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How do you avoid overloading mechanical engines?

Discussion in 'Engines' started by mapism, Dec 24, 2020.

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  1. mapism

    mapism Senior Member

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    Following a drift in this thread (with apologies to Beau about that), I am now following CaptPKilbride suggestion to start a new specific thread on this subject.

    The debate was about how to avoid overloading mechanical engines, particularly in a boat where for some reason (like trimmable Arneson surface drives) it is relatively easy to run her above normal propeller demand load, also while cruising under the max rated RPM.
    I wouldn't focus on Arnesons though, because even if they surely are more prone to demand high engine loads compared to shafts, overload conditions can happen also with shafts, regardless of the reason (wrong or dirty props, fouled hull, excess weight onboard, whatever).

    CaptPKilbride explained me that his way to avoid such risk is, after fitting very accurate digital tachs, to shortly run the boat at WOT in the specific conditions (load, trim, etc) he wants to cruise in.
    If the boat can reach the rated max RPM, all is well and good.
    If she can't, he knows there's something to be adjusted.

    Now, I don't think there's anything wrong in his logic, and I have zero objections to it.
    But I believe that if the boat is fitted with EGT gauges, that is an equally effective way of spotting an excessive load as soon as it happens, with no need to run the boat at WOT.
    In fact, AFAIK, the very first consequence of overfueling the engines, at any RPM, is a noticeable and immediate EGT increase.
    And by immediate, I mean it - in a matter of a few seconds.
    In other words, as long as EGT remains close to its typical range (which can be easily recorder upon a seatrial in ideal conditions), by definition there is no engine overload to speak of.

    So, over to you guys, what do you think?
    Is one approach better than the other (and if so, which and why), or are both good enough?
    Besides, are you aware of any other tricks, maybe?

    Of course, I'm happy to be corrected by CaptPKilbride if in the above summary of his approach I misunderstood anything.
  2. ranger58sb

    ranger58sb Senior member

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    What does it take to fit EGT gauges to mechanical engines? Install a sensor where? Off-the-shelf sensor hardware and gauges? To fit any mechanical engine?

    -Chris
  3. CaptPKilbride

    CaptPKilbride Senior Member

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    @mapism have you ever observed such a phenomena for yourself with your own eyes?
  4. BRyachts

    BRyachts Member

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    For EGT usually a NPT fitting welded into exhaust riser after the turbo and before the water injection for a thermocouple probe. Engine Mfg will have preferred location.

    For general overload, on initial setup I would want to do the max RPM test, and get a baseline on EGT and Boost.

    During subsequent running track EGT and Boost. Possibly have alarm set points.
  5. Capt J

    Capt J Senior Member

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    Aside from Arnesons (which are an entirely different matter entirely and you're not going to get around overloading the engines at slow speeds).

    With straight inboards I REALLY like to see them make 50 rpms over WOT specs with full fuel and water. Propped that way, unless you're running only on one engine, it would be a really unique event that you'd run them overloaded at any RPM point, except in that zone where the boats just about to get on plane and you shouldn't run unless you're on your way to get on plane, or really large seas where you have to. Honestly I wouldn't worry about it.

    Not sure why manufacturers like to prop to barely achieve WOT rpm's. It doesn't make the boat faster. Spinning the props faster generally makes more lift and more speed. So if they hit 2350 WOT instead of 2300 WOT, you're still going to make the same WOT speed and if cruising by load, at 80% load you're make more boost and more HP at a slightly higher RPM and faster cruise speed.
  6. mapism

    mapism Senior Member

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    Not yet, also because my boat doesn’t have any EGT probes/gauges – hence my interest to fit them.
    But let me add a couple of comments on that:

    Firstly, one thing I always noticed while helming boats powered by electronically controlled engines (whose displays can tell you just about anything, as you surely noticed with your new Böning MMDS monitors) is that there is a remarkable correlation between EGT and load.
    Normally, they go up and down depending mostly on RPM, but for instance on a long range cruiser with almost 3k gallons of fuel tankage, I did see a difference in both load and EGT before/after refilling.
    Or in other words, we used to run at the same EGT/load after refill, as we did with empty tanks, but at a lower RPM, hence speed.
    Not a night and day difference of course, but noticeable. Half a knot or so.

    Secondly, the person from which I learnt the theory that I explained in my OP used to be a technical manager at Cummins, where he was also a member of their design team. On Boatdiesel, where he’s a moderator, he’s known as the “diesel engine bible”.
    He literally used to say that he would be happy to run a diesel engine based on EGT alone, rather than on any other instrument.

    As an aside, it’s worth mentioning a partial exception to the EGT/load relationship, and that’s with planning boats with u/w exhaust.
    In fact, EGT doesn’t depend only on load, but also on backpressure.
    And with u/w exhaust, backpressure can progressively increase before the boat is steadily on the plane, and then reduce slightly, when the speed begins to exploit the scavenging effect on u/w exhaust.
    But that’s easy to check and keep into account, by making an EGT baseline as BRyachts suggested.
  7. mapism

    mapism Senior Member

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    I agree that it's pretty rare, but not impossible.
    I've seen straight shaft boats unable to go on the plane due to heavily fouled bottom (both hull and u/w gear).
    Just run such boat for a while with the throttles hammered, and I bet you will see EGT skyrocketing in no time.

