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Fuel burn...

Discussion in 'Technical Discussion' started by mapism, Dec 2, 2022.

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  1. mapism

    mapism Senior Member

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    THIS THREAD WAS SPLIT FROM ANOTHER DISCUSSION

    Agreed, but the main trigger of fuel burn in any half decent planing boat remains sheer weight/size.
    Of course, for any given weight, one hull can be more efficient and burn less fuel than another one, but try to compare two identical hulls and ballast one of them to double its weight, and you'll see what happens to fuel burn!

    That's why, back to your specific example, I really struggle with your numbers.
    In fact, my boat also happens to be powered by twin 800hp MANs, and obviously I am very familiar with her fuel burn.
    BUT, even if she's substantially larger that a Cabo 40' (being a 56.5 x 16.5 feet hull, and weighing 68k lbs loaded), I couldn't make her burn 62 GPH at cruise speed even if I tried.
    Her typical cruising range is anywhere between 1800 and 2000 rpm, burning 41 and 55 GPH respectively.
    Now, a 40' Cabo must weigh what, half of mine, give or take? Pretty sure she can't be anywhere near 68k lbs even if ballasted.
    So, I wonder how on earth can she burn so much more, because it would take a really awful and inefficient hull (which is not what Cabos are known for) to justify such difference.
    Did anyone forget to untie the boat from the dock, before pushing her at cruising rpm?!? o_O
  2. Capt J

    Capt J Senior Member

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    Zues boat "It turns on a dime, thanks to standard Zeus electronic steering that adjusts for boat speed to keep everyone safe. Based on my sea trial and discussions with Cabo’s factory captain, the 40 HTX’s best cruising speed, with respect to economy and engine load, is 2,710 rpm, making 32 knots and burning 45.4 gph for a net of 0.71 nm per gallon."
    Cabo 40 HTX | Boating Mag

    MAN boat burns 63 GPH at 2100 rpms 32 knots

    https://yachtforums.com/attachments/c40-pdf.94861
  3. mapism

    mapism Senior Member

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    I wouldn't fancy running those engines all day at 2100, but I guess that's a choice driven by the usual builder+magazine wish to report the highest possible cruising speed.
    In fact, if you check post #8 in this thread, one owner mentions substantially different numbers.
    BTW, the same guy says that the hull was designed for shafts, which makes the results reported by Boating Mag for the Zeus version even harder to understand.
    Let alone understanding how a boat which does indeed weigh half of mine (according to the article that you linked) and with exactly the same power have a comparable fuel burn... o_O
    But... Hey-ho! I wasn't thinking to buy one anyhow. :)
  4. Capt J

    Capt J Senior Member

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    MAN and Cummins both list rated cruise speed at 80% load, those numbers are at 80% load. Cabo modified the mold for Zues boats so they weren't mounted on the V, same hull just different in the tunnels. I've run boats with pods mounted on the V (same angle as the V) and when you turn a little more than a little all of a sudden they hook and the entire boat leans over and cuts a hard turn. How are the numbers hard to understand? Cummins boat 80% load 32 knots, 45 GPH. MAN boat 80% load 32 knots 63 GPH. I've run dozens of those boats in those combinations. I'd actually get 33 knots out of the Zues boat at 45 GPH, and 32 knots at 62 GPH on the MAN boats, on all of them. Cat C12's 700hp would do 30 knots cruise at 58 gph I think it was. It's a direct comparison. It's a direct comparison as one can get, same boat, same setup, same weight (except lighter engine weight). Different props, different reduction, hotter air and water temperature than where you are. 85-90F air temp and 83-86F water temp burns more fuel. Those are the numbers and 100% accurate. There is no data on Post #8, these numbers are for the express with tower, they also did make a flybridge 40' cabo also which would be slower.
  5. mapism

    mapism Senior Member

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    Why, do you think it's normal that a 40 footer, weighing about half of a 56' with a similar powerplant, has a comparable fuel burn?!? :confused:

    Ref. the data in post #8, I was referring to the Cabo 40 fly owner talking of 48 GPH at 30 kts, which sounds more realistic to me.
    BTW, he also talks of a 38kts top end, which is actually more than in the PDF seatrial that you attached.
    Of course, even if I wouldn't understand why, he might as well be lying, what do I know...?
    Last edited: Dec 3, 2022
  6. leeky

    leeky Senior Member

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    With the same or similar power plant they can have a comparable fuel burn at the same rpm or load; it's the nautical miles per hour that will be different.
  7. Capt J

