Your go to rule for fuel reserve management

[WestCoastMcgyver]

Recent discussion prompted some queries on fuel management. Specifically was around coastal cruising and distances between fuel fill up locations. I have 1100 liters of storage and a burn rate of 7L/hr at 7 knots. My absolute min is 200 L on board. I like to have at least 2x my projected fuel burn on board dependent on trip. Thoughts?

 

[rslifkin]

Regardless of planned burn for a trip, I figure ours as follows. 420 gallons total capacity (split between 2 tanks). I figure the last 20 gallons are unusable, so 400 gallons of usable fuel. I figure 300 gallons can be planned for use, the last 100 gallons is contingency / reserve. For some trips where fuel burn may vary more from expected I may factor my expected burn up by a few percent (which counts against the fuel I plan to burn, so comes out of the 300 gallons of capacity). To think of it as hours of contingency, a 100 gallon reserve for us is somewhere close to 20 hours at slow cruise (6.5 kts) in calm weather and a bit over 3 hours at fast cruise (17 kts).

For local cruising, etc. I try not to get much below ~180 gallons on board.

 

[Portage_Bay]

Recent discussion prompted some queries on fuel management. Specifically was around coastal cruising and distances between fuel fill up locations. I have 1100 liters of storage and a burn rate of 7L/hr at 7 knots. My absolute min is 200 L on board. I like to have at least 2x my projected fuel burn on board dependent on trip. Thoughts?

When working off the west coast of US and Canada I never let the fuel get below 1/3, more comfortable with 1/2. But that came from working in heavier weather than I will recreational cruise in. With the possibility of getting stuck on the outside because the bar was closed. Now, cruising in retirement, avoiding heavy weather I'd be comfortable going much lower.

 

[psneeld]

You really have to ask yourself before anything is....

1. What is my fuel burn under the conditions I anticipate?

2. How accurately can I tell how much fuel I have left?

The more accurate those numbers are the smaller the reserve I will go down to.

3. How easy it to get fuel prior to my destination if #1 goes astray?

Getting acquainted with new to me equipment and shaky fuel burn/quantity measurements...I may only go with 2/3 of known amount of I have personally put in. With equipment of known fuel quantity and burn accuracy (like the helos I used to fly, we on occasion would go down to a 10% (I think) fuel reserve. Running out was bad so we often figured in a bit higher unless it was a very urgent mission, but it was done often enough to give confidence in the equipment.

 

[Shrew]

Here is my .02 on this, just one person's opinion.

The total fuel capacity of the tanks is greater than the usable fuel. I'd like to know what the actual 'usable capacity' is for the tanks. Then calculate my safety margin from that.

 

[PierreR]

I have 700 gallons useable on board and burn 3.25 gph @7.2 knots and includes generator use. The farthest I have pushed it was 1,185miles leaving myself with 165 gallons left or 365 miles in reserve or 24%. I do not have fuel gauges so I dip the 4 tanks every couple of days when cruising.

 

[rslifkin]

How accurately can I tell how much fuel I have left?

That's a good point considering how many boats have questionable fuel gauges, only sight tubes or dipsticks, etc. Fortunately I've got reliable gauges and I've found the past when we fill the tanks, I can typically estimate how much fuel we'll take within 10 gallons or so based on gauge readings and how much we've run. Once or twice my estimate has been within a gallon or 2 on a 200+ gallon fill.

 

[darkside]

It depends on how remote or exposed you are when you run out. When the risk is greater keep more reserve.
How little fuel do you use at dead slow? It can be surprisingly little.
If you have twins can you transfer to one side to maximize usable fuel?
Our fuel pick ups are at the rear of long tanks so we should get most of the fuel out, except when it's rough.

 

[mvweebles]

You really have to ask yourself before anything is....

1. What is my fuel burn under the conditions I anticipate?

2. How accurately can I tell how much fuel I have left?

The more accurate those numbers are the smaller the reserve I will go down to.

3. How easy it to get fuel prior to my destination if #1 goes astray?

Getting acquainted with new to me equipment and shaky fuel burn/quantity measurements...I may only go with 2/3 of known amount of I have personally put in. With equipment of known fuel quantity and burn accuracy (like the helos I used to fly, we on occasion would go down to a 10% (I think) fuel reserve. Running out was bad so we often figured in a bit higher unless it was a very urgent mission, but it was done often enough to give confidence in the equipment.

This. ^^^

When PAE ran their Nordhavn 40 round the world, longest leg was US to Hawaii. They sped up the last day or so because they had excess fuel on board - they arrived with under 50 gals if I remember correctly. Close to 5% reserve. Why were they comfortable doing this with such a short reserve? Because Jim Leishman had made a couple runs from Dana Point to Seattle and back and carefully measured fuel burn. He also designed a calibrated burn tank so he could precisely monitor absolute fuel consumption (still standard equipment on new Nordhavns).

You also have to ask yourself the penalty for being wrong. When I was delivering, I rarely asked owners what their boat burned. They all had an answer but it was always optimistic by a substantial amount. If I trusted their numbers, I'd still be bobbing out in the Pacific waiting for fuel.

Personally, I don't really know my fuel burn. Around 1.0-1.25 gph at 1700 RPMs. I just add more diesel whenever it's convenient.

Peter

 

[tiltrider1]

I track fuel burn pretty religiously. I have a chart that shows my speed and fuel burn at specific RPMs. This allows me to predict very precisely my fuel consumption by hours. My tanks have sight tubes that are calibrated to every 25 gallons. I have run each tank down to 5 gallons as a test for accuracy.

In my normal cruising grounds I am never more than 3 hours from fuel. I also never get lower than 50 gallons as a safety margin.

I was planning a cruise to Cabo San Lucas where I would fill up in Ensenada and arrive in Cabo with 100 gallons of fuel reserve. This gave me 10 hours of of cushion which could be extended just by slowing down. Normally, I would start off slow and then speed up as Jim Leishman did but there are two potential fuel stops I could take advantage of if needed.

My aviation back round probably makes me very conservative. As a pilot I find my self unable to use sky above me, runway behind me or fuel sitting on the ground.

 

[O C Diver]

Slow Hand has 2 identical side tanks with a total of 660 gallons. Being able to transfer all of one into the other allows me to cruise on flat water below 50 gallons reserve. That's fine if you're going through the Dismal Swamp to fuel in Norfolk. Problem is that the weather changes. I get nervous below a third of one tank (100 gallons) crossing large bays and the Great Lakes. Certainly wouldn't go below half of one tank in the ocean.

Then there's the fact that the boat has a nicer ride above half capacity.

Ted

 

[rslifkin]

It depends on how remote or exposed you are when you run out. When the risk is greater keep more reserve.
How little fuel do you use at dead slow? It can be surprisingly little.
If you have twins can you transfer to one side to maximize usable fuel?
Our fuel pick ups are at the rear of long tanks so we should get most of the fuel out, except when it's rough.

Those are good points. For us, due to the efficiency drop off with gas engines, pulling the throttles back from slow cruise to idle saves some fuel, but in terms of fuel vs distance, it's only a small improvement, not as big as you'd typically see with diesels. At low speeds if conditions are fairly calm, we'd likely gain more from shutting down one engine and putting a bit more load on the other, although I haven't tested that for fuel burn. Also because we're gas, there's no fuel transfer capability, although we can run both engines from one tank rather than our normal 1 tank per engine configuration.

Fuel pickups for us are similar to yours, tanks are probably a bit more than twice as long as they are wide with pickups towards the back end.