What can cause diesel to be overloaded intermittently?

Discussion in 'Diesel Engines' started by Northeaster, Sep 15, 2018.

  1. Northeaster
    Joined: Sep 2011
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    Location: Eastern Canada

    Northeaster Senior Member

    Hi folks,

    I know that I am somewhat overpropped now, after adding cupping and a couple of inches of pitch - 2 years ago, I could hit 3300rpms (in gear) on an engine who's max rating is 2800rpm and max continuous rating is 2500rpm. Now I can only hit about 2300rpm except for when I lightened up (removing excess junk / anchors and even waterlogged temporary floorboards etc) and then hit 2600rpm and 20.5 knots. I am not finished building - will add a small cabin roof so I won't have the prop changed until the final weight is on.

    However, my concern is that I can run for a half hour or more at 2200 /2300rpms and hit about 18-19 knots, but for some reason, typically coming home in a following sea, it will start to be overloaded intermittently. I understand that as I am already overloaded/ at max achievable rpms due to the overpropping, that I would experience some black smoke/ reduction in rpms if I go up and down big rolling waves for example. But, once this starts to happen, even if I then change directions the engine will not go above perhaps 1900 or 2000 rpms. Of course, I throttle back a bit and keep an eye for back smoke, but I would like to know if this is just due to the overpropping or could there be any other causes?
    I dive under frequently - neither the prop nor the bottom are fouled.
    Air filter is clean and I even tried taking it off for a minute or so juts to make sure- no difference.
     
  2. Sparky568
    Joined: Jan 2017
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    Sparky568 Junior Member

    If I remember correctly, you had a Cummins 6bta? You need to achieve max rpm’s under load. The reason you may be experiencing this especially in a following sea is “climbing the hill” overcoming it. Overpropping a diesel is the kiss of death. Check out this site.

    Propping a Cummins Mechanical Diesel Engine - Seaboard Marine https://www.sbmar.com/articles/propping-cummins-mechanical-diesel-engine/

    Many other good and informative articles there. You also want to compare fuel burn rates to you specific engine spec. A pyrometer is also a good measure of a diesel working too hard.
     
  3. JamesG123
    Joined: Mar 2015
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    Location: Columbus, GA

    JamesG123 Senior Member

    Compression test?
     
  4. Barry
    Joined: Mar 2002
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    Barry Senior Member

    On our last boat with Cummins, we had an issue with the turbos at the spring start up.
    Evidently over winter, the bearing was failing and hence the turbo would not run up to speed.

    And how we finally determined this
    Fired up the engines, let them reach operating temperature at 400 rpm prox over idle. The boat had sat for 5 months through winter.
    Took it out into the channel, began to increase rpm 500 rpm every couple of minutes.
    The engines are electronically sync'd

    These are not exact rpms as I did not record them but used to illustrate the point.

    Up to 2000 rpm, both engines ran together.
    But as I advanced the throttles to I noticed that one engine was not keeping up.
    Port 2400, stbd 2200 +- No smoke out of the lagging engine, advanced throttles
    Port 2600, stbd 2200 +- some smoke from the stbd engine
    Port 2800 - 3200 prox tstbd 2300+- more smoke from the stbd engine

    I called a couple of diesel mechanical shops and they immediately said that the stbd engine turbo had failed
    They said to pull the filter and you can see directly into the turbo fan, when cool, try to spin the wheel by hand, if it does not spin, then the turbo requires repair.
    And that is what it was.

    So I would try this.

    Also on a more global note, Boatdiesel.com is one of the most comprehensive sources of information on diesel engines, all makes, service advice, trouble shooting advice etc. Last time I
    was a member, the cost was $25 US per year but the info saved me more than that. Prior to purchasing the boat with Cummins, we looked at a Mariner with twin Yanmar's, so I went on this site and found a reference to a manufacturers recall on these engines, researched it and this particular boat did not have the changes made, some quite labor intensive. So we backed away from the Mariner. I informed the broker to let the owner know about the issue as they revolved around excessive heat and possible a fire potential. Several months later, the broker phoned
    me to see if we still had an interest in the Mariner, we declined, went a different way, but he told me that Yanmar came to the table and helped out at a significant level to remedy the deficiency. ( and I think that this was maybe 10 years AFTER the recall)

    The following is what I understand to be accurate, but may be wrong

    Tony Athens who is recognized as a leading diesel mechanic, contributed information to the boatdiesel forum but within the last few years, declined to post. He either works for or did work
    for Seaboard Marine, sbmar.com Seaboard provided free advice on just about every type of diesel engine on the market plus there are many many YouTube videos from their
    shop that shows how to service intercoolers, turbos etc
    Now either Tony is still with Seaboard or is out on his own, but I would look at Seaboard to see what they have on their site, alternatively phone them, spend $5, or pay the membership
    on Boatdiesel to access their information. And if Tony is not with them, then try to find his site for more information
     
    Last edited: Sep 15, 2018
  5. Northeaster
    Joined: Sep 2011
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    Northeaster Senior Member

    Hi guys - thanks for the relies. Engine is a 1991 4BT with about 4300 hours on it.
    I did check the turbo vanes (both intake and exhaust) - they spin freely and have no noticeable play in them, in any direction.

