How low can I go with the fuel level?

[ZZ13]

Now that I am addicted to offshore cruising, how low can I run my diesel fuel level before I should get back in and stop at a fuel dock to avoid whatever problems too low of a fuel level causes?

Secondary question, how do you experts determine where you will leave the open sea and head back in for fuel? Do you calculate based on a fuel burn that you trust and set your route accordingly? Do you have fuel flow meters installed that give you an accurate representation of fuel used/left? Reason I ask is my fuel gauges are not the most accurate indicator of where I am. I hit the 1/4 level on my way to St. Augustine a few months ago, got nervous and came into a fuel dock in Fernandina and found I had 75 gallons of the 175 still left in each tank. Which is roughly 3/8-7/16 full. So, I think I could have gone another 20-40 miles further offshore (depending on the answer to the first question).

 

[dcpringle]

You should be using the 1/3 rule. 1/3 to get out, 1/3 to get back and 1/3 for reserve. For your situation though, you would probably run 2/3 and then keep the reserve for getting/locating fuel... Hope that helps...

 

[FootballFan]

Certainly not an expert, but since that rarely stops me from offering an opinion anyway... LOL

First, I never ever think in terms of how far can I go without running empty on Fuel. My thought is how far can I go before I need fuel, always maintaining reserves.

Where you are cruising certainly has impact how much reserve to maintain. Consider the 3 different scenarios:

Headed off shore fishing, you leave the inlet and head out to open water. The 1/3 rule is universally accepted I think. Would not want to travel further, linger trolling and use more than 1/3 of the fuel I left with. A 1/3 to get back, with 1/3 reserve for problems (bad inlet conditions, have to linger, weather, etc.)

Traveling down the ICW, there is a fuel dock every 20 miles or so. 1/3 rule would not apply here, though I would always want enough fuel to deal with the next marina is closed or out of fuel, and still have adequate reserves. If I am on the ICW, I can go all day and not burn a tank of fuel in most cases - if I leave full, never going to be at that point of being so low I might eat into reserve.

Coastal Cruising: maybe venturing 20 miles off the coast, generally I am traveling along the coast. Still reserves are paramount. Amount of reserves comes back to pre-trip planning.

After some experience I learned how much fuel the boat generally consumes with how I am running it. I pretty well know an average consumption at hull speed, at 80% throttle, etc.

When I am planning a daily leg, fuel planning is at the top of the list. I plan my destination, determine the distance, estimate worst case fuel consumption and I have an estimate on what my fuel level will be at the end of the leg. (when I stop each night, I estimate the amount of fuel I have on board using gauges and smartcraft readings. Over time I am constantly comparing my estimates to actual amount of fuel I take on to continually improve my estimates). I decide what I think the reserves should be for that leg and plan accordingly.

If I know I am leaving in the morning, and do not have enough fuel on board to make the destination: I have fuel options identified and have made schedule plans which accommodate stops.

Along the coast my options for fuel are not where the marinas are, rather where are the inlets that I want to transit to get to a marina.

If due to changing conditions - I feel my reserve might be compromised - I stop and get fuel at my next (previously identified) option.

One of the challenges you mentioned are the accuracy of the gauges - yep that's a challenge sometimes. On some boats the gauges will have different readings based on whether the boat has the bow up in the air - or is moving at hull speed. That's where building some experience with the boat helps.

If I were to err on one side or the other - I would have rather stopped took on fuel and realized it was not as bad as I thought - than have the opposite situation.

Just my thoughts for what they are worth...

Mark

 

[rcknecht]

We ran one time so low that I ready to run out of gas and drop the anchor in the ocean. We used all but 15 gallons total in both of my tanks... We went out into the Ocean I had a full tank. If you want to live on the edge, carry a couple extra 5 gallon containers of diesel in you genny compartment for emergency. I would always recommend a full tank any time you are going out in the open ocean...

 

[ZZ13]

Thanks. All the responses help a lot. I did figure I need to use the boat enough to get comfortable of what my fuel gauges represent. I am just about there with that.

 

[Woody]

Don't rely solely on gauges, they work today but not tomorrow I'd suggest you develop more than one way to monitor fuel levels/usage, learn your fuel gauges, know your fuel burn per hour, know your mpg. By knowing this you can check/compare back and forth between them to be sure of your fuel status at any time and range for trip planning.

 

[my3sons]

Bill, Good info above, only thing I will add is that if your gauges are like mine, they are accurately inaccurate. Neither one reflects the level in the tank, but they are repeatable.
When they say one thing, I have come to know with in a few gallons what is really in them. You'll get a feeling for that after filling them up a few times.

 

[northern]

I have run my boat to 50 gallons remaining out of 300. The boat has fuel flow gauges on each engine and I know how accurate they are. I know how many hours I run my generator between fill ups and how much it burns an hour. At 25 gallons a tank the needles were on E. You need to know your boat if you want to run that close. We were in an area we could fuel so was not too concerned.
Get to know your boat before you push it to its limits.
Will the Coast Guard save you if you run out or will they just call you a tow boat?

 

[Craigalan]

Get Sea Tow. Its reasonable and very much worth it if you need a tow. Thats a huge out-of-pocket expense.

 

[fwebster]

When planning offshore trips, I don't use the fuel gauges and I certainly don't make decisions or trip planning based on them. I use fuel burn per for exclusively since my engines are very consistent. The other factor that hasn't been mentioned is reserve planning as a % of available. Sometimes, the 1/3 out, 1/3 back, 1/3 reserve doesn't work due to the overall length of your trip. Never count on the last 15% in your tanks in smooth seas and 25% in rough seas….then add to that whatever reserve you are comfortable with.

