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Old 07-03-2017, 10:19 PM   #1
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Rpm range for a Perkins 6 cylinder natural

I've had my boat for around two years. I ran it at 1700 my first trip from tx to Ms.

Yesterday I ran it at 1900 foot about 3 hours in the gulf and back. When I got back to the harbor we had a fuel rink around the exhaust. It been doing that for awhile now.

Wondering if I need to get the motor looked at. It seem to run great.

So my question is what rpm should I be running and it is normal to have fuel coming from the exhaust. Doesn't seem like a lot but not liking the looks of fuel around my boat.
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Old 07-03-2017, 11:20 PM   #2
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I'd definitely get at least one mechanic's opinion on the fuel issue.

Have you got a manual? Your rated power and the rpm it develops max hp should be stated. Also an engine speed should be posted as a max rpm for full throttle continious running and this will only be valid if the engine achieves rated rpm under normal conditions .. or a bit more. Often an time limit is given for full throttle operation .. typically one hour or less.

If the engine is overpropped then all you can do is guess.
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Old 07-04-2017, 05:32 AM   #3
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Black smoke can be a sign of an overloaded engine at that RPM.

Simply run the engine at full tilt for about a minuet and note the rpm.

Pull back 10% or about 300RPM and any black smoke should be gone.

Mark the tach with a piece of tape at the 300 lower RPM and use that as the maximum cruise RPM.
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Old 07-04-2017, 07:13 AM   #4
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Perkins 6.35 Raw Fuel in Exhaust

Timetraver...hope you get the issue resolved. Please post a diagnostic/remedy once it is resolved as I have twin Range 4 naturals.
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Old 07-04-2017, 08:20 AM   #5
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Timetraveler:

Your normally aspirated Perkins 6.354 is rated to make 135 hp at 2,800 rpm. If you are getting significant black smoke at 1,900 rpm you have one of the following problems:

1. Something mechanical in the engine and/or its intake or exhaust is not letting it reach proper rpms. This is the least likely cause.

2. The bottom or prop is badly fouled.

3. The boat was delivered with too large of a prop, ie too much pitch meaning that the engine doesn't have enough hp to rev up to 2,800 rpm in gear. Boats are often delivered with too much pitch to reach rated rpm, but usually not as much as you seem to have.

So, first get a phototach- they are less than $50 on Amazon or Ebay and check the wot rpm in gear. I suspect it is in the low 2,000s.

Then check the bottom and prop for cleanliness.

Then see if the engine will rev to about 3,000 rpm at wot in neutral. Don't worry about over reving the engine. Unlike a car, boat engines have governors.

Many trawlers with the Perkins or Lehman engines are "over propped" straight from the factory, with more pitch so that the engine would run slower at cruise and burn a little less fuel. Propping to reach 2,400 rpm and running it at less than 2,000 rpm is ok and should not produce black smoke below 2,000.

But it sounds like you are not making even 2,400 rpm so the problem is likely to be bottom or prop fouling.

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Old 07-04-2017, 09:11 AM   #6
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I'm sorry if I mislead yall but I'm not getting black smoke if any smoke it looks white. Anyways I am not sure what rpm to run my boat full time and not hurt anything.
When I bought the boat I replaced the sea pump, exhaust manifold and elbow all stainless steel. Not to sure how clean the intake is cause it domes not have a filter or anything on it.
When I parked it in the slip it looked like fuel was coming out the exhaust. A nasty oil r fuel ring around the back of my boat.
Never tried to run it at full throttle, going to try that next trip.

Thanks for the imput I really like this site everyone seems to be helpfully to a wanna be cruiser. One day
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Old 07-04-2017, 11:42 AM   #7
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Sorry, I just assumed it was black.

But black smoke and fuel in the exhaust are both related. xzBlack smoke generally means too much fuel for the available air and compression heat and combustion is good. That results in the combustion process burning the easy stuff- the hydrogen on the molecule and leaving the carbon- the soot behind.

Excess fuel in the exhaust generally means gross excess fuel, maybe due to a crapped out injector, but also low combustion temps due to low compression. Also late injection timing can do the same thing.

So, given that your problem is fuel in the exhaust and probably not making wot rpm, the odds of it being engine related rather that propping or bottom condition go up.

Does the engine start easily cold? If it does, that will rule out compression. Then perhaps have the injectors tested.

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Old 07-04-2017, 01:02 PM   #8
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Fuel in the exhaust for a Perkins is normal when it first starts up. Once to temp it should go away. I run mine at 1800, but it easily tolerates 2000. I'm wide open at about 2500. Perkins are notoriously stinky and smokey when first started but otherwise pretty bullet proof. Replacing the exhaust elbow with stainless was a good move.
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Old 07-04-2017, 03:51 PM   #9
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White smoke is water vapor from the exhaust too hot from various reasons. Light gray smoke can be from poorly atomized fuel and need to have injectors rebuilt. If it's fuel the smoke will make your eyes burn. Also will smoke pretty heavy when cold.
I normally run 1700-1800 by the dash tach which runs about 10 percent higher than actual.
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Old 07-04-2017, 04:54 PM   #10
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Quote:
Originally Posted by Egregious View Post
Fuel in the exhaust for a Perkins is normal when it first starts up. Once to temp it should go away. I run mine at 1800, but it easily tolerates 2000. I'm wide open at about 2500. Perkins are notoriously stinky and smokey when first started but otherwise pretty bullet proof. Replacing the exhaust elbow with stainless was a good move.
I'm very close to these numbers with my Perkins 6.354 NA twins. Cruise at about 1800 which gets me about 7 knots with a 2 GPH burn on each engine. WOT is about 2700 depending upon load and sea conditions.
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Old 07-04-2017, 10:09 PM   #11
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Running below 2000 will increase engine life. With clean oil, maybe double the rebuild interval of the hot dogs captains.
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Old 07-06-2017, 11:59 PM   #12
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Quote:
Originally Posted by Timetraveler View Post
I've had my boat for around two years. I ran it at 1700 my first trip from tx to Ms.

Yesterday I ran it at 1900 foot about 3 hours in the gulf and back. When I got back to the harbor we had a fuel rink around the exhaust. It been doing that for awhile now.

So my question is what rpm should I be running and it is normal to have fuel coming from the exhaust. Doesn't seem like a lot but not liking the looks of fuel around my boat.
You didn't mention the age of your engine, but the fuel sheen could be caused if the compression is off a little, they're only (17 to 1 to begin with) or an injector weeping.

But it sounds like your boat is over propped. I had a gillnet boat with single 6.354 na and it would turn up to 2000 wide open. It was grossly over propped. I don't know why they do that, but it was only an 8 knot boat, so we left it like that until the day we sold her.

My current boat is twin Perkins and it turns up to 2800 wide open and normal cruise is 2400. If it doesn't turn up and it's propped right, then you have another issue as was mentioned below.
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Old 07-07-2017, 05:44 AM   #13
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" I don't know why they do that, but it was only an 8 knot boat, so we left it like that until the day we sold her."

Selecting a proper prop for the boats speed and power requirements at cruise is done to add load and longevity to the engine at cruise RPM , and lower the RPM and noise on board.

If a boat requires 2 GPH at cruise that is 30-35HP , so operating at a high RPM that could create 70 HP is a waste of fuel and engine life.

If you operate at least 10% RPM below WOT RPM the engine will not be overloaded.
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