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Old 05-01-2014, 03:59 PM   #1
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RPMS on Cat 3208TA

Looking at a 42' GB with twin 3208's TA (375 hp) and I have no need to go fast. I understand that running at really low rpms in order to conserve fuel can potentially fowl up an engine? At what point could this be an issue?...if at all?
Anyone feel that I would run this risk at 1600? I have yet to try the boat but am guessing it would move along at 7k and burn minimum fuel.......maybe 3 gal/ hr each??
Thanks for you imput.
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Old 05-01-2014, 04:27 PM   #2
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Looking at a 42' GB with twin 3208's TA (375 hp) and I have no need to go fast. I understand that running at really low rpms in order to conserve fuel can potentially fowl up an engine? At what point could this be an issue?...if at all?
Anyone feel that I would run this risk at 1600? I have yet to try the boat but am guessing it would move along at 7k and burn minimum fuel.......maybe 3 gal/ hr each??
Thanks for you imput.
You're about right, based on this fuel burn chart from CAT:
https://marine.cat.com/cda/files/101...Propulsion.pdf


We run ours at about 1100 rpm, which equates to about a 7 knot speed. Every 8 hours of so, I bring her up to speed and let her run for 15 min or so.

As always, boatdiesel.com is the best source of infor for the Cat 3208. I loo ours- they start right up without fail, and run flawlessly.
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Old 05-01-2014, 09:42 PM   #3
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I know these are V8 models, but if they are set up similar to our 3116 TA, the turbo is just starting to spool at 1600, we can run forever at that rpm. Our normal operating speed is 1600-2000, max RPM is 2800.

As Peter says, the CATS are great engines, ours always starts immediately, runs smooth and doesn't smoke. 2000 hours and still purring like a kitten.
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Old 05-01-2014, 09:51 PM   #4
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Let me see, I travel about 90 knt miles to play golf, (Ketchikan to Wrangell) and return or 180 miles. If I were to travel at 7 knots and burn 6 gallons per hour based on
Newbi's calculation.

So- about 13 hours @ 6 gallons=78 gallons priced currently $4.35 per gallon=$339.30 one way or $678.60.
Have to weigh in with "Manyboat" I'd look for a 43 Grandbanks with a single and hopefully a ford. Then while it may cause a controversy here, add a 25 HP outboard on the aft swim step as a "git a home". Same comforts, economy, and backup safety. One could investigate a hook up of the gen set as a hydraulic secondary shaft turner in lieu of an outboard for aesthetic reason.
This observation submitted be a man of modest means were he to duplicate a similar particular boat.
I have to admit that I have seen GB with twin 3208's and they do perform in a hearty method. I wondered about the fuel burn then.

Now I will revert back to my 1.5 gallon 6 knt average on a 27 foot pocket trawler, watch my weather and catch up with you all later.
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Old 05-01-2014, 10:01 PM   #5
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It will probably only take 1,200-1,400 rpm to go 5 kts. At that speed you will be burning 2 gph per engine, maybe less. That is enough to get the engine up to operating temperatures and avoid cylinder glazing, etc.

Just run the engine up to 2,000 rpm or better at the end of the day for 10 minutes to blow any accumulated soot out.

To paraphrase Tony Athens, the moderator on boatdiesel, he has never seen a marine propulsion engine damaged by running slow.

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Old 05-01-2014, 10:42 PM   #6
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From what I have seen/heard, the "low rpm will shorten your engine life" myth is a deviation of the "low load will shorten your generator's engine life".

IMO, there is some truth to the second belief, where the engine runs at a constant speed and load. This results in a constant flow and velocity of engine combustion gases, and a higher ash and silicate buildup on the pistons and valves over a long period of time.

If then after 1000's of hours of this constant buildup, something irregular happens, such as a fast load put on the engine, the buildup breaks free and can cause damage to rings, liners and valves. Sometimes minor wear, sometimes catastrophic failure. I have seen this many times in the power station industry.

With a pleasure trawler diesel, this will only happen if you circumnavigate the globe non-stop at a constant low rpm and load, then immediately go to full throttle coming into the home harbour.
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Old 05-01-2014, 11:18 PM   #7
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One of our members(Giggitoni) has that exact boat and engine combination the OP mentioned so hopefully he will see this thread and give you actual numbers soon. I agree with others that the internet rumors warning against under loading recreational diesel engines is just that, a tall tale. Aus Can's generator example has merit but it is hardly applicable to recreational boats, most of which are lucky to see 200 run time hours in a calendar year unless serious cruising is going on.

I think you are more likely to have your engine damaged by collision with an asteroid than than low RPM usage.
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Old 05-02-2014, 12:08 AM   #8
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RPMS on Cat 3208TA

Actually I have the "na" or naturally aspirated version of the 3208. I have found that I do, in fact, get about 3 gph for each engine at about 1650 rpm and a hull speed of around 8.5 kts. Like others have said above, if one keeps the rpm down below 1600-1700 rpm then the turbo will not "kick in" and fuel burn should stay on the low side.

The important thing is to reach proper temperatures and I believe that those are attainable at lower rpm settings of 1600-1650....ours reach proper temperature at about 1400 rpm.

