Fuel Tank & Gage

[Have Fun Today]

I asked this on the Chaparral forum but didn't get any response. Anyone know much about fuel tanks and their shape?

I have a 1997 Chaparral 2335SS with Merc 5.7LX, all factory original....

I'm trying to gather some fuel flow data and I'd like to know how much fuel I have burned after each outing without filling the tank back up. I check the fuel gage on the trailer before and after using the boat. My manual says the tank is 75 gallons.

1. Can I tell by looking at the fuel gage how many gallons of fuel remain in the tank?

2. What is the shape of the fuel tank? Is it smaller towards the bottom (which would make the fuel gage go down faster on a lower tank)?

3. On E, any idea how much fuel remains in the tank?

Thanks

 

[Silvertip]

Re: Fuel Tank & Gage

A fuel gauge is a "fuel level" indicator, not a "fuel quantity" gauge. Think of it this way. Take an ice cream cone as an example of a fuel tank. Lets say that cone holds a total of 20 gallons of gas. A fuel tank sender unit operates on a float principle meaning an arm has a float on the end and as the fuel LEVEL rises and falls, the arm rises and falls which sends a signal to the gauge. The arm is a linear device but since the cone has much more fuel in the top half than the bottom so fuel QUANTITY changes in a non-linear fashion. So the contents of the cone when the gauge reads 1/2 will represent the fuel remaining in the bottom half of the cone. Therefore the shape of the tank, attitude of the boat, and accuracy of the gauge cannot possibly give you a meaningful measure of how many gallons you've used. The only accurate way to do that is to install a fuel flow monitor such as the Navman/Northstar 2100. It works exactly like the fuel computer in your car. I have one and won't be without it because the fuel gauge on my boat reacts exactly like I just described. It is simply to inaccurate to rely on for long runs.

 

[Solittle]

Re: Fuel Tank & Gage

If your goal is to determine fuel used/remaining, top off your tank, run your normal day, check the reading of your fuel gauge, top off your tank and compute whatever. Or install a feul flow meter.

 

[joed]

Re: Fuel Tank & Gage

You would need callibrate your guage.
When the tank is 1/4 empty on the guage, fill it and see how many gallons that is.
When the tank is 1/2 empty same thing.
Then next time you know that at 1/4 tank, 1/2 tank etc. you have used x gallons.

 

[Silvertip]

Re: Fuel Tank & Gage

Calibrating a gauge is fine as long you have a way of keeping the boat at a consistent attitude. Long shallow tanks cause the fuel level to vary significantly. Depending on load and its distribution in the boat, the attitude changes dramatically and the gauge reflects that. You simply cannot always get accurate readings. Some tanks are more sensitive to attitude than others. Therefore a fuel flow monitor is a good solution. Is it necessary? Not if you are always within five miles or so from the dock. I put on 50 miles or more each outing and need accurate fuel data.

 

[getinmerry]

Re: Fuel Tank & Gage

I bought a brand new boat this past July. I was going nuts trying to get an idea of fuel usage from a guage that bounces all over. For the first month I would simply refuel after each use and pay attention to the guage level and the amount of fuel needed to refill. After a month of doing this I can now tell you how much fuel is in the tank at all times (within 1/2 gallon). All you need to do is study things for a while and you'll learn how your boat acts.

Chuck

 

[j_martin]

Re: Fuel Tank & Gage

There are electronic gauges that can be calibrated to be quite accurate.

BUT, it still depends on the attitude of the boat at the time. If you're concerned about fuel usage beyond avoiding flame-out, or troubleshooting a malfunction, you probably can't afford to own your boat.

hope it helps
John

 

[Silvertip]

Re: Fuel Tank & Gage

Fuel flow monitors offer a number of advantages in addition to knowing fuel remaining -- the most important of those features is instantaneous readout of fuel flow in gallons per hour at any throttle setting. It allows you to get on plane and within a minute determine the engine sweet spot. And it does that regardless of load, attitude, weather etc. If none of that is important to you it is certainly not a necessary item. But it certainly beats the tar out of a dancing fuel gauge.