How much fuel do I have?

[theonecalledtom]

So we just bought a new boat and yesterday took it out for the first time (posted another thread in meets and greets threatening the impending questions).

One of the things that felt pretty wierd was that the fuel gauge is broken. While the previous owners seem pretty trustworthy and thought there was about 50 gallons in the tank being out on the water without *knowing* how much there was is not something I want to repeat next weekend.

One person I spoke to suggested just filling it up until the overflow spouted, with some rags to soak up an excess diesel. Is this standard practice? It seemed odd to me that we can't see into our tank to verify condition and fuel quantity like we can check our water tank. Do a lot of people have fuel tanks without much access? Do many people sail without working fuel gauges?

We have a spare gauge so I guess next I need to find out what is involved getting it wired up.

[hellosailor]

Tom, typically a fuel tank gauge is just a ammeter set up to measure the amperage flowing through a variable resistor--which is the actual sensor. Temperature sensor, pressure sensor, whatever, they are all fairly similar although the details of each system ARE important and they are not all interchangeable.

It will be set up something like this: Power from the ignition/master switch goes to the fuel gauge, and from there to the tank float sender, and from there to ground. As the float moves up and down in the tank (very similar to a toilet bowl tank float) it "wipes" a contact arm on a variable resistor that is at the top of the float assembly. That resistor value changes, the amount of power flowing changes, the reading on the gauge changes.

So, the gauge can be perfectly good--but if the float was hollow, and has developed a leak over the years, it may not be floating any more. Result? Tank shows "empty" all the time. Or the variable resistor at the top of the float may have worn out, or been damaged by too much motion. Result? Erratic gauge or "stuck", etc, no telling.

From what I've seen, the gauges themselves rarely fail, but the float assembly in the tank often does. It is simply subject to more physical wear. If you can access the float assembly on top of the tank, you can remove it and use a multimeter to test it. And to test that it is getting power in the first place, and that it has a good ground. Sometimes the only problem is a bad contact or broken wire--you really need to check all the way from one end of the circuit to the other.

In the meantime...you can check the tank the old fashioned way. Tap on the side, see how low you can go before the tone changes. Or put a stick in the fuel filler nozzle (if it is reasonably straight) and see how much of it gets "wet" by the fuel. Be careful--if the fuel inlet hose is old, it may need to be replaced.

If the boat is "new to you" this may be a good time to inspect the fuel tank, either through an inspection port or by pulling that float and looking in. There may be enough gunk in the tank that you'll want to start your new relationship by having the tank cleaned and the fuel "polished" so it can be relied on. Old old fuel, unknown tank levels, these things can make for a long expensive day with a diesel engine. :-(

[dacust]

OK, rewrite. hellosailor just said most of it. A coupla additional things, though:

They make a little bottle with suction cups to go over the fuel vent to catch the spill. Fuel spills, even drops, are serious stuff and can result in fines.

Keep close while you pump with your ear close to the filler and listen. You should be able to hear the change when it's about full.

Depending on the type of fuel tank, you might be able to use a gauge that is just a strip you put on the fuel tank itself. It works by temperature difference. Works best on metal tanks but I've heard of it working on fiberglass.

But getting the existing gauge working is your best bet for the long run.

-dan

[Eleven]

I've heard of a tank contents gauge based on pressurising the tank. You calibrate the gauge for each tank (empty) and it works out the volume of air space by the pressure change as it pumps air in.
Works on any tank, it's non-invasive and it's just a rumour unles some one somewhere has one in use.

[Whimsical]

The Tank Tender - by Hart Systems, Inc.
Nice but not cheap

[Charlie]

another thing to remember is that tanks in sailboats are often of an odd shape so that if the level of the tank reads 50% but the tank is triangular in shape you may be at 50% of the height of the tank but the volume of fuel may only be 25%. The only way that I know of to find out this is to start with a near empty tank and then put specific amounts of fuel in the tank say 5 gallons and then see where the tank gauge reads as you increase the amount of fuel in the tank.

[Eleven]

The tank tender as listed by Whimsical. Well done.
400 dollars but it does more tanks at 100 dollars a tank so kitting out a cruising boat is not bad.
I like the non-intrusive aspect and the no cross contamination.
And it actually measures spare air volume so tank shape is of no concern nor is getting a consistent reading in lumpy water.
Draining, stripping, re-fitting and sealing a diesel tank. Urgh.
But then to calibrate you have to drain the tank or run dry, calibrate, de-sludge the filters, top up from cans, desludge the filters again. Is this what sailors do for fun!
Allowing ready access to Black, potable, and diesel tank contents must be worth some aggravation. Depends on your type of sailing I suppose.

[David M]

You could always do it the old fashioned way depending on how straight your fill line is. Take soundings by using a piece of wood like a yardstick or even an old dipstick.

For topping off I would use some oil sorbants and not rags. For most boats you can hear when the fuel level reaches the fill pipe and then you know its time to shut off the fuel before it spills.

[Zach]

I don't like electric gauges, particularly on tanks that are of unknown volume/shape.

Try to track down others with your boat to see what size tanks they came with is the easiest. Top up once till she's full. Track down the fuel burn info for the engine at cruise rpm... and see if your mileage (in reasonable conditions) is reflected at your next fill up.

If the tank is unknown and you can get a stick to the bottom of the tank, and the tank will gravity drain... drain the tank into jerry cans until it is empty. Fill back up with as small or big of jerry cans as you feel suitable and mark your stick.

If the tanks are square, measure the size and calculate how many square inches are in the tank. 231 inches to the gallon. If the side of the tank is easy easy to see, you can add a sight glass to the system by having a bung welded in to the tank at the top and bottom with valves attached, clear hose running between them. If you know how wide and deep the tank is, figure out the area... and mark the inch and half inch intervals alongside the sight glass. Trapezoids are harder to judge.

Folks smarter than I can use trig and find the volume if you've got access to measure the sides.

[hellosailor]

"If the side of the tank is easy easy to see, you can add a sight glass to the system "
I've heard multiple times from multiple sources that a sight glass is against USCG and ABYC regulations for small craft and if your insurer finds out about it--you'll hav no insurance. Apparently they are considered a fire danger, since breaking the glass can put the fuel in the boat.

[Zach]

Haven't heard that before...

I'll give the coasties a call and see what they have to say, working on a project that has twin 600 gallon tanks with sight glasses, plumbed with clear hose. Would prefer not to engineer a solution to go electric, as the tanks are five feet tall with only side access. Hopefully its different for an 83 footer!

[David M]

Larger vessels have what are called settling tanks or day tanks....with sight glasses. Sight glasses always have a shut off valve at the bottom and one at the top in case the glass breaks. There is also a guard that protects the glass. The question is, how big must a vessel get before sight glasses are approved? Sight glasss cannot give a wrong reading which is the nice thing about them.

At work one of our gasoline boats has an opaque fuel tank made of some ABYC approved plastic where you can see the fuel through the tank. This obviously is another fool proof method of telling the fuel level. Its the same material I think as sanitation tanks...polyethylene or something similar. I never said to use a sanitation tank for fuel...don't.

[AnchorageGuy]

Downeaster 38 should be a standard tanks size unless the previous owner expanded it. You to YachtWorld or on of the other listing sites and pull up a few Downeaster 38s. Should be a few on the market. Look at the tank sizes and you should get an answer to your question.

[coot]

Originally Posted by theonecalledtom

One person I spoke to suggested just filling it up until the overflow spouted, with some rags to soak up an excess diesel. Is this standard practice?

I don't do that. If you listen at the vent hole, you can hear the fuel making noise as it falls into the tank. The noise will change as the tank fills. I found that if I pay attention and there isn't too much background noise, I can tell when the tank is near full.

On my boat, this is _better_ than using the fuel gauge because 1) the needle points above the full line when the tank is full, and 2) it is 15 feet away from the fill hole.

If you use this method, you at least know that you have as much fuel as what you put in the tank.

It seemed odd to me that we can't see into our tank to verify condition and fuel quantity like we can check our water tank. Do a lot of people have fuel tanks without much access? Do many people sail without working fuel gauges?

On my boat, you can easily remove the dip tubes to see into the tank, but you wouldn't want to do it too often because it is metal screws into a plastic tank. I've done it only a few times in 5 years.

In my previous boat, the fuel tank was inaccessible when full -- you could only remove a wall, drain the tank, then remove it from the boat.

I currently don't have a working fuel gauge because the float is jammed. I'll probably fix it when it gets a little warmer, but I don't need the fuel gauge enough to justify letting the diesel smell into the boat while I work on it right now. ( In another month or two, I won't mind leaving the window open. ) This is mostly because I'm not going anywhere lately, but I've put maybe 10 hours on the engine since the float jammed. I know that is a small portion of the fuel in the tank. If I were planning any big trips, I would fill the tank and count consumption more carefully.

[sailawayjon]

I think enough people have suggestions about fuel level but, I just want to say assuming were talking diesal here just make sure you dont run out that can be hours (or 5 mins) work to get that engine started again.