Fuel gauge question.

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Fish Guts

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Hello all... I have a question about how accurate my fuel gauge is. Let's say I'm sitting at the dock, start my motor and my fuel gauge reads 1/4 tank. I start cruising get up on plane and now I read almost half tank, ride for a while, watch the gauge drop a bit. When I stop to anchor, I read just about empty. Get back on plane again and it reads up again. Which reading should I trust. I know to be safe I should use the lower reading, and I will be buying a fuel flow meter soon. I guess my question is... Is the gauge meant to be read on plane or at stop?

Thanks
 
When you're on plane the fuel flows to the rear of the tank, Depending on where your sender is that may indicate either a false full or empty reading. Gauges will give you the best reading at rest.
 
At stop but often they will be very repeatable i.e. they will give the same reading under the same conditions.

You can also calibrate the gauge by recordings the reading when you add fuel.
 
That's the thing... I put in 20 gallons in a 64 gallon tank, and at stop it showed barely an 1/8, on plane it showed the 1/3 I knew it should have.
 
my gauge is only accurate at full and at empty
 
My boat has a 25 gallon tank and I get 3&1/2 mpg. I put my GPS trip odometer back to zero when I fuel up. When the trip odometer shows around 50 miles I will take 14 or 15 gallons. That to me is a more accurate way to keep track of my remaining fuel supply than reading the gauge.
 
Most fuel tanks are not square, but belly shaped for boat hull configuration. The fuel senders read volume of fuel based on depth, not actual amount of fuel. With these conditions, real fuel on hand is only an estimate. The amount of fuel in the upper third of a tank can be ten times that of the lower third. I like to judge fuel use per hour, this allows me to keep a close eye on both engine condition and fuel usage.
 
Absolute, I agree with your way of judging fuel, however, without a flow meter this is near impossible, which is why I am getting one. Pocket change... how did you determine 3.5 mpg?
 
quote:
Originally posted by pocket change

My boat has a 25 gallon tank and I get 3&1/2 mpg. I put my GPS trip odometer back to zero when I fuel up. When the trip odometer shows around 50 miles I will take 14 or 15 gallons. That to me is a more accurate way to keep track of my remaining fuel supply than reading the gauge.

Click to expand...


That's good as long as you tend to average the same type of usage(RPM), as 50 mi at cruise will use a considerably different # of gallons than 50 at under hull speed ( trollers , sight seers) or near WOT.

But of course I'm just jealous you average ~3 x the distance on a gallon of gas as my boat.

Fish Guts- Even with a FloScan system it pays to have a decent gauge & sender. If for any reason the tank is not fully filled at gas-up or F/S totalizer is not reset to 0 at that time, the "Gallons Used" displayed will be way off.

A vertical sender such as the calibratable Centroid Products (no moving parts) sender , or vertical float-type like WEMA can both be much more useful than typical scheap swing-arm types, though Rochester Gauge makes a pretty good mechanical swing-arm sender and optional electric conversion module to send to the gauge.
No matter which kind of sender it's important that it is of the correct length for your particular tank depth and that it sends level from that Min. Usable level to top/full.

And CruzPro makes a fuel gauge that can "learn the tank shape" and compensate for the change in rate of drop at different levels.

Bruce makes an excellent point though, nearly any properly working sender and gauge can be used to surprisingly accurately assess gallons used or remaining if... you diagram the gauge needle position in a log book just before full filling the tank, then record the pumped gallons at that particular needle position each time you fill until you have a really good representation of what your gauge is really telling you when boat is at rest. If you add more register marks on the bezel or lens with tape or fine M/M that will help with accurate diagramming.
 
The shape of the tank doesn't matter, nor does the relative accuracy of the gauge, the repeatability is what matters.

The easy way to measure the volume of an irregular container is to measure how much it takes to fill it.

For fuel tanks with an uncertain but not zero amount as a starting point you measure what it takes to fill from any start point and work backwards with the gauge. The gauge reading at start will be the tank volume minus how many gallons you added.

Read the capacity plate to find actual volume. Start with the tank low, it does not have to be empty. Then as you fill the tank record the meter reading every 5 or ten gallons. Anyone can eyeball estimate 1/8th intervals. At full figure out where the gauge was when you had filled the tank 1/2 its spec'd volume. That is the actual 1/2 tank reading and will be very close every time. do the same with 1/4 and 3/4 and as many points as you want.

You can practice on your car. Start by guessing how much gas it will take based on the gas gauge before you fill up.
 
Sandy, thanks for the leads on new senders. Bruce, I already do this with my truck, not that I need to. I think what I need to do is bite the bullet to fill the tank so that I can get a starting point. Thank you all for the input

Richard
 
quote:
Originally posted by getakey

my gauge is only accurate at full and at empty

Click to expand...

Mine is not accurate at "full". I can keep pumping and the needle goes beyond the full mark. I haven't tried it at empty.

To the OP: It's best not to cut it too close on fuel on board. You never know what might happen to cause you to use more fuel than you expected to.

If you run out, you can't just step out and walk to a gas station.
 
quote:
Originally posted by pocket change

My boat has a 25 gallon tank and I get 3&1/2 mpg. I put my GPS trip odometer back to zero when I fuel up. When the trip odometer shows around 50 miles I will take 14 or 15 gallons. That to me is a more accurate way to keep track of my remaining fuel supply than reading the gauge.

Click to expand...


Your GPS is measuring distance over land, not distance over water. If there is no current or wind, they are the same. If you are travelling against a current, you will have to travel further over the water than the GPS indicates.

Your method could leave you stranded.
 
quote:
Originally posted by Fish Guts

Absolute, I agree with your way of judging fuel, however, without a flow meter this is near impossible, which is why I am getting one. Pocket change... how did you determine 3.5 mpg?

Click to expand...

I got that number the same way you would do it with a car. I fill up and set the GPS trip odometer to zero. When the GPS shows around 50 miles I fill it up and divide the milage by the gallons. I did this 4 times to get the average MPG. This takes into account wether I'm sightseeing, trolling, or going 20 miles on plane. The boat is 18' with a 3.7L 4 cylinder. At 50 miles the most it ever took was 17 gallons, usually 14 or 15 gallons. This method may not work with a larger boat, especially one with twins but it works for me.
 
Tides, currents, waves, wind and load all affect speed over ground but GPH is less dependent on outside conditions. That may be why it is more commonly used than MPG though the conversion is easy.
 
quote:
Originally posted by rawidman

quote:
Originally posted by pocket change

My boat has a 25 gallon tank and I get 3&1/2 mpg. I put my GPS trip odometer back to zero when I fuel up. When the trip odometer shows around 50 miles I will take 14 or 15 gallons. That to me is a more accurate way to keep track of my remaining fuel supply than reading the gauge.

Click to expand...


Your GPS is measuring distance over land, not distance over water. If there is no current or wind, they are the same. If you are travelling against a current, you will have to travel further over the water than the GPS indicates.

Your method could leave you stranded.

Click to expand...


Just to be more clear, here's an example from real life:

I normally cruise my trawler at 2000 RPM. In slack water (no current), I am travelling over land (and over water because it's not moving) at seven knots. If I am running into (against) a two knot current, I only make five knots over land (that's what the GPS displays), but I'm still making seven knots through the water. It takes me longer to get from point "A" to point "B" and I burn more fuel

If I'm running with a two knot current, I'm making nine knots over land, but still seven knots through the water. I get there quicker and burn less fuel.

Using a GPS to decide if you need fuel will leave you stranded unless you boat where there's no current (or wind).
 
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