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TOPIC: 2001 3788 - Cummins 330B - Correct Cruising Speed?

2001 3788 - Cummins 330B - Correct Cruising Speed? 09 Oct 2017 22:26 #1

  • W8N4SUN
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Did a search and got overwhelmed with the number of topics here so please excuse me if this has already been covered?

Since owning this boat last March I have noticed diesel soot on on the back. We normally cruise at 1600-2000rpm. This past weekend one of our dockmates told me I should be running at 80% max rpm, which translates to 2400rpm for us. So we tried it out for 1.5hrs at 2400rpm with a speed of 22kts. We got back to our slip and low and behold - NO SMOKE residue!

The downside is too much speed for relaxed cruising and I'm sure we won't be getting 7.5-10gph consumption.

Q1. What lower rpm can I run without loading up the engines and causing a lot of exhaust residue?
Q2. Is there a recommended rpm to cleanly run at the 8-10kts range?
Q3. What are others running this same engine combination running at to have no exaust residue?

Thanks for putting up with me!

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2001 3788 w/ 330 Cummins
Seattle, WA

2001 3788 - Cummins 330B - Correct Cruising Speed? 09 Oct 2017 23:44 #2

  • smitty477
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Your 330 Cummins should be rated for 2,800 rpm at WOT>
As a brief test can you achieve 2,800 rpm of preferably 2,880 - 2,950 fully loaded on a hot and humid day?
That would be the first 'test' I would check before recommending a safe cruising speed.

Even better is to run that test and also add EGT and boost gages for not too much money to help protect those engines.
I see you are also running a lot of weight way out back on the stern.

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Northport NY

2001 3788 - Cummins 330B - Correct Cruising Speed? 10 Oct 2017 14:30 #3

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I have checked to see what WOT rpms she wil reach and have gotten both engines to sing at 3000rpm. 80% is 2400rpm but I really don't want to cruise at +22kts as I can't relax and the fuel burn will be higher than I prefer. But I am torn because at lower rpms the diesel soot on the back is terrible.

Is there a safe lower rpm cruise speed?

I'll ask our mechanic about installing the EGT and boost gauges as you suggest. Thanks

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2001 3788 w/ 330 Cummins
Seattle, WA

2001 3788 - Cummins 330B - Correct Cruising Speed? 10 Oct 2017 14:58 #4

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Even thought your WOT achieved 3000, I believe Smitty is correct that the 330 is rated at 2800, so use that as your "denominator" for your calcs. Besides, as you say it'll get expensive racing about at WOT not to mention the engine strain. The spec number is on the engine label attached to the timing gear housing near the pump.

Black smoke/soot is unburnt fuel. I'd be looking at your injectors, removing them and testing for pop pressure and spray pattern at your local injector pump shop as they may be dripping. If it's a Bosch pump, there is no fuel calibration adjustment - not sure about the other Nippondenso brands etc. At the same time, check the valve clearances. Also keep an eye on your oil levels as well as you may also be making oil from an accumulation of the excess diesel.

Hope this assists. Cheers

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John H
Brisbane QLD Aust
"Harbor-nating"

2000 - 4788/Cummins 370's

2001 3788 - Cummins 330B - Correct Cruising Speed? 10 Oct 2017 15:03 #5

  • The Other Gary
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It sounds like you are nicely propped for engine longevity, not working too hard at any rpm.
Your choices are simple, hull speed which is about 7.5 knots for your waterline length or nicely on plane where you may need a little tab
to keep the nose down. Soot is hard to avoid as there is a station wagon effect from your transom. Servicing air filters frequently using the K&N kit if you
have Walker or Racor air handling will reduce soot but never eliminate it.
Here is a fuel burn chart for your engines. As you are slightly underpropped you may achieve better.

www.sbmar.com/docs/performance-curves/6B...%2000,M-90208%5D.pdf

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"Adios Dinero"
1997 3988 with new 330 Cummins
Photo Credit: Whiskywizard

2001 3788 - Cummins 330B - Correct Cruising Speed? 10 Oct 2017 17:06 #6

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Appreciate the feedback and advice for everyone. Sounds like there will always be a little soot to deal with and I'll resign myself to that. The air filters are new but the injectors have not been serviced yet so I'll give that a shot as well, at least prior to next year's heavy cruising season. The oil levels are good and she doesn't burn oil, a few drops here and there but nothing major. For being a 16 year old boat she runs well.

