fuel consumption

andy jeffrey · 29 September 2011, 06:52

Hi is there anyone out there who can advise me on the rough distance/time i should get from a yamaha 115 4 stroke on a ribeye 600 as cruising speed ie 20/25 knots , tank holds 90lts , the fuel gauge is all over the place when at sea ??????

Poly · 29 September 2011, 07:33

Search the forum you will find loads on fuel consumption. However at cruising speeds almost all ribs run at roughly 1 L/NM (+/- 20%). DO NOT rely on this as your minimum amount of fuel for a journey though.

wavelength · 29 September 2011, 11:43

no such thing as mpg at sea of course. We usually work on1l /mile in reasonable conditions as a pessimistic figure, but that could go up considerably if the weather turns against you. Is it an underfloor tank or an above deck thing?

andy jeffrey · 29 September 2011, 13:22

hi its an under floor tank

wavelength · 29 September 2011, 14:44

problem is you can't dip it then. Thats one of the various reasons we try and keep clear of u/floor tanks whenever we can. The big boat I am sat on at the monment has the sight tubes checked every day cos I wont trust just guages if I have any choice in the matter. We get on a few of the smaller boats in the fleet to deliver them occasionally and they are just on lcd readouts for the fuel tanks-not good for the nerves when you step onto a strange boat and set off relying on them and the good log keeping of the previous skipper.
Lot of good discussion on here in the past year or two re fuel sensors and flowmeters. Need to have a look around or some of the lads will be able to put you right.

9D280 · 29 September 2011, 16:09

if you have time best bet is fill to the cap (or a known point o nthe filler line) go for a burn (not too far!) and use your GPS to record average speed / distance etc. (can use a cheap handheld if you don't have a fancy plotter) Then when you get back, top it up to the same place, & take a note of how much went in.

Only after a few trips like that will you get a feel for how much it actually guzzles. That won't take tide into account numerically, but unless you are really unlucky you should get a rolling avearage of with & against you.

I'm about to start that game with mine, as I only stopped burning 25:1 last trip out and have no idea how much it will now drink after the rebuild on "design mix". For what it's worth, on the 25:1 mix I was getting 1.1L/Nautical Mile, but that did include 3 hrs at displacement speed.

Courageous · 29 September 2011, 20:29

9D has it right. I'm still in the dark with mine and carry 25L in a jerry can "just in case".

dnv · 29 September 2011, 22:05

I prefer to calculate consumption in'liter per hour' at a certain rev, rather than per nm. Reason is tide:
If you run at 30kn, 5kn head or tail tide make a difference of 33% in speed over ground, in time, in liter per nm and consequently in consumption for same distance (35kn vs 25kn over ground at same revs).

Jan

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Leapy · 30 September 2011, 04:21

Quote:

Originally Posted by 9D280

if you have time best bet is fill to the cap (or a known point o nthe filler line) go for a burn (not too far!) and use your GPS to record average speed / distance etc. (can use a cheap handheld if you don't have a fancy plotter) Then when you get back, top it up to the same place, & take a note of how much went in.

Only after a few trips like that will you get a feel for how much it actually guzzles. That won't take tide into account numerically, but unless you are really unlucky you should get a rolling avearage of with & against you.

...and if you do that for a season (say) all the effects of wind, tide and everything else are averaged out. I've kept a simple fuel added/GPS distance travelled log for over 1000 miles now: 8.6mpg plus/minus naff all (can't be arsed with nm, litres or fathoms)...just add in your favourite safety margin for any trip you make

9D280 · 30 September 2011, 09:11

Quote:

Originally Posted by dnv

I prefer to calculate consumption in'liter per hour' at a certain rev, rather than per nm. Reason is tide:

At risk of sounding like a trainspotter, I also calculate that! Thing is L/mile makes trip planning a lot easier!

Over a season, as said by leapy you get a warmer feeling that your guess is going to be accurate!

eilean mor · 30 September 2011, 09:37

I'm with Wavelength on this one- tanks you can see/dip

I've looked at different boats and worked with a 20/25 litre portable can so I can work the basic logistics easier than with big inbuilt tanks.
100 on a destroyer 5.8;
I'm currently working on £1 per mile,
between 1-1.5 mile/litre, although, in planning, I'm working on 1 l/m and carrying a bit of spare.
inbuilt gauges/ angles of tanks all too variable, imagine being on a boat where you can't actually access over 100 litre of fuel ( a third of the tank! ) - or another where the gauge states quarter (40litres! ) of a tank remaining, and it stops- new boat/ gauges, and busy working it out and carrying spare fuel