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  1. #21
    Join Date
    Apr 2015
    Posts
    196

    Default Re: Addicted to the fuel burn gauge

    Quote Originally Posted by Portofino View Post
    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 .
    So my maths makes it for the 6 miles you cruise to:

    D at 8.3 knots at 30 litres per hour: 21.68 litres used
    P at 27 knots at 180 litres per hour: 40 litres used

    Probably insignificant on the 13 minute cruise like you do...but over a few hours it's a huge difference...even if you just take there and back it's saving about 37 litres on a tiny journey.
    Last edited by Paul_S123; 01-10-19 at 15:30.

  2. #22
    Join Date
    Apr 2015
    Posts
    196

    Default Re: Addicted to the fuel burn gauge

    Quote Originally Posted by BruceK View Post
    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)
    LOL...I think you need to work with some real figures...taking your best D mpg and compare it to the best P mpg you can get from your boat..then factor in the additional cruising time and figure it out.

    I can categorically state that I just completed a 75 mile trip past gib to Ceuta, then down to Morocco , then back to Sotogrande and used under 200 litres of fuel. I would get less than an hour with that fuel usage at 20 knots...so would have gone 8 or 9 miles up the coast, turned around and came back and burnt the same.

  3. #23
    Join Date
    Feb 2015
    Location
    Conwy
    Posts
    4,203

    Default Re: Addicted to the fuel burn gauge

    Party-pooper. Trust me it's a helluva difference 5knts and a pinch under 9 knts. At 5 knts I have to suffer the crippling indignity of some WAFI in a sailboat overtaking me and asking if I need assistance. It's not the done thing!

  4. #24
    Join Date
    Apr 2015
    Posts
    196

    Default Re: Addicted to the fuel burn gauge

    Quote Originally Posted by BruceK View Post
    At 5 knts I have to suffer the crippling indignity of some WAFI in a sailboat overtaking me and asking if I need assistance. It's not the done thing!
    Point taken

  5. #25
    Join Date
    Apr 2011
    Location
    Boat- Western Med
    Posts
    5,356

    Default Re: Addicted to the fuel burn gauge

    Quote Originally Posted by Paul_S123 View Post
    So my maths makes it for the 6 miles you cruise to:

    D at 8.3 knots at 30 litres per hour: 21.68 litres used
    P at 27 knots at 180 litres per hour: 40 litres used

    Probably insignificant on the 13 minute cruise like you do...but over a few hours it's a huge difference...even if you just take there and back it's saving about 37 litres on a tiny journey.
    Yes on a long trip D pays off .
    Say 100 miles at D @8.3 ish round to 10 hrs so 300 L , used out of a fill up of 2200 L
    Or P gun it in @27 knots =3 hrs round up 180 x3 = 540 L used a difference of 240 L , a quarter of the fill up burnt .
    But 10 hrs instead of 3 hrs is a big ask .Then again 240 x€1.58 = €379 buys the €80 / night berth and leaves €299 the man maths blow on a restaurant.
    Or anchor off and cook in and bag the saving .
    Yep my 6 mile swim trip is neither here or there in terms of D vs P running costs .
    But a 100 mile trip out and another back with a bit of movement at the destination could easily = €1000 saving .

    I think we all know deep down P boats are not on Greta Thunberg ,s wish list .
    Wonder how much fuel would be used if she had P ,d to NY in something like Bransons “ Challenge “ boat ?
    A lot !
    Last edited by Portofino; 01-10-19 at 17:37.

  6. #26
    Join Date
    Nov 2012
    Location
    West Sussex
    Posts
    39

    Default Re: Addicted to the fuel burn gauge

    Surprised that the large sophisticated plotters etc. aren't showing consumption per mile in real time as you wallow or bounce along accordingly. I thought that level of data would have been routinely available nowadays. I'm certainly looking for that sort of data if I re-engine my twin outboard boat with modern engines, or am I being naive?

  7. #27
    Join Date
    Dec 2005
    Location
    Live London
    Posts
    3,843

    Default Re: Addicted to the fuel burn gauge

    Quote Originally Posted by 2hullsonemarinaberth View Post
    Surprised that the large sophisticated plotters etc. aren't showing consumption per mile in real time as you wallow or bounce along accordingly. I thought that level of data would have been routinely available nowadays. I'm certainly looking for that sort of data if I re-engine my twin outboard boat with modern engines, or am I being naive?
    As long as you have fuel flow many do. I had new Garmin stuff that read litres per NM. My more elderly Raymarine just shows LPH but converting this in my head to lt per NM given the display shows speed and consumption is easy ( albeit I am surprised it does not do it for me)

  8. #28
    Join Date
    Feb 2015
    Location
    Conwy
    Posts
    4,203

    Default Re: Addicted to the fuel burn gauge

    In my case large sophisticated plotters etc are largely useless when it comes to engine management of old analogue engines. Next argument. Would I want sophisticated and complex ECU driven engines? Probably, but I'd no longer be comfortable working on them and boy do they seem to throw a lot of issues judging from the forum queries. There is a lot to be said for the KISS approach inclusive of a blissfully ignorant belief structure. Is the engine hot? No. Cool, life's peachy.

  9. #29
    Join Date
    Jul 2003
    Location
    River Itchen, Southampton
    Posts
    7,226

    Default Re: Addicted to the fuel burn gauge

    Lpm is what matters for me. Obviously you use less at d speed but often 20 knots uses less than 18 and definitely less than 14. A read out of lpm lets you trim the boat and make real time range decisions for diversions, weather changes etc.
    My old engines don’t measure fuel flow.
    On my last boat I fitted navman fuel flow meters and they were great.
    They’ve stopped making them.
    Where can I buy fuel flow meters other than floscan which are pricey and AFIK don’t do lpm.

  10. #30
    Join Date
    Feb 2015
    Location
    Conwy
    Posts
    4,203

    Default Re: Addicted to the fuel burn gauge

    Quote Originally Posted by Elessar View Post
    ....... other than floscan which are pricey and AFIK don’t do lpm.
    They do one have metric units but you are correct they are only lph http://www.floscan.com/html/blue/ser...?sid=4&catid=2

    You say pricey, I tried to find out more from their listed UK and European dealers. All links went to dead ends so apart from importing I dont know how you'd get your hands on one.

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