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Old 06-06-2020, 09:17 PM   #1
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How many hours do gas inboard engines go?

Hi Folks. I have a friend buying a 34 foot boat which is 15ish years old driven by two mercrusier’s. He was wonder what is a high hour engine for these gas engines and I didn’t know since I have always had deisel’s where 1000 hours means you change the timing chain. I know it comes down to how they are maintained, but is 950 hour engines in their middle of their lives? They would be coming from fresh water location. I would love to hear your thoughts. Thanks
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Old 06-06-2020, 09:31 PM   #2
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All depends how hard they were run. If the were run north of 3800 rpms, probably only 1000 hours. If the were run south of 3,000 rpms then probably 2,000 hours. However good oil and good temperature can extend any engines life.

Compression test will tell you a lot about a gas engine.
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Old 06-06-2020, 10:49 PM   #3
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Even with raw water cooled engines, it is really rare that one dies from "old" age. They overheat, or the oil pump gives up, or low compression causes such hard starting or so much smoke that they get rebuilt. Even if they are run "hard" for 950 hours, that may be the equivalent of 50,000 miles in a car. And I am not sure what running "hard" means. Generators are intended to be run at a continuous 80% or so of their rated output, and tend to last forever. If it were me, I would get the engines surveyed, and if they aren't smoking, don't have much blowby, run at the thermostat temp when at 90% load for an extended period, then I would expect the engines to outlive me. Or at least until something stupid happens like the impeller goes and they overhead badly.
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Old 06-06-2020, 11:02 PM   #4
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On the Great Lakes, most need rebuilds at around 2,500hrs.
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Old 06-07-2020, 12:30 AM   #5
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How heavy is the boat? How hard was it run? What are the maintenance records showing? I’m considering downsizing from our 4788 to a 35 Tiara open (can 8 still fly my TF burgee?). Dry weight is 14000, engines are Crusader 8.1 MPI, 385hp. He king torque curve, doing research, that’s about top end for displacement (for grins I figure about 18500 loaded and with crew). A Tiara 35 express dry weight is 18000 and I would only purchase with diesels. There is a reasonable amount of good info on the web. One thing I did learn is that Mercs might be hard to work on because of the way they marginalized the blocks. Also newer gas engines are more robust than older pre EFI engines and Crusaders seem to be preferred over Mercs. That’s just what I discovered after 2-3 hours of research.
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Old 06-07-2020, 01:21 AM   #6
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That would be 50,000 miles of continuously running up hill.

The nice thing about gas engines, long blocks cost less than $5,000.
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Old 06-07-2020, 05:31 AM   #7
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Beaverlake, I hear good things about the Tiara that have the traditional diesel, shaft and props.
Also, do a bit of research on the American Tug. I’m sure you can find a well maintained one.
There are main brand smaller boats that can satisfy your requirements.
The only difficulty you will discover is, what to do with all the stuff from 47ft when you downsize.
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Old 06-07-2020, 05:34 AM   #8
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Gas engines seem to die of bolt-on stuff. Carburetors, heat exchangers, water pumps, fuel pumps, valve springs, and exhaust risers don't last like diesel stuff. Once you get to 1000 hours it is a non-stop replacement parade. Blocks and heads go 1000 hours if you cruise at 3000ish RPM. Maybe 2000 hours if you troll a lot. That has been my experience in a saltwater recreational fishing boat environment. Also I have never owned a fuel injected gas inboard. My current FI four stroke Yamaha outboard is at 1695 hours and still runs like new. A local charter skipper retired a pair of the same OBs at 6000+ hours.
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Old 06-07-2020, 05:55 AM   #9
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Thanks folks for the insight. I will pass it along.
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Old 06-07-2020, 06:01 AM   #10
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I agree w comments so far.
I had a pair of gas Crusader converted GM 305s that I sold with about 1500 hrs.
I did compression checks in the later yrs I had them and results were pretty consistent and within acceptable ranges. Comp check will reveal some otherwise hidden issues and fairly easy to do on a gasser.
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Old 06-07-2020, 06:21 AM   #11
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Depends on which gas engine and how run....

On one end, I have personally run a 454 to over 4000 hours of very hard, commercial use.

Then others have died from under use sitting in boats, suffering all sorts of indignities from non use to abuse and died around 1000 hrs.

Even engines that some laughably call "bulletproof" die early deaths......so as always....sometimes luck of the draw wins over every other variable.
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Old 06-07-2020, 08:16 AM   #12
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I'm with Psneeld on the big blocks having a pretty good lifespan. One of my 340hp 454s was replaced after a previous operator lost an oil cooler line and didn't get it shut down before the main bearings were toast. The other is original, about 1650 hours on it. Can't tell any difference between the 2 in how they run, the older one still has good compression, only uses a tiny bit of oil. And that's an engine that's spent plenty of hours up on plane at 3300 - 3400 rpm, although on this boat, that's still 200hp or less continuous output, so not abusive to the engine.

The higher output big blocks don't last as long though. One of my dock neighbors had his 502 Mag MPIs rebuilt over the winter at just under 1000 hours due to high oil consumption and somewhat low compression. He never ran them overly hard, but who knows what the previous owner of the boat did.


In general, I'd say if oil consumption isn't excessive, compression is good and oil pressure at hot idle is adequate, it's got plenty of life left.
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Old 06-07-2020, 09:32 AM   #13
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How about some opinions on older gas engines vs. newer ones ??
Older one, almost always when they had problems it was electrical related. 1st thing, tune up. Plugs, wires, points. If not that then fuel. Filters, carb, pump.
Newer gas engines with electronic ignition and fuel injection generally don't seem to have as many problems. But they do come with their own set of problems...
That leaves mechanical. I'd hope newer engines would be made with better metal but I wouldn't bet on it.
I'd rather have an engine with moderate hours run every week than one with low hours that was run hard once in awhile.
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Old 06-07-2020, 09:42 AM   #14
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I think you are all diesel worshipers.

Even old cars like 40’s and 50’s lasted 100000mi. How many hours is that? 2500. But they wern’t run like a boat engine. Women and kids drive’in them and 20 times as many cold starts and most people drive a car the same way cold and warmed up. And they had so little power people climbing a hill had their foot on the floor till they downshifted then back To foot on the floor.
High Wire has a good point that bolt on stuff needs replacement often. But I remember people getting their pistons knurled to make them fit the bore again. And they had “soft blocks” especially Chrysler products And they were the most underpowered.

Now when you put on fuel injection, better radiators, alternators, electronic ignition, better lube oil and antifreeze and many many more automatic transmissions and in the mid-late 50’s much more power so engines didn’t have to work so hard it’s amazing the old engines lasted 100000mi. But as I recall most did.

But now w all the modern advancements like automatic transmissions and electronic ignition engines should last much longer. Probably twice as long. Modern cars last 300000mi if not abused. That’s about 7000hrs in a boat.

On TF the diesel engine is a thing to be worshiped. Even the racket they make is viewed as a good thing. But if one suggests they cease overpropping their boat they say no way. It makes too much noise at higher rpm. But mention that a gas engine is much quieter and they say OMG it will blow up and kill me. But in the 50’s all pleasureboat engines were gas and almost nobody refused to go boating out of fear of exploding.

Different times different attitudes over the same things reminds me of how subjective our minds are. In my hang gliding days fliers were jump’in off mountains w little or no concern for safety. Then somebody created a deployable (throwable) parachute and many fliers quit flying until they could afford a parachute. But the year before .... everybod’s doi’n it. w/o the chute. Maybe diesel engines are related to parachutes.
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Old 06-07-2020, 10:03 AM   #15
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I dont do things now that I did when I was young and didn't know better or there are now alternatives.

Some of us can remain objective....even about parachutes.

Kinda like boating without life jackets...well except for costs.

I prefer a diesel much of the time but not all applications warrant one.

Many opinions....pick one.....
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Old 06-07-2020, 04:02 PM   #16
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People usually kill them before they "wear out". Abuse, neglect, water intrusion, rust, creative destruction.
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Old 06-07-2020, 04:49 PM   #17
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When we had our Chris-Craft with the 350 engine, we rebuilt at 3200 hours. The new owners rebuilt at just over 3000 hours. Both rebuilds were long block and the engine had an enclosed cooling system.
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Old 06-07-2020, 05:34 PM   #18
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My Mitsu diesel is quite civilized and if it wasn’t for resale - and if an old uncle paid for the switch I would probably switch. But my diesel engine being small dosn’t have the energy to shake much or shake other things like bulkheads. So not much noise either.

As a fantasy the engine I’d really like is an in-line 3 cylinder BMW motorcycle engine found on a K75 model BMW. It’s too much power (at about 70hp) but gas engines are happy being underloaded as long as it’s not too extreme. The K75 engine is water cooled and runs perfect on it’s fuel injection. It’s so smooth in Wyoming I pulled onto the road and went through the gears but my brain tripped and I didn’t make it to 5th gear. I ran (w wife on back) for at least a half an hour before I realized I was only in 4th gear.
There’s no torque either as the flywheel turns the opposite direction as the crankshaft. But one would need a 4-1 or so gear to get the prop to the right speed. Cruise at 4-5000rpm.
Well enough dreaming it ain’t gonna happen.
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Old 06-07-2020, 07:13 PM   #19
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I can add this about gas engines... because most items have already been clearly stated.

Well cared for gasoline engines usually last best/longest when their total hp is rated at 75% or less than their total cubic inch displacement.

Thus... a 260 hp 350 cid engine = 74% ratio. Mercruiser, Crusader and other engine manufacturers base engines [of all cid sizes] usually have nearly this ratio. There are hp "hopped up" same size cid engines available, but, their lifetime hours are usually reduced, in comparison to the base engine ratios.

Reason for the 75% mark: Their power capability is not sufficient to easily rip themselves apart too quickly... of course, depending on owner use/maintenance.

I recommend, for longest engine life and least needed repairs, to often use a 75% hp to cid ratio engine at 50% or less of their WOT rpm.

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Old 06-08-2020, 05:47 AM   #20
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Newer engines do last longer than the old mess produced iron.

The main reason is the assembly , parts selection , is now computer organized.

With the shape of a few hundred pistons to chose from a far better fit for todays oval pistons is easy.

Same for connecting rod weight and length , and most other engine parts.

Back 50+ years ago race cars used seasoned blocks as the heat and stress of running reshaped the block, machining would true it , so it could last longer at high RPM.

Today computer design and experience allows better, more accurate machining.

THe new engines are as easy to kill from neglect , the right oil and antifreeze are required.

I believe most boat engines are killed by not following Da Book, (the maint handbook) about proper out of season storage.
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