Long time at low loads = dead diesel?

Discussion in 'Diesel Engines' started by Gashmore, Oct 5, 2017.

  1. Gashmore
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    Gashmore Junior Member

    I'm coming from the sailboat world where it is preached that running your 50-80hp diesel regularly at low loads charging and making water is a sure way to an early overhaul. In my old age I am now looking at the trawler world. In particular a 70' 50 ton semi-displacement hull with a pair of 1400 hp Cat 3412's. She hits hull speed at just a crack above idle (~900RPM & ~10 GPH) and that is where the majority of time on a long distance passage will happen. Also, I understand Cat estimates engine life more by gallons burned rather than running hours. Is there something different about these big engines?
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    FAST FRED Senior Member

    The way you are going to operate is NOT what the boat builders had in mind when installing (2) 1400HP CATs.

    The service life probably will be reduced at idle operation.

    To mitigate this a 15 min of high throttle operation every few hours may help , but the long term damage can not be avoided.

    Eventually the cylinders will be burnished and the engine will slobber.

    THe real question is how long will it take to begin to slobber , and weather it matters to you..

    A couple of thousand hours of clean operation , more with more frequent oil changes , and some heavy loads may extend that.

    Perhaps long enough for whatever cruising you desire?
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    How about running on one engine ?
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    FAST FRED Senior Member

    The hassle is there is huge drag from a non moving prop , and a slow turning freewheeling prop has even more drag.

    The stern bearing , and stuffing box may require special help (water under pressure) , and some transmissions do not freewheel with out lubrication assistance

    To be efficient a set of CCP props would be needed , so zero pitch , no induced drag , just prop blade surface area drag.

    Plan B is to haul the boat before an ocean crossing and remove one prop.

    A boat this size and weight 50 ton will need 2-3 HP per ton at displacement cruise speed..

    Even at the high end 150HP on a 1400HP engine is still almost idle.

    The only rational answer is to live with it and take precautions.
  5. Lepke
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    Lepke Junior Member

    Any decent marine diver can change props in the water. I'm a former diver and commercial fisherman for tuna and salmon. When trolling for salmon engine rpm was 700 and tuna (with a prop change) was 1000. Usually for 12 to 14 hours a day for 6 months. Usually ran in at night salmon fishing, but doing tuna, shutdown and drifted so the engines were not run at high rpm until the trip back. Maybe every 2 weeks. Engines were Detroit, so HD compared to recreational engines. Most other people were using Detroit mains at the time (early 80s) and many were WWII surplus. Some were never overhauled. Most would easily go 10-20,000 hours.
    Glazed cylinders and sleeves come from high rpm with light loads. Not so much from low rpm. More soot in the oil because the engine isn't running at the designed temp or rpm. Glazing is especially bad on high rpm generators. 3600 rpm the worst. 1800 better, 1200 best. @ 60hz.
    Diesel rings shape is designed to seal well at high loads. Light loads may not fully expand the rings so more blow by goes down the cylinder/sleeve walls and strips some of the oil that would normally lube the upper areas and more oil contamination.
  6. CatrigCat
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    CatrigCat Junior Member

    This is not a prompt reply but I hope it helps.

    Low loads most of the time lead to glazed cylinders for two reasons.

    First is the mechanical fuel injection. For low loads the engine requires very little fuel. Since the time that fuel is injected cannot be shortened, the fuel pressure is reduced (a lot). This leads to bad fuel atomization, thus more soot and unburned fuel that may dilute the cylinder wall oil film and contaminate engine oil.

    I am not a fan of common rail systems (dislike is a polite term) but they do help with cylinder glazing problems.

    Second reason and much more important is the cooling system temperature. Low loads without a sophisticated cooling system will lead to low coolant temperature and this is the major factor for cylinder glazing.

    If you can modify the cooling system (for example install a second thermostat) so that engine coolant is at normal operating temperature even when you idle the engine for days, you will have minimal cylinder glazing. Make sure the engines do not overheat at cruising speeds.

    Idling the engines for days will lead to engine oil contamination. Shorter oil change intervals is a must. Because an oil change for your engines is quite expensive, you may want to send out oil samples (for example to Blackstone) and they will inform you if you can keep your engine oil or it is time to change it. If it was a car, you just change the oil but with lots of gallons of oil for your large Cats, testing the oil is cheaper.

    Just keep your engine coolant at normal operating temperature and you will have almost no problems with cylinder glazing. Pushing the throttles once or twice a day for a few minutes at cruising speed will also help.

    Check the fuel consumption with only one engine running, the boat will probably need less fuel. If it doesn’t (rare) run both engines.

    Happy idling,
  7. baeckmo
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    baeckmo Hydrodynamics

    That's nonsense, both timing and injection duration vary depending on loading and rpm. The injection pressure is determined by the injector opening setting, which is constant.
  8. gonzo
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    gonzo Senior Member

    The pressure should be constant or pretty close to it. The time between start and end of delivery is what determines the amount of fuel injected.
  9. Steve Wright
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    Steve Wright Junior Member

    Nah, I hear this nonsense traipsed out all the time and it's wrong. I've run diesels (non-marine) professionally for 30 years, and 99% of the time they're idling unloaded. All my engines I've destroyed (quite a few - lost count) have either had failed timing chains or just plain worn out. I get at least 10,000 hours out of any engine.
  10. Joakim
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    Joakim Senior Member

    Old post, but just read it. A freewheeling marine prop will always have less drag than a fixed one. See fig. 6 in the appendix. https://strathprints.strath.ac.uk/5670/6/strathprints005670.pdf

