­est horse­power for the least weight and space–not nec­es­sar­ily the best recipe for a long life. En­gine life can be ex­tended in these higher horse­power con­fig­u­ra­tions by lim­it­ing the load­ing and the hours, in other words, by com­ply­ing with the recre­ational rat­ing. For full-dis­place­ment trawlers and for boats that will al­ways run at dis­place­ment speeds, you could make a good case for “de-rat­ing” the en­gine by go­ing for a lower horse­power ver­sion with a higher rat­ing.

In ad­di­tion to rat­ing choices, many en­gine mod­els can be pur­chased within a range of horse­power of­fer­ings. The well­trav­eled Cater­pil­lar 3208 be­gan as a 210-horse­power en­gine and even­tu­ally reached a 435-horse­power model.

At this point we can draw our first two con­clu­sions:

1. The lower the load fac­tor, the longer the life of the en­gine. For cruis­ing boats, the load fac­tor should be less than 35%.

2. All other fac­tors equal, the lower the horse­power for a

given model, the longer the life ex­pectancy.

Now, you might be say­ing to your­self, “If higher load fac­tors over ex­tended pe­ri­ods of time re­duce en­gine life, then if I put in a big­ger en­gine and run it very slowly, it should last a long time.” Un­for­tu­nately, how­ever, that sce­nario cre­ates a new set of prob­lems.


Chron­i­cally un­der­load­ing an en­gine cre­ates a dif­fer­ent set of prob­lems, es­pe­cially on pre–com­mon rail engines: car­bon buildup in the ex­haust sys­tem and in the en­gine. Terms that de­scribe this, such as “ex­haust slob­ber” and “wet stack­ing,” sound ap­pro­pri­ately un­ap­peal­ing.

Engines rely upon high tem­per­a­tures to thor­oughly com­bust the fuel in the cylin­ders. At light loads cylin­der tem­per­a­tures may not reach the thresh­old needed to burn off all of the fuel dis­persed by the in­jec­tors and lube oil residues on the cylin­der walls. The un­burned fuel and oil cre­ate car­bon, which can build up in the cylin­ders, on the valves, in­side the ex­haust man­i­folds, and in tur­bocharg­ers. In the cylin­ders, car­bon buildup re­duces the ef­fec­tive­ness of the rings and can lead to cylin­der and pis­ton dam­age. Valves with car­bon buildup be­come less ef­fi­cient. In the ex­haust man­i­fold, the buildup can re­strict air flow, and on the turbo car­bon buildup can cause a loss of boost pres­sure or cre­ate an im­bal­ance that causes a fail­ure of the turbo vanes. In short, run­ning chron­i­cally at light loads can cre­ate a host of prob­lems that lead to pre­ma­ture en­gine fail­ure.

Newer com­mon rail engines are bet­ter able to com­pen­sate for un­der­load­ing than older engines, which in­ject fuel when the in­jec­tion pump cre­ates enough pres­sure to “pop” the in­jec­tor with­out any ac­com­mo­da­tion for other pa­ram­e­ters. The in­jec­tors on modern com­mon rail engines dis­perse the fuel in three or more doses for each in­jec­tion, and the en­gine’s com­puter ad­justs for the fact that loads are light and tem­per­a­tures are low. These newer engines are there­fore bet­ter able to ad­just for the un­der­load­ing con­di­tion, but they are not im­mune to the prob­lems it can cause.

It should be noted that chron­i­cally un­der­load­ing does not mean you will nec­es­sar­ily en­counter these prob­lems, just that the risk is greater. If you have been run­ning this way for years, it would be wise to have a me­chanic in­spect the turbo and ex­haust com­po­nents and pull an oil sam­ple. You might find that your con­fig­u­ra­tion is work­ing fine. In any case, if you do run most of the time at light loads, the fol­low­ing pro­to­col will pro­mote en­gine health: For ev­ery 8 hours run at light loads, run at about 75% to 80% load (or about 10% be­low max­i­mum rpm) for one hour. In ad­di­tion, pe­ri­od­i­cally run­ning at wide open throt­tle (WOT) brings ad­di­tional ben­e­fits.

One more point about en­gine horse­power. Most en­gine war­ranties stip­u­late that your en­gine must reach the rated WOT rpm. An un­der­sized en­gine or over­sized pro­pel­ler can void your warranty. The warranty is­sue should be rea­son enough, but even if you are out of warranty your en­gine will last longer if you meet this cri­te­ria. Achiev­ing WOT con­firms that the pro­pel­ler has been prop­erly sized for the en­gine, prop­erly match­ing load to rpm through­out the range. Based on these fac­tors, we can draw two more con­clu­sions:

3. En­gine life can be ex­tended by siz­ing your en­gine and pro­pel­ler so that you can reach the rated WOT rpm.

4. Chron­i­cally run­ning your en­gine at light loads may re­duce en­gine life. If you do run at low rpm most of the time, check for car­bon buildup and pe­ri­od­i­cally run higher loads to burn off car­bon de­posits.