What would be ideal is to be able to run at any speed and keep the soot levels down. At this point the only time I've run and not had soot is 2400rpm but I will experiment the next few times I go out and see what the magic lower speed rpm nets a cleaner exhaust level.

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2001 3788 w/ 330 Cummins
Seattle, WA

2001 3788 - Cummins 330B - Correct Cruising Speed? 10 Oct 2017 17:15 #7

  • p91473
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On my older 3788 with well serviced Cummins 250s I used to get good soot build up at cruise (2150 RPM). I added additional trim tabs to get the bow down and now zero to very little soot. I am not saying you need more trim tabs, just that the soot is related to the engine load factor for these enginees. My Hinos never created soot regardless of load.

The comments above are good. You maybe able to reduce the soot by having the injectors tested, but may always have a little soot.

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1997 3788/Cummins 6BTA 5.9 M2s
2014 Sea Fox 22 Center Console
2006 Boston Whaler 13 Sport

Vero Beach, Fl.
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2001 3788 - Cummins 330B - Correct Cruising Speed? 10 Oct 2017 19:37 #8

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Let me offer a different 'angle' on this question. ;-)

Your boat (and any boat with a 'planing hull') is essentially a 'two speed' boat. There are two 'correct' speed ranges, the first is your slow (displacement) speed range and the second is your fast 'planing' speed range. Either speed is dictated by your running angle, and within each of your two speed ranges there will be a sweet spot where your fuel burn is minimized, giving you the best fuel economy. At your slow speed you should not exceed a 2 degree running angle. When you are up on plane, you should have a maximum running angle of 5 degrees or less, and lower is better. Going faster than displacement speed, any planing hull is most efficient when the boat is well 'up on the plane'.

The smoke you were seeing was because your engines were overloaded at an "inefficient" speed for your hull. When you sped up, the smoke went away because you got the hull up into her planing sweet spot.

I agree with a previous poster -- it looks like you have a lot of weight aft. I think it is very likely that you are going to want to take an inch (or probably two) of pitch off of your props, but here is how to determine if you need that:

First, get yourself something you can use as an inclinometer, a calibrated bubble level is best. Many smartphones also have a digital inclinometer, but I find them unreliable. Drop your dinghy and set up the level so it is showing zero degrees sitting at the dock. Also, grab yourself a grease pencil.

Second, leaving your dinghy at the dock and with your trim tabs all the way up, take the boat out starting at about 1,000 rpm and gradually increase your speed until the bow just starts to come up and you see about 2 degrees on your inclinometer/level. I guess you will see 2 degrees at about 1,200 rpm or so. If you like, you can use your grease pencil to make a small mark on your compass dome at the edge of your compass card for reference -- your compass card is an excellent inclinometer!

Next, power up and keep an eye on your running angle. You will go through what is politely referred to a the 'transition speed' but commonly known as 'plowing water', where you might see a running angle that is 6 degrees or more. In your boat (without the dinghy) I'd guess you'll see 6-7 degrees when you are really plowing -- looking back you'll see a tremendous wake and engines will probably be smoking.

Then, increase your speed to maximum (WOT) and note your lowest running angle. Make another mark on your compass dome and note your RPMs. Now throttle back and bring your RPMs down until you see one degree more than your lowest running angle and this will be your hull's best cruising speed (best miles-per-gallon). I would expect to see something like 3.5 degrees in this 'sweet spot', but less is better. Note your RPMs again. Looking back you should see minimal wake and no smoke. If your engines are operating as they should, and your hull bottom and props are clean your engines should be turning at about 80% of maximum WOT RPMs or hopefully less.

If everything above is 'working' as described, then you are correctly propped for the boat -- as it was ORIGINALLY designed.

Now, with your dinghy back on the davits, repeat the WOT test and make sure you are able to get the same RPMs you got in the previous WOT run without the dinghy. Note the increase in your running angle. At this point you will most likely need your trim tabs (a little), but if you need lots of trim, remember that the drag you are creating is also going to load up your engines (and excessive trim is dangerous in following seas). Bring the RPM's down until your running angle is once again in the sweet spot. Now you will likely see it takes ~200 more RPMs (and some trim) to attain the same running angle.

If you cannot get to the same RPMs at WOT with the dinghy on the back, you are overloading the engines and will need to re-pitch your props. The last time I put a heavy dinghy on the swim platform, I needed to take off 2" of pitch.

The bottom line here is that your proper cruising speed 'on plane' is dictated by your running angle, which in turn determines the load on your engines. Note -- it is not the weight of your dinghy that is the problem, it is the effect it has on your running angle, being that the weight is so far astern.

While I'm at it here, I can't resist taking a swipe at the bogus 'marketeering' practices that have typically been used by manufacturers of planing hull powerboats. The props specified by new-boat manufacturers are always chosen to maximize top-speed for the boat when it is brand new, lightly loaded and optimally balanced fore-aft, as such they are almost always over-propped. Once the boat is in use, loaded up with supplies, belongings and a dinghy on the back, it becomes a different boat. As a matter of standard practice, I would always take pitch off the props anytime additional weight is added beyond what the marketers intended, and this goes double for boats with weight added aft.

Hope this helps!
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RiverGuy's Fleet:

2001 Bayliner 4788
2003 Mainship 390 (available for Charter)
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2001 3788 - Cummins 330B - Correct Cruising Speed? 10 Oct 2017 19:47 #9

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Note...your engines should NOT be producing any soot at cruising speed. If they are, then the most likely reason (by far) is that they are overloaded.

By the early 1990s, all marine diesels were using high-tech injection systems carefully calibrated to run a 'clean' exhaust. Resigning yourself to a sooty exhaust will surely lead to early engine failure.

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2001 3788 - Cummins 330B - Correct Cruising Speed? 10 Oct 2017 20:19 #10

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3,000 RPM at WOT is good, according to Cummins you should be seeing 2925 minimum at WOT. Lots of confusion on the 2,800 RPM number -- this is the RPMs for maximum rated horsepower, not for WOT.

One thing to reiterate (I went into this in more detail previously) is that it is your running angle that really matters here. If you are needing to run at 2400 RPM to get your bow down (which is likely with your dinghy setup), that's ok, and you are actually burning less fuel per mile than you would at a lower speed with a high running angle. Use the lowest RPMs you can to keep the bow down, but you should expect to need more speed when you have a heavy dink on your stern.

re: "Is there a safe lower rpm cruise speed?"

If your engine is making soot, you hull is going too slow...
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2001 3788 - Cummins 330B - Correct Cruising Speed? 10 Oct 2017 20:29 #11

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Whoa, excellent information. I've got some homework to do.

Thank you for such in depth explanations!
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2001 3788 w/ 330 Cummins
Seattle, WA

2001 3788 - Cummins 330B - Correct Cruising Speed? 10 Oct 2017 21:16 #12

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While you are at it, look at these fuel consumption curves from Tony Athens. Note how fuel burn rate drops dramatically -- even at the same RPM -- when you unload the engine!

Tony Athens Charts: Fuel Burn vs. Engine Loading

A high running angle (eg. 'plowing water') will overload engines just the same as if they were over-propped, so getting the speed up and the bow down will increase your fuel efficiency. Not to mention reducing your wake ;-)
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RiverGuy's Fleet:

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