 

[IanBat]

Plus one on Franks post above

I run petrol, but I do a lot of offshore runs,.......40-60nm straight out, a long way from the coast, so no option of a quick duck in for fuel.

I run fuel flow meters and am checking them constantly, I also watch the gauges purely as a secondary indicator, as I do not have faith in them.

With the fuel flow meters, I normally have them set on lph, so burn rate. However I am constantly switching to fuel left and monitoring to ensure I am leaving around half my fuel for the run back, as we can get him with a 25kn plus wind and 2-3m (6-10') Swell with waves on top, in that scenario I need a buffer.

The last 2 years in particular here the Tuna season during winter, sees the adrenalin pumping on a lot of inexperienced skippers with small boats, they here the coordinates on the radio where the fish are and away they go. We have heard several calls as people get to the grounds, asking if anyone has any spare fuel!,!

And they are 40nm off shore

Don't underestimate the importance. As you are doing, get to know the boat.
Know your fuel burn
Understand what different speeds do to your fuel burn
Plan your trips, particularly speed to burn rate
Monitor the the burn rate and fuel left.
If it's a long run, know the points where you can pick up fuel if needed
Monitor weather and swell, currents and tide

Oh, and relax and enjoy,


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[blaster]

Get Sea Tow. Its reasonable and very much worth it if you need a tow. Thats a huge out-of-pocket expense.

My guess is he has that covered.

 

[ZZ13]

Very, very helpful, all. Not quite confident on my burn rate just yet. I'm around .8-.9 mpg at cruise based on about 6 multi-hour runs. Seems to match the 6CTA performance curves I've seen and Boattest on similar weight boats with the same engines (haven't found a test for my exact boat /engine combo). And yes, Blaster, I have a couple of tow plans. And Frank that's the bottom of the tank recommendation I was missing. I just didn't know where I wouldn't be able to deliver fuel to the engines. I don't plan to get near there, but I needed to know.

 

[IanBat]

also make sure your carrying spare fuel filters and you know how to change them


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[northern]

We have never run out of fuel but things have broken. I would be more concerned about the engines failing. We have had fuel pumps fail, raw water pump seals go, oil coolers leak, and worst was fresh water pump failing so heads did not work.
Know your boat and get some spare parts before you go place no one will save you.

 

[Turtlesboat]

mpg is pretty much useless, I only use GPH. know your GPH and stick to it. when you get to the bottom of the tank, all hell can break loose, you'll be sucking up all kinds of junk so bring spare filters.

 

[ZZ13]

mpg is pretty much useless, I only use GPH. know your GPH and stick to it. when you get to the bottom of the tank, all hell can break loose, you'll be sucking up all kinds of junk so bring spare filters.

For in route monitoring I can see that (by tracking RPM to GPH). But why would that be the case for planning the trip? If I am planning an offshore cruise I need to know how long it is going to take me to get to a location if I am using the GPH approach. So to know how long, I gotta know my average speed since I know the distance. If I know my average speed and distance then I can use MPG to do the same planning since my MPG is fairly consistent for an average speed. Am I missing something?

 

[FootballFan]

Speed through the water? Or speed over the ground? I like working from the assumption of gph, then validating distance remaining. With proper reserves, the risk is minimized.

 

[Turtlesboat]

For in route monitoring I can see that (by tracking RPM to GPH). But why would that be the case for planning the trip? If I am planning an offshore cruise I need to know how long it is going to take me to get to a location if I am using the GPH approach. So to know how long, I gotta know my average speed since I know the distance. If I know my average speed and distance then I can use MPG to do the same planning since my MPG is fairly consistent for an average speed. Am I missing something?

Yes, you're missing tides, currents, wave height, wind etc... In coastal waters, your MPG can be off by as much as 30% or more as a result of these factors. The only thing that will remain constant is GPH. How far you get on those gallons is the variable. I plan my day based on hours, not miles. My speed over ground will be between 15kts-24kts. As you can see, that's a huge difference in miles over a 12 hour day. So at the 7-8 hour mark, i'm looking for a place to call home for the night. I might have traveled 100 miles or 250 miles, doesn't matter to me. I've used my fuel that i'm comfortable using with enough in reserve. So in planning my trip, I make reservation at 3 places in my transient places (nearer, medium, far). Then I know I have a place to stay no matter how far I was able to get.

 

[k9medic]

For in route monitoring I can see that (by tracking RPM to GPH). But why would that be the case for planning the trip? If I am planning an offshore cruise I need to know how long it is going to take me to get to a location if I am using the GPH approach. So to know how long, I gotta know my average speed since I know the distance. If I know my average speed and distance then I can use MPG to do the same planning since my MPG is fairly consistent for an average speed. Am I missing something?

I always use the GPH calculation. For my particular boat, at 1100RPM I burn 3.5 gph. Most of the time that gets me an average of 8kts so I plan on 7.5 kts. Up on plane I calculate my fuel burn a little on the high side compared to the charts just in case and round down .5 kts. for an average.

These calculations have served me well and I have taken several long distance trips using this method with a plan to start looking for fuel when I get to 1/4 of a tank. I have even cross flowed fuel to balance the tanks due to the generator burn or for unequal fuel flows.