    Capt J Senior Member

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    Because it's doing 32-33 knots at that fuel burn. Better efficiency. NMPG
  8. mapism

    mapism Senior Member

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    'Course my 56' is slower, with the same power pushing twice the weight!
    She does 29 kts at 2100rpm, but even at that speed, she still doesn't burn 63 GPH: 61 is more like it.
    So, in terms of NMPG, we're talking of 0.51 for a 40' vs. 0.48 for a 56' - which one would you describe as "better efficiency"?!?
    In fact, the 0.63 NMPG (30/48) reported by that owner of a MAN powered 40' fly seems to me more consistent, when compared with the 0.71 of the Cummins/Zeus version.
    But hey, if you confirm that your PDF file of the seatrial is accurate, I take your word for it.
    As I said, I'm not thinking to buy a 40' Cabo anyhow - and pretty sure I'd never remotely consider buying a pod powered one even if she would burn half of what she does!
    Which interestingly, is a viewpoint most folks agreed with also in the thread I previously linked.
  9. Capt J

    Capt J Senior Member

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    The difference in fuel burn is because your boat is in the med, cooler water, cooler air, leaner air fuel ratio. Could also be props. That's what the man 800's burn, any set of 800 hp diesels burns 60-65 gph at 80% load. You could always run the cabo slower and get better mileage, like any boat. But yes, the seatrial is accurate. I've done deliveries on at least 20 different 40' Cabo's over the years. For efficiency the Volvo's D12's in the newish Cabo 41's were very fuel efficient for shafts and ran good....... 41' is same boat as 40'.
  10. mapism

    mapism Senior Member

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    I think you're largely overestimating the possible effect of the different environment, even if you add the more salty water to the other elements you mentioned.

    As far as props go, I'd expect Cabo to use something as efficient as anyone else, also because while the builder of my boat used to fit Rollas on open boats, flybridges like mine have plain vanilla 4 blades Radices, which are nothing to write home about.

    OTOH, I agree that a pair of 800hp diesel engines typically burn between 60 and 65 GPH at 80% load.
    In fact, I didn't say anything different in this respect, when I said that at 2100rpm (whose ballpark load is 80%, though with mechanical engines you can only estimate that) I'm around 61 GPH...
  11. Fiammetta42

    Fiammetta42 Member

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    @ MapishM.
    Can you see fuel burn on a screen live ?
    In the absence ( which I suspect ) using dry manufacturers graphs with hypothetical prop curve is where the error , the mismatch of your findings and the others on here lies .

    To comment on fuel burn one needs live instrumentation.
    My MAN 2876 le 401 rated 700 Hp have MMDS so I can see it live .
    The numbers I witness are different from the manufacturers graphs flying about online which are let’s face it done on a test bed not in a real boat .Those manufacturers graphs are over optimistic.

    This explains your wildly optimistic fuel burn numbers on this thread and others when you quote your numbers .
  12. mapism

    mapism Senior Member

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    Nope, I should retrofit flow sensors for real time measurement. Doable, but not worth the hassle/cost.
    OTOH, I've got calibrated and very accurate sight gauges, where I know that each centimeter corresponds to 27.5 liters.
    So, I ran the boat cruising for one hour at every hundred rpm and checking the tanks before/after.
    I guess you might dismiss this method as rough, but I can assure you it isn't, also because it proved pretty consistent over the years.
    Feel free to call it "wildly optimistic" anyhow, if that makes you feel better.

    Incidentally, you are aware that your engine displays do NOT show the actual fuel burnt, but rather what the ECU thinks it is, aren't you?
    If you would have one engine with an injector stuck or pissing fuel, you would never see any difference with the other engine.
    You might as well have the fuel hose of one engine leaking a lot of fuel, and you will never notice that, till the bilge alarm goes off or the leak is large enough to starve the engine of fuel.
    Only flow sensors on both send and return lines (e.g. Floscan and Maretron) can measure the real world fuel burn, net of any electronic guesswork.

    Ref. the manufacturers charts, I agree that for most builders they aren't worth the paper the brochure is written on, but I found mine remarkably accurate.
    Unsurprisingly, since back in the days, the builder made a point of making those charts on a normal production boat, fully equipped and fully loaded - including ballast equivalent to the max number of people the boat is rated for.
    In fact, believe it or not, a couple of years ago, in a trip at the beginning of the season (with hull+props clean, a very light fuel load, empty fresh water tank and only myself and the Admiral onboard), I actually burnt a fair bit less than I should have based on those charts.
  13. Fiammetta42

    Fiammetta42 Member

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    ^
    A fuel hose off would trigger some alarm or other .If it doesn’t then simply lack of rpm or some other parameter would be brought to the helms attention.
    Coincidentally since you mention injector timing I have had a faulty injector sensor ( the only thing btw in 7 seasons taking them from 480 to 1080 hrs ) .Well you get a sensor error for starters and some of the numbers do alter .Alter enough to make it obvious further investigation is necessary.Also a rpm limit is triggered .I can’t remember exactly which .But noticed for example the EGT s no longer sync d .The affected motor switched to a stored algorithm for its fuelling instead of using live data = some parameters on the screen lost there synchronicity.This is when the sensor alarm is shut off .