    I have read almost all of Tony's tips on the Seaboard Marine site, on a variety of topics.
    I should break down and join the boatdiesel site- I know there is a lot of knowledge and help there.

    To be clear, I plan on having some pitch taken off at some point ( I have 2" of pitch and moderate cupping added a couple of years ago, which brought my max acheiveable rpms down from 3300 to about 2300. But, when I tow the zodiak (which is often) and/ or have 3-4 adults on board and 3-4 kids and beach gear, I start to bog down to around 2000..
    Just not sure if I will have it done this year or wait until the cabin roof is done and final weight is added. I will replace my waterlogged temporary wood floors with aluminum of a bit less weight, and wont be adding more than a couple hundred more pounds, I don't think..
     
  6. baeckmo
    Joined: Jun 2009
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    Location: Sweden

    baeckmo Hydrodynamics

    When overpitched like this, the propeller power versus rpm curve lies very close to the engine power curve; they follow each other closely. This means that there is very little margin for increase of propeller power requirement due to increased wheight et c. Sparky's comment "...overpropping a diesel is a kiss of death" is spot on. There simply is not power enough at low to medium rpm because the turbo is not producing enough pressure.

    What gearing do you have, and what was the propeller dia, pitch and blade number before modifications?
     
  7. kapnD
    Joined: Jan 2003
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    Location: hawaii, usa

    kapnD Senior Member

    What an cause a diesel to be intermittently overloaded? (Or show similar symptoms)
    Dirty fuel and or filters, lack of air, overheated ER, overheating engine, dirty bottom and running gear, overloaded vessel, poorly matched power, gearing, prop, out of tolerance engine parts (worn out), added engine driven accessories, out of whack throttle and shift controls or cables, loose belts, to name a few.
    An infrared thermometer, anEGT gauge, fuel vacuum and intake pressure are good diagnostic tools.
    Recalling your previous thread, you had cobbled together some parts enough to make the engine run, did you ever get the motor really up to spec? Coolant capacity is important in maintaining even temps.
     
  8. Sparky568
    Joined: Jan 2017
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    Sparky568 Junior Member

    Tony,s site has a forum. Describe you engine and issues, someone will respond and it’s free. :)

    A quick check of your engine specs shows output around 150hp. Seems you’re asking too much from that little 4 banger.
     
  9. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    No doubt running with the sea, and your speed not being that far different to the wave train, has you climbing the back of waves, a lot of the time, that will load the engine up.
     
  10. Northeaster
    Joined: Sep 2011
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    Northeaster Senior Member

    Ok - thanks for all of the replies! Sorry for the long post but trying to answer questions and give relevant info..
    It is a 4BT, 150hp
    gear is a 1:1 72C Velvet drive
    prop is a used 3 blade 15x14 I believe. I had a very experienced prop guy add moderate cupping and 2 " of pitch - results before and after below.
    Prop had some pitting but he said is should work fine.

    the boat is still a work in progress but now has:
    - proper OEM gear driven seawater pump
    - permanent new Moeller fuel tank
    - all new fuel lines
    - new large capacity primary filter/water separator, and new engine filter put on last years and have less than approx 50hours or 150 gallons of fuel through them. I could try changing filters them but would they really be causing issues with this little use since starting with a new tank, lines, filters, etc.??

    Prop and bottom are pretty clean - bit of slime abut no barnacles, etc - been in our cold water 2 months now.

    I had issues with overheating and loosing coolant / steam in the exhaust for the last 2 years (ran water alot rather than coolant while troubleshooting)Issue was found and fixed (crack in water cooled exhaust manifold - new one put on this year and since then no overheating and no coolant loss)
    Did replace head gasket myself earlier but this was not the issue.
    - Engine runs at 180 -185 degrees consistently on once warmed up.
    - The heat exchanger is from a gas engine and water hoses inlet/ outlet ports are 1" diameter vs my seawater pump and likely an OEM Heat exchanger with 1.25"connections- but, as it runs at 180 even at 2300rpms could the smaller cooling capacity matter - I mean is is running at proper temps??
    - I have never checked compression but it has always started immediately....

