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  1. #11
    Join Date
    Feb 2015
    Location
    Conwy
    Posts
    4,175

    Default Re: Addicted to the fuel burn gauge

    Vas there is no easy way for me to measure actual fuel burn reliably. I have these figures and when it comes time to refuel and I look at my journeys and speeds then they seem to bear out. I appreciate it is not exact and is a guesstimate at best but the driving point seems that over short hops the savings in fuel vs time seemed to largely even out. I have though about getting inline flow meters but cost vs accuracy seems to be moot. Can you suggest a valid test short of re-plumbing the engines to portable tanks?

  2. #12
    Join Date
    Feb 2015
    Location
    Conwy
    Posts
    4,175

    Default Re: Addicted to the fuel burn gauge

    Actually, thinking about this, please dont disabuse me of my little belief structure. It's working for me just fine

  3. #13
    Join Date
    Jun 2011
    Location
    Volos-Athens
    Posts
    4,847

    Default Re: Addicted to the fuel burn gauge

    Quote Originally Posted by BruceK View Post
    Vas there is no easy way for me to measure actual fuel burn reliably. I have these figures and when it comes time to refuel and I look at my journeys and speeds then they seem to bear out. I appreciate it is not exact and is a guesstimate at best but the driving point seems that over short hops the savings in fuel vs time seemed to largely even out. I have though about getting inline flow meters but cost vs accuracy seems to be moot. Can you suggest a valid test short of re-plumbing the engines to portable tanks?
    nope Bruce, you need flat waters, no tides (or very accurate paddlewheel, yes that's a joke I know...) and a few longish trips at constant speed brim to brim.
    I'm not familiar with outdrives performance but fuel burn vs lpnm is definitely NOT linear in the 40-45ft range that I do have experience with twin shaft drives

    V.

    PS. okay lets say outdrive boats are completely different consumption wise and be done with

  4. #14
    Join Date
    Feb 2015
    Location
    Conwy
    Posts
    4,175

    Default Re: Addicted to the fuel burn gauge

    Quote Originally Posted by vas View Post
    PS. okay lets say outdrive boats are completely different consumption wise and be done with
    Absolutely! And the chorus of "Are we there yet" from two prepubescent brats for 10 hours vs 2.5 is worth twice the fuel burn should I be shown massively wrong anyway.
    If anyone can give me fuel burn from their KAD300/44 if they have figures for displacement speeds and just comfortably on the plane (not balls to the wall) I'd be very appreciative. I cant see there being that big a difference to it's older sibling.

  5. #15
    Join Date
    Apr 2011
    Location
    Boat- Western Med
    Posts
    5,321

    Default Re: Addicted to the fuel burn gauge

    I know what Bruce means . It's not as clear cut for some depends on how optimised the particular hull is
    . Some will fair better @ D compared to P than others more so if the motors are on or near the "edge" power and thus burn rate when planing
    My maths give a marginal saving but not really bothered as it depends on the conditions.
    Only D of calm and flat . In a sea chop best to P
    My favourite anchorage for a swim is 6 miles one way .
    D at hull speed 8.3 @ 15 L/h or 9.3 @ 18 L/h
    Plotter says 45 mins so 3/4 of the L/h 12 to 14 L used
    On way each motor

    P @ 27 knots plotter says 12 mins so 20 % of 1 h @ 90 L/H so 18 L burned in 11 /12 mins as opposed to 12/14 in 45 mins
    X all by two
    If it's rough and choppy then all done n dusted in 11/12 mins
    And over time if D becomes more norm there's hours put on 11/12 mins compared to 45 - 4 x the hours
    You can extrapolate up for longer passages . Sure there's a saving but is it that big to sacrifice 27 knots to 8 or 9 ?

    I don't think so with my boat .

    This is because they are only doing 1750rpm when WOT is 2150 .
    Obviously at 2000rpm the burn rate is north of 200L /h
    Real figure not internet btw .I don't need to push it as most other 14 M boats like the FL T47 and S/Ske Porto 46/57/48 have D9 or VP 74 at 575 hp or worse still 480 hp.
    I have 700 hp motors which I can run at an economic rpm for a given speed
    Those 575 boys above will be at a steep part of the consumption curve keeping up with me @27 knots .
    Last edited by Portofino; 01-10-19 at 12:23.

  6. #16
    Join Date
    Feb 2015
    Location
    Conwy
    Posts
    4,175

    Default Re: Addicted to the fuel burn gauge

    Bless you Porto I started to question my sanity

  7. #17
    Join Date
    Aug 2004
    Location
    A tiny Island, Caribbean
    Posts
    16,940

    Default Re: Addicted to the fuel burn gauge

    Quote Originally Posted by EugeneR View Post
    I would be too, if I had one.

    What I do not understand is why people use litres per hour, which is not really a good indicator of efficiency unless you consider speed as well, rather than litres per nautical mile which is what we are really trying to optimise.

    Other units are available gallons, km, etc.


    I get on the plane and throttle back with 2 x 85hp 2-stroke outboards, on a 22' Mako centre console

    +/- 2.5 mpg.
    RELAX, Rum may not be the answer, but nor is water or juice. Now what is your Question?

  8. #18
    Join Date
    Apr 2015
    Posts
    196

    Default Re: Addicted to the fuel burn gauge

    All figures approx......but on my P50 going 20 nm takes 1 hour at 20knots and 2.35 hours at 8.5 knots. So 220 litres at 20 knots (220lph x 1) and 42 litres at 8.5 knots (18lph x 2.35).

    Huge difference (415 litres) over a days cruising of say 50 nm.

    Edited as my man math was wrong
    Last edited by Paul_S123; 01-10-19 at 13:55.

  9. #19
    Join Date
    Feb 2015
    Location
    Conwy
    Posts
    4,175

    Default Re: Addicted to the fuel burn gauge

    Quote Originally Posted by Paul_S123 View Post
    All figures approx......but on my P50 going 20 nm takes 1 hour at 20knots and 2.35 hours at 8.5 knots. So 220 litres at 20 knots (220lph x 1) and 99 litres at 8.5 knots (8.5lph x 2.35).

    Huge difference (302 litres) over a days cruising of say 50 nm.
    WATCH THAT FUEL METER!
    I can see your point clearly and it was also my natural assumption until I started playing Man Math. What's interesting here and whether it has relevance or not I dont know is the difference in speeds between the two boats D and P. For me the difference is nearer 4:1 and you are closer to 2:1. I suspect you are closer to SD bias where as I am pure planing and so our efficiencies will be markedly different. (question and questionable. I'm still hanging on to my belief structure. Be gentle)

  10. #20
    Join Date
    Aug 2009
    Location
    Hamble
    Posts
    1,032

    Default Re: Addicted to the fuel burn gauge

    Quote Originally Posted by Paul_S123 View Post
    All figures approx......but on my P50 going 20 nm takes 1 hour at 20knots and 2.35 hours at 8.5 knots. So 220 litres at 20 knots (220lph x 1) and 99 litres at 8.5 knots (8.5lph x 2.35).

    Huge difference (302 litres) over a days cruising of say 50 nm.
    A that rate, I can do a trip from Cowes to St Samson to fill up, get back, and still save compared to filling up on the Hamble. And get 24 hours' free boating along the way. Man maths

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