    So the multiple bits of data fed into the black boxes by the arguable excessive sensors MAN use , then transferred to the screens kinda cover each other .

    I don’t think your argument that Capt J s , mine and others sensors are erroneous when we run our boats and happen to flick about through the various pages on our screens ……fuel burn being one , is validated.

    If they were as I said the boat tells you either via alarms or you see plane differences in the pairing of the parameters.

    For any credibility discussing fuel burn comparisons on boat forums we need like for like independent electronic ( does not matter which the manufacturer has adopted) read from a screen .
    I will add the caveat if I may from fully working engine(s) not alarming off .

    like this below .
    51F02E4A-40FF-4684-A159-0BB9D9CB34A8.jpeg

    My normal cruise ^ keeping under the 80 % load in the interest of engine longevity.Equates to a 28/29 knot cruise .Or a bit lower with a dirty bottom .
    I can’t exactly remember when this pic ^ was taken in the season ? I have seen 80 L early on in the season lightly loaded .So the consumption does vary sometimes.I suspect this pic is later in the season + heavily loaded fluid wise ?

    5EE38D86-BB34-4497-9AE2-1163B1C537B1.jpeg
    A fast cruise ^ 31-32 knots .

    0A4BECCD-BB54-485A-A101-7DD348B08CCA.jpeg

    WOT test. Some 3-4 months after it’s annual service / antifoul .
    The rated rpm is 2200 . So pretty pleased with that 20 y old MAN 2876 le 401 s @ 700 Hp , in-line 6 s . Tells me the propping is right .No alarms ( touch wood ) the motors are in rude health.

    I have sight glasses on my tanks with tape from the previous owner marking them in 1/4 increments.I just casually “ sight “ them during daily checks especially if I plan a long ish trip .Yes agree with you partially years of experience tells me you can’t beat the MK 1 eyeball .With electrotwackery there’s always a lurking suspicion it’s gonna fool you .

    But it’s no doubt the best thing , the only way to compare boat fuel burn rates .Well to get any credibility.
  14. mapism

    mapism Senior Member

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    Well if you feel you NEED that, don't consider my posts.

    My point exactly.
    But as I said, if you don't trust my comments, just ignore them.
    It's that simple!
  15. Fiammetta42

    Fiammetta42 Member

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    It’s not or ever was a case of ignoring yours or anybody else’s post .

    It a case of accountability for your outliner consumption figures .
    Explaining the real reasons for the differences.

    As it is you claim your 16 L V8 s running at or around 1900-2000 rpm ( or what ever ) in a far bigger / heavier boat 56 ft vs theCabo 40 or my 14 m lighter (heavily built granted coming in @ 18 - 20 T depending on fluids carried ) boat with inline 6 s at 40 % less cubes , 2 less pots users the same fuel more or less .It can’t .Worse still burn less fuel typically running .


    Assuming my numbers I have posted and Capt J he quoted ( assuming he can read his screens ) are correct .Also if others chime in with there screen data we will collectively form a hierarchy whereby the Hp , the size ( boat + motors ) , drive systems IPs or shafts , Arnesons , jets etc broadly form industry standard order .
    So others without “ screen “ fuel burn data can make a meaningful estimation where there boat fits in this general order .

    Your calibration must be waaaaaay out .

    Your numbers are outliners .

    Have you never considered that’s the case ?
  16. volfkan

    volfkan Active Member

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    pershing 82 after this summer plance mti 52 race boat order but wife 50-60 ft yacht need (don't want to spend a lot of money)
    If I can't convince, this topic will be useful. =)
  17. Fiammetta42

    Fiammetta42 Member

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    So a 1 M tall tank hold 2700 L ? That feels a bit large in a 56 .
    Can you expand on those numbers ? They don’t feel right .

    How tall are your tanks and what’s your boats tank arrangements?
    Just intrigued by your claimed outliner fuel burn numbers .Let’s get to the bottom of this anomaly.