Most boat own­ers keep up with oil and fil­ter changes. And while ne­glect­ing most of the other en­gine main­te­nance tasks may lead to a break­down, it will not nec­es­sar­ily re­duce en­gine life. Based on what we see in our boat­yards, the most com­mon area of ne­glect that im­pacts en­gine longevity in­volves the en­gine cool­ing sys­tem. Un­like ne­glect in other ar­eas, fail­ures in the cool­ing loop can lead to sud­den fail­ures and ma­jor dam­age.

In­tro­duc­ing sea­wa­ter into the metal com­po­nents in­vites a host of prob­lems, mostly hid­den un­til too late. Keep in mind that rec­om­mended ser­vice in­ter­vals do not de­pend on run hours alone–pas­sage of time counts, too. Sea­wa­ter re­sides in­side the var­i­ous cool­ers, whether you are run­ning the en­gine or not. The main­te­nance sched­ule in your owner’s man­ual will usu­ally state the re­quire­ments this way: “Ev­ery 1,000 hours or ev­ery two years.” You might only have 800 hours on the en­gine, but if it has been more than two years since you had the af­ter­cooler re­moved, cleaned, and in­spected, you are danc­ing on the edge. If the af­ter­cooler fails it can lead to ma­jor en­gine dam­age. Print out the main­te­nance in­ter­vals for your en­gine, re­view them care­fully, and fol­low them closely.

One more fac­tor im­pacts en­gine life–fre­quency of use. Al­though lack of use min­i­mizes hours, sit­ting for months at a time with­out run­ning has harm­ful ef­fects. Changes in tem­per­a­ture cre­ate con­den­sa­tion and mois­ture ac­cu­mu­lates in the cylin­ders, in the tur­bocharg­ers, and in other com­po­nents. Block heaters will help re­duce mois­ture dur­ing cold months.


One piece of in­for­ma­tion crit­i­cal to de­ter­min­ing en­gine longevity is of­ten miss­ing, and it con­cerns the man­u­fac­turer’s de­sign in­ten­tion. An en­gine might be rated for 30,000 gal­lons of fuel burn be­fore a re­build. Or the man­u­fac­turer might have a tar­get of 10,000 hours within the rat­ing guide­lines. Num­bers get tossed around anec­do­tally: “The fish­ing guys I know get 20,000 hours on those engines.” Per­haps, but with­out know­ing if it was a com­mer­cially rated en­gine or a recre­ational one or how it was used and main­tained, those num­bers are mean­ing­less.

If you know the pro­jected life of a given model, you can ad­just your pre­dic­tion based on the con­clu­sions listed above. For most engines in the range of 200 to 600 horse­power with load fac­tors un­der 30%, en­gine life of 10,000 hours would not be un­usual and 20,000 hours would be pos­si­ble. Un­for­tu­nately, due to op­er­a­tional con­di­tions (run­ning too hard or too light too of­ten) and due to main­te­nance ne­glect, for many of these engines 6,000 hours would be a good run be­fore need­ing a re­build.

n Steve Zim­mer­man is the pres­i­dent of Zim­mer­man Marine, which op­er­ates five boat­yards in Mary­land, Vir­ginia, and North Carolina, and South Carolina. Zim­mer­man has been build­ing and re­pair­ing boats for more than four decades.

Even­tu­ally, time and wear catch up and the sit­u­a­tion will call for a re­build, a re­man­u­fac­tured en­gine, or, in this case, a new en­gine like these from John Deere.

Even­tu­ally time and wear catch up and the sit­u­a­tion will call for a re­build, a re­man­u­fac­tured en­gine, or, in this case, a new en­gine.

Chron­i­cally run­ning at light loads can lead to a build up of car­bon de­posits in the cylin­ders and ex­haust com­po­nents. Over time this can lead to pre­ma­ture en­gine fail­ure. This en­gine re­quired a re­build af­ter only 700 hours. (Photo by Nigel Calder)

Well main­tained, prop­erly sized for the ap­pli­ca­tion, and op­er­ated within the op­ti­mal min­i­mum and max­i­mum loads, this en­gine has a good shot at run­ning for 10,000 hours—and pos­si­bly much more.

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