    Then again, if you would argue that it doesn't take any EGT gauge to understand that such boat desperately needs a good clean, I'm not disagreeing, of course! :)
  8. alvareza

    alvareza Member

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    A fully canyon loaded sportfish might be operating in an overloaded condition if propped to barely make rated rpm. If setup to turn up 50 rpm over rated rpm probably ok with even extra people, gear, bait, ice and full fuel bladder.
  9. CaptPKilbride

    CaptPKilbride Senior Member

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    The entire crux of my discussion had to do with engines with mechanical injection pumps. I lightly touched on the topic of the propeller power absorption curve in relation to the fuel curve, and how in a mechanical engine at less than WOT the rack could be advanced to create an over fueling condition with the potential for damaging the engine if allowed to persist over time.

    Now you are throwing electronic engines into the mix, which changes the conversation significantly. For what it’s worth, on an Arneson drive boat the practice of spooling up to verify the drives are set correctly is still good practice in my opinion. The downloaded engine load blocks support this as well.
  10. Capt J

    Capt J Senior Member

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    Well, you always need to have a clean bottom. A dirty bottom will overload ANY engine no matter how lightly it's propped.
  11. Norseman

    Norseman Senior Member

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    A pyro gauge has been on my list of things to do for a long time:
    (Yanmar turbo diesel)
    Hate drilling holes and running wires however, also running out of space on the dash for a pyro gauge:
    (Pod mounted to the left of chart plotter perhaps)
    B8AF490D-8E48-4A42-BF52-B30D0A6AF65A.jpeg
  12. Capt Ralph

    Capt Ralph Senior Member

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    I am sure by now that N2K senders are available. Most new displays can display engine data.
    Your Yanmar is probably CanBus/N2k ready.
    BTW, Is that Lev-O-Gage for you or the boat?:p
  13. mapism

    mapism Senior Member

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    I mentioned electronic engines because only with them you can monitor the load.
    And in my experience with them, the higher the load, the higher the EGT.
    Consequently, if you follow the same reasoning backward, whenever you have higher EGT (which you can monitor also with mechanical engines, as opposed to load) it's safe to assume that it's due to higher load.

    The fact that you can't see the load with mechanical engines is a red herring, because the logic actually remains exactly the same, and it is indeed possible to overload also electronically controlled engines.
    In this respect, I agree that the max RPM check is as much a good practice as it is with mechanical engines.

    OTOH, keeping an eye on EGT (possibly with the support of alarm set points, as BRyachts suggested) is in my opinion equally effective if not even better, and doesn't require spooling up at WOT every time you want to adjust the drives setting for whatever reason.
  14. Norseman

    Norseman Senior Member

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    Aye Capt Ralph:
    The level gauge is there because my previous boat, a Glacier Bay 2770, coming back from the Bahamas with quartering seas from the Northeast I though the **** thing was going to roll over: Average 15 degrees, max 30 degrees and this was NOT a sailboat.
    For reference I glued a similar Lev-O-Gauge on this here Albin, had no idea a power boat would rock and roll that much.
    Fortunately it did not, hardly see much angle on the gauge, but being an ex-pilot and control freak I need as much info as possible even if running out of panel space.
  15. mapism

    mapism Senior Member

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    If so, maybe Norseman already used the N2k interface to feed the Floscan instrument above the callsign?
  16. Norseman

    Norseman Senior Member

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    Not sure?
    The boat came with Floscan when I bought her 3.25 years ago and it was installed 2001, when I was just a youngster. :cool:
  17. mapism

    mapism Senior Member

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    Ok, that's irrelevant vs. your interest to fit a pyro sensor, anyway.
    I believe the point CaptRaplh was making is that by fitting a N2K sensor you might be able to display EGT directly on the Garmin plotter, with no need to install a dedicated instrument.
  18. Norseman

    Norseman Senior Member

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    If I could display pyro numbers on the Garmin, great, but would prefer a dedicated gauge. With the lack of panel space I may have to instal a pod:

    D97038B0-C929-4C2F-A32E-A5313876ABED.jpeg

    But only as a last resort. :D
  19. Capt J

    Capt J Senior Member

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    Just mount the pyro gauge sideways on the starboard support bracket for the garmin.
  20. DOCKMASTER

    DOCKMASTER Senior Member

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    I was surprised when I installed my new C-18’s that EGT was not a standard feature with the engine package. I assumed the computer read and used EGT as a parameter to control the engines. But this is not the case. I wanted to monitor EGT so ordered the probes as an option. The wire harness is set to accommodate them and the displays can be programmed. The only thing that surprised me more than them not being a standard feature was the cost of the probes