Cruising distance - fuel planning

[Trimix]

I know how much fuel my tanks hold and how much I use at a cruising speed. But wonder how much I need to keep in reserve for when I do longer distances.

Logically, I would assume enough to get to the next fuel supply, but how much extra on top of that to take into consideration variation in conditions / route / messing about ?

1/3rd, 1/4er, enough to get back to the start ?
What sort of variation do you get in usage if the conditions change ?

(where I am cruising there is very little boat traffic, so next to no chance of a tow)

[Pikey Dave]

The general rule is 1/3rd out, 1/3rd back & a 1/3rd in reserve. Saying that, I have been known to return on fumes before now, very stressful


.....sh1t happens.......

[Trimix]

Bit like cave diving then. I'm actually tempted to run out just to see exactly how much fuel the tanks hold.

I've been told they hold 95 litres each, but I would like to know for sure.

If I do run out I assume I just top up and pump the rubber bulb on the fuel lines and all should be well ? (its not a diesel after all)

The other thing is the gauges never reach the full mark, they only show 3/4 full when I brim out the tanks - so I assume that when the show empty there is a 1/4 left. But that's only an assumption, one Id like to test for real.

I'm planning on going from Corsica to Italy, so I need to take it more seriously.

[Pikey Dave]

Originally Posted by Trimix

Bit like cave diving then. I'm actually tempted to run out just to see exactly how much fuel the tanks hold.

I've been told they hold 95 litres each, but I would like to know for sure.

If I do run out I assume I just top up and pump the rubber bulb on the fuel lines and all should be well ? (its not a diesel after all)

The other thing is the gauges never reach the full mark, they only show 3/4 full when I brim out the tanks - so I assume that when the show empty there is a 1/4 left. But that's only an assumption, one Id like to test for real.

I'm planning on going from Corsica to Italy, so I need to take it more seriously.

I don't know what your electronics setup is, but assuming that your engines are recent & you have a nmea2000 compatible plotter or gauges on board, I'd ditch the standard fuel gauges & go down the engine fuel flow route. There are a couple of threads kicking about at the moment regarding same. Way more accurate once you have it all setup. You might have to chuck a few quid at it to get it working, or you might already have the necessary bits installed to make it work & its just a case of setting it up.

[Trimix]

Actually I do have them set up to show the fuel flow rate. So I know what its using as I go.

I just use the gauges to glance at when I cant do the maths in my head, or if I'm messing about and not at a steady pace.

Annoyingly cant seem to get the Garmin to show the range left, despite it knowing the size of the tanks and the rate being used.

When cruising I'm using 1.2 litres for one Nautical Mile - so I should get a maximum of 158 NM till I run out.

Italy is only 116 NM away - I should be fine then

[bedajim]

Originally Posted by Trimix

Actually I do have them set up to show the fuel flow rate. So I know what its using as I go.

I just use the gauges to glance at when I cant do the maths in my head, or if I'm messing about and not at a steady pace.

Annoyingly cant seem to get the Garmin to show the range left, despite it knowing the size of the tanks and the rate being used.

When cruising I'm using 1.2 litres for one Nautical Mile - so I should get a maximum of 158 NM till I run out.

Italy is only 116 NM away - I should be fine then

A 25L can strapped to the deck as a reserve may make your day feel so much more comfortable

[Trimix]

Your right. On the last long distance trip I took two 20l cans which fit very easily on one of the three lockers Ive got.

Never needed them, but it was nice having them along for the ride.

When I didn't take them every time I opened up the throttles and glanced at the fuel flow rate I got nervous. There are not that many marinas with fuel along the coast.

I could very easily store 4 x 20l cans without noticing it. But Two of them should give me an extra range of 30+ Nm.

[Trimix]

Oh, the trailer mods are still working out great BTW:

[bedajim]

Originally Posted by Trimix

Oh, the trailer mods are still working out great BTW:

[jonp]

Way to many variables to take into account. If Im correct and you are running twin engines your calculations are for good to reasonable conditions, if things become bad and you are punching big sea's obviously the consumption will increase. Slower speed causes more prop slip giving less range.

If one engine fails (fuel, mechanical or prop damage etc) will the boat still plane on one engine if so will it still do so in rough conditions. Most twin engine boats Ive been on don't perform very well under a single engine ( talking about cats and single v boats as my experience in twin ribs is limited).

Jon

[ShinyShoe]

You presumably need to decide if reserve fuel is to get you to a safe haven (with or without fuel) and let you work out how to get more fuel or if its to let you RTB.

Also to decide how your driving style changes if fuel becomes low - can you be more efficient than 1.2l/NM - some people get 0.8l/NM...

Fuel gauges are 100% useless! Even if they work they often aren't linear so you get a false impression of fuel use and that assumes the tank shape is linear and the boat is horizontal on both X and Y plane.

The best fuel gauge I've ever used was a mahogany stick, marked in 1 inch increments. You opened the fuel filler, stuck it in, pulled it out and the dark section on the mahogany was your depth. Count inches, multiplied by 10 (tank shape was rectangular and must have been the correct size) and that gave you litres. As long as your tank is a cuboid and you know the dimensions you can do something similar. Wont work if boat at angle but most people wont open a tank while underway as likely to get spray in it...

Do you feed from Tank A to engine A and Tank B to engine B? Or A then B or A & B at same time? Each has advantages. A then B means you know when you got to half full. If you are doing there and back without refilling you can switch to B before departing from halfway point and check how much is in A, but doesn't help redundancy... A then B means more likely to run out when underway which generally should be no big deal just switch over, prime and start. But run out in the wrong place its a problem so you do need to watch for places you don't want to get caught out and check fuel before...