    I mean mine are twin 800 L marked in 1/4s so 200 L between marks .
    The tanks guessing are about 1.4 M tall by eye and estimating. The marks 1/4s are about 30 cm + apart .Each tank feeds a motor .So running guzzling 90 L per side @ 80 % or under load I have autonomy of over 8 hrs .

    Additionally there’s an optional 600 L “ long range “ tank .This has no sight glass or gauge .Buts it’s connected by cocks / valves to the others so I can top them up . About another approx 4 hrs planing .3 to be safe .
  18. mapism

    mapism Senior Member

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    I've got two sight gauges, one on each of the side tanks, and when I mentioned the 27.5 liters/cm I meant one centimeter on both sight gauges.
    Which BTW is yet another way to check the consistency.

    I don't think anyone is interested in further details on the tank arrangement of my boat - or any other boat, for that matter. Christ, I'm not interested in that myself!
    All I know is that every time I filled 500 L, anywhere from almost empty all the way to fill-up), I get just a tad more than a 18 cm raise on both sight gauges.
    So, it doesn't take a math degree to reach the 27.5 L/cm that I quoted.
    And which proved consistent over 6 years of boat ownership.

    Of course it can, and your reasoning just shows how unfamiliar you are with the way diesel engines work.

    Firstly, when comparing the same output produced by two different engines, assuming that the one with less cylinders and/or less displacement must burn less makes no sense whatsoever.
    Besides, just check the MAN charts related to your D2876LE401 engines, and you will see the following:
    1) even if rated for 700 hp @ 2200 RPM, your engines are capable of producing full power already at 2000 RPM, with a fuel burn of 139 L/hr.
    Before you mention it, of course a boat shouldn't be propped to demand full power below the max rated RPM, but as a matter of fact, if requested, the engine can deliver it.
    2) the actual fuel burn at 2000 RPM based on a "normal" propeller demand curve is instead 101 L/hr.
    Now, whether at 2000 RPM you are burning 139 or 101 L/hr each side (or possibly even less, since the prop demand curve is just an estimated average), that is entirely up to the engine load.
    Which in turn depends on the boat weight, hull profile, transmissions, props, cleanliness, you name it.

    Bottom line, it's absolutely normal that any diesel engine can burn significantly more or less fuel at any given RPM depending on the boat they are powering.
    And it's a well known fact that deep vee hulls like yours, good as they may be in some ways, are NOT the most efficient by any stretch of the imagination.
    So, I really can't see what your point really is.
    Aside from having a funny feeling that you are more interested to dismiss what I said ("Your calibration must be waaaaaay out"), rather than in a constructive debate.
    But in this respect, I can only reiterate what I already said: if you don't trust my comments, just ignore them.
    You are entirely free to think they are just BS, of course.
    Just don't expect me to lose any sleep over it, let alone trying to convince you of the opposite.

    PS: incidentally, the photos you posted prove what I just said nicely, because while your engines are right on the money vs. MAN charts at WOT, they are burning more than they should both at 1800 and 1900 rpm. In fact, according to "normal" prop demand data, they should burn 75 and 87 L/hr respectively, vs. 90 and 103 shown by your displays.
    Last edited: Dec 8, 2022
  19. YachtForums

    YachtForums Administrator

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    Gents,

    The discussion went from boat recommendations to fuel burn so we split the above posts into a new thread.
  20. Fiammetta42

    Fiammetta42 Member

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    Precisely as I mentioned and I think you agreed - manufacturers charts with ave prop demand curves cannot be relied upon .Indeed underestimate the real world consumption.
    The pics I posted show the load which I use around 80 % to set the cruise rpm .The variability of load I see due to fouling / weight etc I can assure you I am familiar with how diesels work .I see it .Also EGTs on a another page .Having screens and watching changes actually stimulates a greater understanding than otherwise .

    It’s still a pity in terms of credibility and comparability of your consumption you cannot access real data from a screen .

    Or your consumption figures fitted in the gen norms / patterns of other 56 ftrs with 16 L V 8 s .
    10 tons heavier ( near enough 50 % more ) and despite my 23 degree deadrise on a 14 M boat your 17 M must have at least 2x under water surface area planing compared to mine . Bigger engines etc older mech injection .
    Mines pretty near the Cabo 40 maybe a 1 M bigger ? .= similar consumption.
    Well closer to the Cabo 40 than your DP 56 in terms of consumption.

    I just can’t see it like the Cabo 40 being close to your consumption or indeed worse as you claimed ^
    For a like for like cruising rpm say typically 1870 , 1900 rpm .It must burn more , possibly @ more “ load“,
    We can never see your true load figures .
    Mine will travel faster obviously which it does , which improves its gallons / Nm .
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