    old numbers before prop change:
    4.5 kts at idle 800 rpms
    14.5 kts at 2200 which is where it hits peak torque
    About 17 kts at 2500rpms which is max rated rpm for continuous use
    About 19 or 20 kts at around 2800 rpms which is max rated rpm intermittent use.
    23 kts full out which is I think around 3300 rpms - have not been out again since cleaning sensor. ** I know 3300 is more than rated - thus was same as achievable in neutral***


    new numbers after adding cupping and 2"of pitch to prop.
    5 kts at idle 800 rpms
    17 kts at 2200 which is where it hits peak torque
    About 20 kts at 2500rpms which is max rated rpm for continuous use
    max out at About 21 kts at around 2600 rpms which is 200 rpms less than max intermittent use rating due to load on engine.
    ** However - these numbers were from last year before adding a bit more weight and I can now only achieve about 19Kt at 2300rpms when alone and not towing a zodiak....
     
  11. Sparky568
    Joined: Jan 2017
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    Sparky568 Junior Member

    When your boat is complete and fully loaded with the most weight/people you would eve want on board you should see 50 to 150 rpms above rated rpm to achieve max efficiency and longevity. Not sure why you went through the trouble and expense of adding cup before completing.
     
  12. Northeaster
    Joined: Sep 2011
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    Northeaster Senior Member

    I guess I made a mistake... was not my first, nor my last likely.. I don't think I will be adding more than a couple of hundred pounds in the future so it is close to final weight. When I weld in aluminum floor panels this winter I should actually save weight over the current, waterlogged (in places) temporary OSB/cheap ply floors.
     
  13. Northeaster
    Joined: Sep 2011
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    Northeaster Senior Member

    I know it is not a great video, but this was earlier this year, albeit me alone in the boat and not towing a zodiak. I think the 4 banger moves the boat along nicely - I juts have added to much weight and changed the prop too much to handle more weight and towing..

    The hull should only weigh 1800 lbs had I used 1/8" aluminum per plans. As It would have been harder to weld and had much more distortion, I went with 3/16" hull sheet topsides as well. I justify this somewhat as I stretched the boat abot 10% (allowable per plans) from 23 ft to about 25ft. I felt that this added hull size and extra bouyancy would partially opffset the increase in weight - i.e. I would expect a 25ft boat to perhaps have a thicker hull than a 23ft boat, albeit not 50% thicker.. The engine was on the high side of allowable engine weight.
    these speed numbers are from the Glen L website:

    6 knots 10 SHP
    9 knots 25 SHP
    13 knots 40 SHP
    18 knots 70 SHP
    22 knots 105 SHP
    27 knots (*) 150 SHP
    31 knots (*) 200 SHP
    (*) Attempting to obtain these speeds with the Inboard Version, due to the deep skeg, may cause excessive fuel consumption and may not be practical.
    A local builder of the aluminum version is running at 40 mph with a 200 hp outboard.

    ** I have a full box keel**

     
  14. kapnD
    Joined: Jan 2003
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    kapnD Senior Member

    Don’t recall ever seeing 25 knot winds with no whitecaps, but I’ve never been to Canada, maybe it’s different there?
    Your undersized heat exchanger could definitely be restricting the cooling system. It might be fine for very low speeds, but if you’re going to run at high speeds, knowing that the vessel/engine is overweight, your redesign should target the high end of the spectrum, and a healthy dose of overkill, rather than under is in order.
    Do you have an infrared thermometer, and have you taken readings on all inlets/outlets of the heat X at say every 1000 rpm throughout the range? How about at cruise speed every few minutes for a half hour or so?
    Exhaust gas temperature graphing would be a good idea.
    Quoting the numbers from the website is totally meaningless, as you have altered the design, apparently without engineering the changes you made.
    Until you are certain that the motor is making its advertised horsepower, and that that number is adequate for the application, you are shooting in the dark.
     

  15. Northeaster
    Joined: Sep 2011
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    Location: Eastern Canada

    Northeaster Senior Member

    Yes, our cold water keeps the white caps from appearing in most home videos. Not surprisingly, it also lets us run with undersized heat exchangers.
    No, I do not have an IR gun at the moment. We only have a few nice weekends left of boating up here, so I may not get a chance this year. (have 2 small kids to keep me busy and a cottage to paint before it's too cold to paint this fall)
    This may be a stupid question but if the heat exchanger were too small, would then engine temps not climb beyond the 185 degree mark? I fail to see that the engine's performance could be hampered if it is running at the correct temperature...
    Now, I do see how having added to much pitch to the prop is affecting performance (given the current weight of the boat, and when towing or many extra persons are factored in... No question I am overloading the engine at times which is not good for it.
     
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