Addicted to the fuel burn gauge

[Paul_S123]

Just took a trip over to N Africa with a d speed buddy..and I have become totally addicted to the fuel burn meter...plodding behind him took my fuel useage down from 220 l per hour to about 18 lol...albeit at a slower speed...but doing the calculation, it's a huge difference. Even an extra knot doubles the fuel burn, so I spent the entire time maximising fuel burn for knots lol...Love driving at 8.5 knots too...saw tons more dolphins etc...and lets face it, never in that much of a hurry to get anywhere.

 

[Bouba]

I’m also addicted to it :disgust:

 

[Greg2]

We had a coupe of fantastic passages along the east coast this year on hot, sunny days and flat calm conditions. Ran with the tide at displacement speed and the autopilot on - no inclination to burn a load more fuel and foreshorten the experience. To be honest I think it is the future for us as fuel prices will inexorably increase and we near retirement - currently musing whether it will be a yacht (we enjoy sailing) or a displacement mobo, possibly in steel.

 

[EugeneR]

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.

 

[markc]

I don't have a fuel burn gauge so I made a spreadsheet that gives a range of outputs, including cost per NM, which is the most useful. Range is from 3.85€ - 14.00€ per NM, which is quite motivating to keep it slow!

 

[BruceK]

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 think because consumption per hour will be largely constant where as distance travelled will be subject to currents, wind etc forces. But I agree. When I worked out how much fuel I saved by going 6knts vs 20 knots over a 50nm hop the difference was disappointing in actual £ per hour saved. Would need to be long distance to make significant benefit

 

[Paul_S123]

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.

It's watching the burn going from 18l per hour at 8.3 knots to 36l at 9.3 knots...very encouraging to keep the speed down, as while I realise it's going to get us to the destination quicker...we are only talking about tiny difference in speed = massive difference in fuel used. So the saving in fuel used in a shorter time..is greatly outweighed by the amount of fuel it takes to get there at that speed.

Just my thoughts...no exact science..it did really help me stop nudging the throttles a little forward lol.

 

[Bouba]

This morning we moved our boat from it’s summer to it’s winter mooring. A distance of less than three nautical miles in flat calm warm sunny conditions. 5.1 knots @ 2.8 lph. And that’s with a whole season of growth.

 

[BruceK]

Not having a fuel consumption capability I look at my stern wake to judge efficiency. As soon as it gets more than what I see on my 280, 3.5 hp tender I back off. It's probably all in my mind but the thought process goes I cant be wasting more that 3.5hp On my 34 footer that equates to just over 5knts in still waters and about 1000rpm (out of 3900 at WOT).
I have a figure from the internet (cue unreliable) that I can expect a fuel burn of 4.5 litre per hour per engine at these revs and speed. 9 lph
At 3200 rpm, (again an internet figure) I am doing 22 knts and a fuel burn of ~20lph per engine. 40lph
Given on a weekend jaunt I may go 25 miles to an anchorage for a total of 50 return then rough maths makes : displacement burn ~10 hrs x 9 = 90 litres
planing burn ~ 2.5hrs x 40 = 100 litres
So where is my Math going wrong?

edit: these internet figures appear to be born out when averaged against amount refuelled over the course of several trips.

 

[vas]

Not having a fuel consumption capability I look at my stern wake to judge efficiency. As soon as it gets more than what I see on my 280, 3.5 hp tender I back off. It's probably all in my mind but the thought process goes I cant be wasting more that 3.5hp On my 34 footer that equates to just over 5knts in still waters and about 1000rpm (out of 3900 at WOT).
I have a figure from the internet (cue unreliable) that I can expect a fuel burn of 4.5 litre per hour per engine at these revs and speed. 9 lph
At 3200 rpm, (again an internet figure) I am doing 22 knts and a fuel burn of ~20lph per engine. 40lph
Given on a weekend jaunt I may go 25 miles to an anchorage for a total of 50 return then rough maths makes : displacement burn ~10 hrs x 9 = 90 litres
planing burn ~ 2.5hrs x 40 = 100 litres
So where is my Math going wrong?

edit: these internet figures appear to be born out when averaged against amount refuelled over the course of several trips.

Bruce,

why don't you do some measurement, I VERY MUCH doubt the dif from 5kn to 20kn is 9lph vs 40lph that's bloody analogous, no way! one of the two lph values is way wrong

V.

 

[BruceK]

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?

 

[BruceK]

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

 

[vas]

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

 

[BruceK]

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.

 

[Portofino]

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 .

 

[BruceK]

Bless you Porto :encouragement: I started to question my sanity

 

[Rum_Pirate]

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.

 

[Paul_S123]

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

 

[BruceK]

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)

 

[EugeneR]

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 :encouragement: