Fuel gauge accuracy?

Empire · 05-03-2002, 06:45 AM

Took my new boat to the gas pump for second time. Going in, the gauge read less than 1/4 of a tank. I have a 39 gallon tank, so I figured it would take about 30 gallons. At about 20 gallons it was full and gas went spilling everywhere. Does anyone else have a less than accurate fuel gauge?

Lured In · 05-03-2002, 06:59 AM

I have a similar, althought not as extreme problem with my guage. I remember a thread awhile back that referred to a "flow meter" that went in line with the fuel line. Rather than telling you how much you have left, it tells you have much you have used. Check it out.

Predator Dawg · 05-03-2002, 06:59 AM

Two things.

If you have an under floor tank, it will be long and flat. This will make having an accurate gauge very tough do to the total heighth of the tank.

Also, if the boat was not perfectly flat or slightly up hill to the fill hole, you could have a big trapped air bubble in your tank and you are not actually full.

Propwash · 05-03-2002, 07:02 AM

Mine has a fourty two gal tank and it does about the same thing. I think they all have thier own oddities. Not a big item for me.

Pete · 05-03-2002, 07:08 AM

Fuel guages on boats are notoriously inaccurate since only a slight tilt in the boat will drastically affect where the gas is in the tank. I rely on a guage that tells me how many hours I've put on my engine and some experience to know when I need to fill. I recently heard of an in-line guage that accurately measures the flow ... that's probably the best measure. But the bottom line is that you don't want to run out of gas on the water, so don't run too low and keep it topped up.

AngleThis · 05-03-2002, 08:33 AM

We all have that problem. If for no other reason than your sending unit is sensitive to the angle your hull is at, you can get very different readings at different speeds. Even how you load the boat can effect the readings.

If you are going to do long trips and push your range to the edge of the envelope, I suggest a fuel flow meter. There has been a lot of talk about this on the Salty Dogs threac because we are all preparing for halibut next weekend.

Empire · 05-03-2002, 09:02 AM

I know it was not a bubble in the lines or tank. After a few hours, and after washing the boat, I could still smell gas fumes quite strongly. I removed the panel that covers the fill line and vent hose connections to the tank. I could see gas seeping from the connections. I removed one of the vent lines, and that tank was FULL! (I lifted the trailer so the vent lines were high up). I siphoned off some gas, then tightened the worm drive clamps to stop any more leaks.

How much $$$ are the fuel line meters?

TJB · 05-03-2002, 09:16 AM

I had the same thing happen to me. The vent line was plugged and gas filled all the way up the filler spout. The weight of the gas in the filler spout forced it out the fittings. Very dangerous situation if you don't know what happened. Your bilge will be full of raw gas.

kevray · 05-03-2002, 10:20 AM

I would also like to know more about these fuel line meters. What line do they attach to and how about those of us who run both the kicker and the large outboard off of the tank.

As another possible form of measurement and I am just experimenting. My depth finder has a milage gauge that may be cleared at the pump. After several fills I hope to get a rough miles per gal. We will see.

Bait O' Eggs · 05-03-2002, 10:33 AM

I have never seen one of these, but I just checked West Marine and it looks like they get about $300 for one.

West marine Fuel Flow gauge

bait boy · 05-03-2002, 12:03 PM

Check out the prices from boaters world on this type of fuel flow computer

NAVMAN

Thanks to threemuch for finding this little gem...

SouthCoastStu · 05-03-2002, 04:58 PM

My fuel gauge is the same way. It only reads accuratly when the boat is at just the right angle. I've figured out about how many miles/gallon I get and use my GPS's trip meter to keep track of when I need to refill (I never run it past 1/2 a tank). I used to fill it everytime, but it's a hassle to get just $5 worth of gas.

HOGTIDE · 05-04-2002, 07:23 AM

Same story as everyone elses, here. My 37 gal. tank either shows full or empty, never an accurate reading in between. I started keeping a log of my standard trip distances and then refilling each time, to estimate gallons used.

I know I can get 4 of my favorite 'local' excursions in on a full tank without a worry...then it's time to refill.

When I'm in question...I always fill er up!

Hook-up · 05-04-2002, 09:02 PM

My friend has a fuel meter on his boat. One of the additional benefits besides showing how much fuel is used, is that if you watch your RPM's and the flow of fuel it is amazing the differance a few RPM can make. While trolling for tuna we noticed that our fuel use doubled when we bumped out RPM up by just 200 (from 1800 to 2000 RPM). It also works to figure out your best cruising speed with regards to your fuel use.
Also consider installing a bypass in case the fuel meter gets plugged. We have never had that happen but it was suggested we install it with one.

jcarufo · 05-04-2002, 11:41 PM

I also had the same concerns with a constantly bouncing fuel gauge on our Boston Whaler. I just purchased a Flowscan fuel meter that arrived yesterday for $264.00 from DB Marine that I found doing an on-line search. The guage was drop shipped from Floscan in Seattle in 1 day. The sending unit goes between the filter/bulb and the fuel pump/engine. The guage is the size of a tach and analog and shows a rate of gallons burned per hour with a digital window for total gallons used. It should be easy to determine best fuel use by watching the tach vs. GPH used. They advertise a typical 10-15% fuel savings which should pay for the meter and provide greater peace of mind. The guages are sized based on 1/10 of total HP, so in my case 1-10 gph for a 90hp Honda. They also have 1-20 and 1-32 gph meters. The 5500 series I purchased is the most basic with 2 engine and GPS tied meters for inboards and outboards also available. The Floscan meters are also used in the Airplane industry, so I would guess they're pretty accurate. They even have as part of their guarantee that they will pay up to $500.00 for a tow or gas if you ever run out while using one of their meters. Their site is rather informative; I think it's floscan.com. By the way I'm a consumer with no ties or business relationship with the company. Tight Lines
Jean

Threemuch · 05-05-2002, 01:32 PM

My guage is whacked. It reads full with 76 gallons in, but empty after I burn 20 or so. Sending unit is not easily accessable, so I bought a fuel flow meter which I installed yesterday.

Boaters world has the NAVMAN for 150ish, but when I ordered they said it was backordered for a week. Then two. Then three. With halibut approachin I went elswhere. I bought a Standard Horizon unit for 186 at online marine which is the same as the Navman with different badging. Installed yesterday and tested. It tells fuel flow, gallons used, gallons remaining (assuming you tell it how much you have to start), and low fuel alarm. It's accurate to 5% out of the box, and 2% with calibration. I will post again on it's use after halibut.

If you want to play on the big blue, no better way to increase your range than adding tankage and knowing how much fuel you have left with a flowmeter. I have 76 in the main below deck, and I put in two 12 tanks above deck with a threeway valve. 100 total for a range of about 150-175 miles. Sounds crazy, but I want 50 mile range for albacore plus trolling fuel. The bonus here is that if I have fuel problems like moisture or other contamination, I have a completely separate fuel supply to get back to the dock with. Luckily, my boat is big enough that I can store the fuel tanks in the bow when not in use.

KB

Pilar · 05-06-2002, 12:47 PM

You can also use an engine manifold vacuum gauge as a cheap way to optimize your fuel burn rate. This is the method used by aircraft with variable pitch props to set the engine load during flight.

It also has the advantage of immediate indication. When you run, trim the boat for max vacuum for a given RPM. You can also adjust engine speed for max vacuum to achieve a certain speed. Just a touch on the throttle or trim makes a big difference in vacuum reading. My 'Pilar' runs at between 7 and 10 inches cruising at 20 mph. To get 25 mph the vacuum falls to 3 to 5 inches and gas use goes up alot.

One other use is that you can tell if your engine has issues when you run at a lower vacuum than normal. Then it's time for a tuneup or time to weigh the boat.

$30 by Sunpro at Baxters, 2-1/2" gage face and manual mechanism, no power or wiring required.

One other comment is about fuel tanks in general. To prevent water condensation in your fuel, keep it full This means stopping for gas after every fishing trip. A fuel/ water separator is also a good thing to have. Ask Threemuch about this point.

[ 05-06-2002, 01:49 PM: Message edited by: Pilar ]