If you can, run the boat at 1400 and see if your temperatures reach at least 175 degrees F. Good luck. Where is the boat located?
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Old 05-03-2014, 07:12 PM   #9
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We are traveling north back to NC from the Keys, and currently north of Daytona on the ICW. We have been running our 3208 TA's (375) around 1200 rpms for past couple of days, mainly due to all the rain we have had. We get around 7 kts. +/-, and temps stay around 175-180. Our boat is a heavy 46 Grand Banks. As most others have suggested, we run them about 75-80% of WOT for 15 minutes or more near the end of each day if possible. Some days that does not happen. I am also one of those who does not buy into the myth that under loading these engines will shorten the life. Hope that helps.
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Old 05-04-2014, 07:38 AM   #10
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We chartered a 42 GB MY with 375 hp Cat 3208's. 200 miles at 6.5 kts @ 1300 rpm and 30 hours gen use consumed 275 gal of fuel (measured fill-up to fill-up) and one gal of lube oil. I would never buy a trawler with those engines!!!
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Old 05-04-2014, 10:50 AM   #11
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Quote:
Originally Posted by newbie68 View Post
Looking at a 42' GB with twin 3208's TA (375 hp) and I have no need to go fast. I understand that running at really low rpms in order to conserve fuel can potentially fowl up an engine? At what point could this be an issue?...if at all?
Anyone feel that I would run this risk at 1600? I have yet to try the boat but am guessing it would move along at 7k and burn minimum fuel.......maybe 3 gal/ hr each??
Thanks for you imput.
Look for fuel leaks in the "V" between the cylinder heads. Injection pump leaks are not uncommon, and the intercooler has to come off to get at the pump. Expensive fix, particularly if Cat Service techs do the job. The Sabre next to us had both engines done last summer for more than $4K...included flushing intercoolers....which were incredibly filthy (about 3000 hours on the engines) Two day job for two Cat techs. The Fleming on the other side of us has 375's and they are serious smokers. I'm a fan of turbocharging diesels, but not with intercoolers. There are reported (expensive) problems with intercooled Cummins here on the forum as well. Finally, the engines are not the most fuel efficient.
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Old 05-04-2014, 11:08 AM   #12
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We chartered a 42 GB MY with 375 hp Cat 3208's. 200 miles at 6.5 kts @ 1300 rpm and 30 hours gen use consumed 275 gal of fuel (measured fill-up to fill-up) and one gal of lube oil. I would never buy a trawler with those engines!!!
Must be more to this story. Those cats are fairly efficient at hull speed, not stellar but close to the pack. At 6.5kts that boat should get at least 2nmpg. You got 0.7!! Super fouled bottom, or some pulling something sneaky with the fueling, stealing fuel, etc.
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Old 05-04-2014, 11:15 AM   #13
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Must be more to this story. Those cats are fairly efficient at hull speed, not stellar but close to the pack. At 6.5kts that boat should get at least 2nmpg. You got 0.7!! Super fouled bottom, or some pulling something sneaky with the fueling, stealing fuel, etc.
Indeed- something is not right.
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Old 05-04-2014, 01:03 PM   #14
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The bottom was pretty rough. I did not note the exact beginning fuel level other than "it's full" from the ER walkthru (my fault = learning: Take photos of begining and end tank levels, and hour meters. If it wasn't full to the top, I would not be able to dispute later). I was guessing 170 gal used for the engines and 20 for the gen during the trip. I used 30 engine hours and 30 gen hours. Totally shocked at the 275 gal fillup. Afterwards when I asked about the fuel, the charter capt said the engines normally cruise at 10 gph total and 1 gph for the gen. The Cat perf curves show 5 gph at 1500 rpm per engine. Still, way too much HP for a 42 ft trawler with fuel at $4+/gal.
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Old 05-04-2014, 01:58 PM   #15
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I see over and over claims about fuel consumption at a certain rpm. Fuel consumption is a result of engine load NOT engine speed. If you're even 100 rpm off or different from rated rpm at WOT noticeable fuel burn variations will occur.

This is not a rant about overloading or underpropping but one must realize that a boat running at "X" rpm burning "X" amount of fuel may not be the same or even similar to another.

If you go 7 knots w a boat underpropped 100 rpm fuel burn will be way different from the same boat w the same engine over propped 200 rpm at an engine speed of (for example) 1600rpm. The actual load on the engine could be even close to 1/2 the load of the overpropped engine That will equate to 1/2 the fuel burn.

These are all numbers totally off the top of my head but when you compare two boats (same hull and same engine/s) at a given rpm very different fuel burns may result. And what they imply may not be obvious.

Marin Faure was overpropped w his GB w twin Lehmans. Went to a well established prop shop and they convinced him to prop for rated rpm. He did so and experienced a reduced fuel burn at a higher (yes higher) rpm and at the same speed. Also I think he went from 4 blade props to 3 blade. Anyone please correct me if I'm wrong as the above is from memory alone.

My point is that a boat with a certain engine burning X gallons of fuel an hour at a certain rpm is meaningless unless it is compared to the same boat with same engine AND a wot rpm that is also the same.

Re the boat in question the actual engine load may be very different than the norm and thus have very a different fuel burn.
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Old 05-06-2014, 04:59 PM   #16
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Here are my stats from Oakley CA to Longview WA with my 3208 TAs

Total mileage from Oakley to Longview WA: 740.1 miles
Total fuel used: 848 gallon
Total cost of fuel: $3486.00
Average Fuel burn: 14.62 gph
Average speed: 13.2mph
Total Time: 57 hrs 44min
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