Catalina 315 Fuel Gauge

[Skipper John]

Greetings, sailors!

On our six-week old 315, the fuel gauge is not well calibrated and I wonder if anyone has a similar experience.

After about 23 hours on the engine, when the fuel gauge was just below 7/8, we added 12 gallons of diesel, which is almost half in a 27-gallon tank.

Our helpful dealer explained that this is because the tank is wider at the top.

I just wonder if other 315s have a similar fuel gauge complication, or if this is something that needs more investigation.

Thanks for your input, John

 

[SJN]

John, congratulations on your new boat. My experience with boat fuel gauges has shown them somewhat inaccurate. Your dealer is correct about the tank, it has a sloped bottom so the gauge will not represent reality.

I suggest keeping close track of engine hours and fuel consumption over trusting the gauge. Twelve gallons in 23 hours seems close to normal, albeit load and rpms matter.

Be careful not to overfill your tank. Before the overflow line emits fuel, the fill pipe and overflow line will hold fuel. Leaks may occur where the lines connect to the tank, even though things are tight and new. Be conservative, estimate fuel use and give the tank no more than a 7/8 fill. Your bilge will be a better place because of it.

 

[Davidasailor26]

It is very common for boat fuel gauges to be inaccurate like that. I just go by hours and keep a log of fuel consumption so I can estimate what I’ll need. I also pump slowly and listen for the change in vent tone as the fuel begins to fill.

 

[rpwillia]

The fuel gauges they use are pendulum type affairs and they are notoriously inaccurate. The above advice is spot on.

 

[Skipper John]

Thanks, all. I was hoping for a better fuel gauge reading... the one on our Catalina 30 was perfectly accurate for 27 years.

 

[jssailem]

Wow John. That, seems like a pretty good run for a gauge.

They are notoriously unreliable for fuel measurement. It is a learning process to understand the gauge and how it relates to fuel usage. For several years I had no gauge. I dipped the tank when I wanted a understanding and counted hours of fuel consumption to get a expectation of fuel capacity. When I got with in 20% of what I thought was low low, I bought a measured amount of fuel. Then I started the log again. Not saying there wasn't some error but for the most part it works.

I would suggest you examine the type of fuel gauge you have and how it sits in your tank. You may not have the gauge down to within an inch of the bottom. This may explain the measurements you are reporting. I would try to put the gauge into the deepest area of the tank. There are also tank sensors that are out side of a plastic tank they may be a better tool than the internal units.

You may also find if the sensor in the tank is an armature float, that reorienting the arm may help to lessen the inaccuracy. If the arm is hitting the edge of the tank that could easily cause the disparity in the gauge readings.

 

[stephen_i_w]

I will repost what I wrote before on fuel guage calibration:

As a former fuel system engineer in the automotive industry that actually did a great deal of work on fuel gauge calibration, I can tell you that fuel gauges, on automobiles, are notoriously inaccurate. One factor that needs to be accounted for that there is always a certain amount of fuel that is unreachable in the tank. Personally, I assume that amount is one gallon. Another consideration is that boats heel and pitch from wave action etc. This causes the fuel pickup to potentially suck in air when the fuel level is low. I recommend a more conservatism in your estimations when traveling on bumpy water.

To calibrate my fuel gauge, I carefully log my engine hours, and the fuel needed to fill the tank. I do this understanding that each time the tank is "filled" will be somewhat different. Yanmar publishes fuel usage vs. RPMs for its various engines. Hopefully your manufacturer does as well. I can compare this to the calculations I make using my fuel log. This gives me confidence my estimations.

Here is an example:

My fuel tank is 27 gallons, subtracting 1 unreachable gallon is 26 gallons. If it takes 10 gallons to fill my tank, from my last fill, when the gauge reads 1/2, then, 26 - 10 = 16 gallons of usable fuel remain in my tank. According to my log I put 12.5 hours on my engine. I calculate that I used 10 / 12.5 = 0.8 gallons per hour. I cruise at 3000 RPM. The published curve for my engine shows the same fuel usage per hour. This gives me the confidence that I can travel for 16 / 0.8 = 20 hours on flat water or 120 NM at 6 knots when my gauge reads half full. If there are rough conditions, I would half that.