    Might be a different story for very narrow blades used in airplanes and windpower.
  11. chinaseapirate

    chinaseapirate Previous Member

    I run 2 trucks part time also. I rarely idle them though. Mostly I run them at the lowest speed possible for traffic. If on national hiways, which i seek out, Im usually doing 51 or 52 mph at 1100 rpm which is within sight of its peak torque (my cruise control wont let me go below 1050 anyways- probably for some reason which someone knew was beneficial) . Interstates I feel 56mph is ok, don't get honked at too much. Are you sure your only a crack above idle at 900 rpm? I have a 3406B on the 2nd truck which is half of your engine (set at 400 hsp), and it idles at 650 . With an empty trailer it will stll idle in 10th gear no black smoke (but no acceleration without tons of black smoke). 11 th gear It will hold idle by itself but obviously not running right. That's what I was told to is to run a diesel at highest loading that doesn't make black smoke for max efficiency. Its nice if that matches your peak torque - 1200 rpm on my 60 series detroit. I thought it was the same for my 3406 .

    If I was NOT dragging nets through the water and just cruising around in a overpowered trawler, I would get the props off them ASAP. If i couldn't sell or trade them for new "lighter"props supplied and installed for free, I would order new from India and sell old ones for scrap price. Then pull out 4 cylinders on 1 engine and 6 or 8 out of the other. Run single engine anytime you can. Clamp the shaft with an vise and 4 x 4 if you have to . I know you can adjust power and torque on many older diesels but I think yours are only 700hsp each, and that you have a "rack" which you can set to minimum just turn the screw controlling it all the way in. I ran a diesel at sea once just about at 800-900 i'm guessing (tachometer wasn't included in price) for about 300hrs it always burned appox 2 - 2.5 gallons per hour and it was 5.7 liters. 1/5 the size of a 3412 CAT. I'm not sure if "hull speed" varies with type of design or what because there are more instances of the term being misused than not, if in fact there is a single definition of the term. But you should be doing about 12 knots easy with of 66% of one of those engines and a matched prop at 1100-1200 RPM or 8 knots even easier with 33% of the other engine at 900RPM (assuming the engine starts) with a second different prop. If your just living aboard and not going anywhere but out of the harbor and back...nothing wrong with running a single diesel at low rpm, whether max loading factor or not.

    You need an accurate displacement to match a prop, they are probably giving you registered tons info which means little. I've never met a propeller dealer (at least 5 that i remember) who knew anything about sizing a prop according to the propeller curve that designers use. And unfortunately none of shops ever have the size I specify... you may be best off with something that looks like an airplane propeller (which they wont have).
    Last edited: Apr 7, 2018
  12. gonzo
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    gonzo Senior Member

    Airplane propellers are completely wrong for use on boat, unless it is an airboat. All reputable propeller shops will have no problem using propeller application curves. You can also go on the Michigan Propeller website and get the calculations done.
  13. chinaseapirate

    chinaseapirate Previous Member

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    Two marine propellers -top. One airplane-bottom. Although even the general public knows not to buy an airplane propeller and expect it to work on a boat most of them wold be surprised to know that in other parts of the world outside of professional "propeller" shops and exponentially so marine "shops" (who's salesmen, usually do the ordering, without use a propeller chart) some propellers look to appear to be "airplane propellers".

    The chart above for the two bladed prop above is the lowest blade area ratio commonly available at the blank factory. Corporate America has/does/will not offer boats to the public capable of of using efficient props with "low blade area", and when they occasionally do its in the form of a canoe/kayak or sailing catamaran. Otherwise it will affect their marketing of gas guzzling outboards and extremely over priced bathtubs. However in your case, should you choose to go forward with your trawler deal, and you do not care about speed during passages, your most efficient prop (non available at your diameter, what 60") would be the two bladed example. 60 x 30 - 60 x 36 with 8 cylinders removed 700 RPM single engine 6-8 knots 3 MILES per GALLON. Not 3 gallons/mi like originally set up to trawl. And if your prop is already this low of pitch. Then just reshape it with much lower blade area. Be prepared for it to break the prop if you rev the engine. You got two so...your safe.
  14. Joakim
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    Joakim Senior Member

    The problem with low blade area efficient propellers is cavitation, which does not exist in air. Especially with airplane propellers, which on top of having low blade area have profiles causing very low pressures on suction side, since that is desirable for giving high efficiency. In water they do work, but only at low power and rpm used in mostly electric or human powered extremely low drag vessels at rather low speed. For them to work it is necessary to have low tip speed and low loading. Anything outside that will cause a lot of cavitation ruining the efficiency and potentially the propeller. The OP is interested in trawlers, which is the last vessel type to consider airplane propellers or marine propellers with very low blade area. With less power used it may be possible to reduce blade area and get a bit better efficiency and also clearly less drag for a fixed propeller.

  15. chinaseapirate

    chinaseapirate Previous Member

    Did I mistakenly say somewhere to "use an airplane propeller" tell me. I'll edit it...my "guestimate" was based on 60" prop size existing. The tip speed is the same as it was. If it isn't an 60" existing prop then the same pitch apples. 50 x 25 or 50 x 30. As I said before the yacht surveyor chimed in with "perfect words of wisdom". And I find no misinformation in what you have added. I know nothing of airplane propellers other than what they look like. I'm sure a refurbish of a trawler involves tossing a lot of heavy equiptment, #1 the drag of the net. And why would dropping the Rpm from 1500 original to 700-900 increase cavitation?? If an 80 hsp 28 liter V12 can't pull 6 knots than i lose my license as a prop selection guide....
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