[Searider]

I'd suggest letting the level drop below half (say, 60 litres per tank used) and then pumping all of the fuel into one tank, using the fitted fuel piping - as if the engine had been using the fuel. See what the fuel gauge for that tank reads.
Then fill the empty tank noting how much you can get in. This will give you the useable fuel capacity of that tank.
Alternatively if your fuel system allows both engines to draw from one tank simply run it out that way - then refill etc.

Typically a rule of thumb is to make sure you've got 20% (minimum) more fuel on board than you calculate you'll need.
Or, make sure you've got enough to get to the next port if your first cant be entered due to poor weather, plus 20%.
For the crossing from Corsica work out what the point of no return is and be aware of this on the day.

To complicate things, at sea, you may not be able to draw as much fuel out of the tank as in the test above - the fuel may be splashing around too much and the engine starts taking in air. On the round the IOW trip in the 4M we could only run the tanks down to about 1/4 full before the engine started missing due to air.

Once you know how much you can use and you have your 20% (or more) margin you know where you are!

If the weather is good there's no problem in using some of your margin towards the end of the trip if for example you want to go faster.

On a delivery from Gibraltar to lanzarotte on a 22m Princess we planned at 10 knots, a full tank and 6 200 litre drums on the aft deck.
Towards the end of the trip it looked like we would make lanzarotte in the dark. An unfamiliar port, poorly lit.
At the 0800 watch change the Skipper asked me to work out how much fuel we would have on arrival if we increased to 25 knots and therefore could arrive at 1500. 18% I reckoned - as did he. The weather was good, so 25 knots it was and an early arrival.
We paid no notice to the fuel gauges but knew the useable tank capacity and the consumption at the speeds we had been doing and worked from there.

[Trimix]

Cheers, that's great advice guys. Lots to do and think about. I will check how the tanks supply the engines, I suspect they go direct to just one engine each. Good point about draining them out to check the size and your comments about sucking air when almost empty.

I think in reality my safety margin will depend on weather. As you say I need to decide if I return to base, or head to the next marina.

For long distances I'm not going to go out if the weather looks iffy. Also its not like the Solent where it can change in less than an hours. It seems to take a day to change every time I've been there, which helps a lot.

Thanks.

[Maximus]

Originally Posted by bedajim

A 25L can strapped to the deck as a reserve may make your day feel so much more comfortable

X2 ..Simlpy the best & easiest way to ensure safety and "comfort"

[Mollers]

Originally Posted by bedajim

A 25L can strapped to the deck as a reserve may make your day feel so much more comfortable

Not when you lose it out of the trunk.

Slip a square of sheet rubber or an old rubber car mat under plastic can. The slightest movement over time and the non-slip can wear through the fuel container, as Jim will testify.

[bedajim]

Originally Posted by Mollers

Not when you lose it out of the trunk.

Slip a square of sheet rubber or an old rubber car mat under plastic can. The slightest movement over time and the non-slip can wear through the fuel container, as Jim will testify.

Wondered where the smell was for coming from

320 litre tank this time

[willk]

I do a reasonable amount of cruising in waters with fairly sparse refueling options. Over similar distances to the OPs planned trip too. A few thoughts:

1. Forget the gauges - they're never going to provide useful information,
2. You REALLY need to know the exact volume of the tank/s. BTW, 95L a side seems really small for an 8m RIB, so I reckon they're bigger.
3. Using the engine fuel figures is a good idea - it's good for monitoring the burn if your conditions/plan changes.
4. Having good historical fuel burn figures is very useful for planning.
5. Carrying a known reserve in tanks. I carry a tank/s that give me just over an hour at cruise speed - enough to get into somewhere safe and worry about fuel there. Mollers has a point about fresh non-slip paint - it eats plastic
6. If you can dip a tank - it would be very useful. To make it work you would need access to determine the depth of the tank proper - so you'd need to get the fuel filler line off and dip from there - maybe not viable. Dipping tanks is foolproof if calibrated correctly the first time.
7. Fuel usage can change a lot - bad weather is the main reason but a bit of hooning around in calm weather can dramatically alter your tank level too. I know of lots of people who have run out of fuel. It's one of the main reasons for RNLI callouts to motor boats.

[Trimix]

Good points.

I think I will drain a tank and then fill it to be sure of its size as you suggest.

I have also been collating fuel usage and have safe storage for at least 80lts without problems.

On a really calm steady cruise in the Med I got 1.3 l/Nm, but in the Solent on a choppy day I got 2 l/Nm

Cheers,

(I will let everyone know when I run out)

[Jeff]

I don't know about Suzuki's but mercury smart craft can show fuel used (and the ability to reset to zero) which is very accurate. I'm guessing Suzi's must have similar capabilities?
Floscan fuel meter with flow per HR and a totaliser is either accurate once calibrated ,,,, you need the model for litres not gallons ! (Yanks don't know what a gallon is )
I've had the same one since 2001 on 3 engines and 2 ribs and still going strong.

Jeff

[jeffstevens763@g]

Being somewhat cautious learning the hard way over the years I always carry double the fuel I am going to use and a split system in case of fuel contamination I might be over the top but peace of mind for me too many factors to change how much fuel your gonna use, once towed a boat Bach with small kids in would have hated to say got to go not enough fuel to get you Back.