Fuel gauge calibration

[Jack236]

I have a brand new Fisherman 236 with a 115 gal fuel tank.
The Command Link Plus display never shows a fuel level between 95 and 115. It stays at FULL for a long time then
drops right to 80 or so. I watched it at the fuel dock: It started at 80, then went to 85, 90, 95 briefly, then FULL. I then added another
15 gal until it over flowed. CLP configured for 115 gal, ABYC sensor.

Another test: Filled the tank, reset the CLP engine statistics, went for joy ride, and came back to the same fuel
dock. Stats now read: Available 80, Used 21.8, Trip 54 miles, Ptrip 4.6 hrs. It took 23.8 gals to refill. Same fuel dock,
same gas pump, just me in the boat standing at the helm both times. Its like the float gauge thinks it is a 100 gal tank.

Explained this to dealer service manager, he said there is a fill limiter, and a whachamagigger, and its a float style, etc..
Bad answer, My lawn mower float gauge works fine, shouldn't my $100K boat tell me how much gas I have?

Anybody else's boat behave this way? Or is this a sloppy calibration job?

Anyone done the Single or Multipoint calibration described in the manual?
(I saw the tank when the boat was being built. Its basically a rectangular box with a slight V at the bottom,
so a float sender should be fairly linear except near empty where I never plan to go)

Also, can I really put 115 gal in a 115 gal tank or is there some Limiter to provide expansion room that the dealer referred too?

Emptying the tank to do the calibration seems like a real pain, but I don't trust the dealer to do it right.

Independent of the fuel gauge,
in the example above, the "Used" fuel as measured by the engine's flow sensor differs by the amount
of fuel I refilled it with by about 10%. I reset it at every fill up and get about the same 10% difference each time.
I saw another post where the guy's numbers differed by about 1%, any idea what accuracy to expect?
I saw a menu item for adjusting the flow calibration, should I do this?

I see where most folks use the engine's consumption number over the float gauge. I get it.

All experience and/or guidance appreciated!! (My first boat was an 18' Sea Ray 3.0L Merc , very simple.
This is a huge step up for me to the "big" GW, far more complicated . . . and fun.)

 

[suzukidave]

before pulling the float, maybe get the make and model and call the maker and ask if that is normal. they are not precision devices. my truck gauge hangs at way above full for a while before abruptly dropping off to the full line. i have no idea what it does when i am filling since i never look.

 

[SkunkBoat]

Like yours, My main tank reads full 15 gallons or so before it gets full. My Aux tank never reads full and has lots of gas when it blinks empty.
Amounts vary based on load of the boat at the time of filling and which tank I fill first if they are both empty.

Whether you can actually put the max amount in a tank depends on the they way the boat is sitting relative to a level tank and how level the tank is mounted relative to the waterline.
Again....Amounts vary based on load of the boat at the time of filling and which tank I fill first if they are both empty.

If you are on a trailer...all bets are off!

The electronic senders are way better than the old float arm.

If you do a calibration, post and lets us know how it works out

 

[seasick]

Gauges are notoriously inaccurate. I can't say for sure if you gauge is reading correctly but I do understand why it seems that at when 'full' you can add another 15 gallons'
If the angle of the tank slopes aft, the rear of the tank fills to the top before the front of the tank. You can see this happening just by looking at the gauge when you stand far aft and then move far forward. The level will go down.
So if you tank slopes just a bit aft, it will read full even though there is space forward for more gas.

As to the comment about the aux tank reading empty even though there is a lot of gas, there are several explanations. If the sender is a rod type (slide on shaft), it reads empty when the slide gets to its lowest position which could be a good distance above the tank floor. If the sender is a float type, it could be that the float arm is too short and needs to be adjusted (bent down a bit). Note that if the arm is not too short, bending it will cause the gauge to read full earlier than before.

Don't rely on the gauge, you will end up running out of gas one day. Fuel flow /usage monitors are a much better indication of fuel levels when calibrated correctly. Over time, based on trip times and conditions, you will be able to approximate the gas used and remaining remaining.

I would expect your new boat to have a WEMA or equivalent sender (the rod with slide float type). Those don't have as many issues as arm type senders.
One more thing to note: The geometry of the tank affects its volume at different levels. Although senders are relatively linear (especially shaft types), a tank with a pronounced v bottom will not be half full when the fuel level is half the tank depth. The level will be less than half. How much less depends on the actual tank shape on its bottom.

 

[seasick]

I am by no means an artist or anywhere near to that but I took a stab at a drawing to explain what I said previously. The left image is a level tank with partial fuel. The middle image is the same amount of fuel but the tank is slanted making the sender think there is more fuel than the left image case.
The right image shows how an angled tank can read full but still have room for more fuel.

Don't tell me to keep my day job. I'm retired

 

[Jack236]

YES! SkunkBoat and seasick, I get it. I've been staring at seasick's awesome diagrams for an hour thinking my tank angle was opposite because at rest water flows down the
deck toward the aft drain holes. BUT, bilge water collects midship due to the SeaV2 hull design, so I bet the front of the tank is lower than the rear when the boat is at rest, consistent with the drawings.
I think the sender is in the front, and the pick-up must be in the rear closer to the engine. So like SkunkBoat, its perfectly normal to have more room for fuel after the float has topped out.
So for the fuel to displace the air, the vent must be in the rear of the tank (pardon my nerdy physics problem, but this is so interesting)

Great, 1 mystery solved and I learned a lot. Do you think the calibration is still off? I don't remember seeing any gauge readings between 100-110 even on plane.

 

[SkunkBoat]

Usually, the fill and vent are fwd and the pickups and sender are aft. I would imagine there would be bad problems if not like this.

The vent is next to the fill (usually just aft of it). The vent is a much smaller diameter so the fuel comes up the vent before it comes up the fill.

I don't think the old Yamaha digital gauges that I have are capable of calibrating, but I could be wrong.
There are 8 LCD segments so in a perfect world each segment for my Main tank is 130/8= 16.26 gals but on my Aux tank they are 120/8= 15 gals.
When I switch the readout from Main to Aux there is always a long delay before it reads correctly.

My fuel flow meter seems to be fairly accurate except that once in a while the stbd one stops working due to an intermittent connection that I have not spent enough time troubleshooting.

 

[Jack236]

OK, that makes sense, you would want the pickup to be at the low end of the tank while under way, and if the sender is also in the back, then the gauge would tend to read false high on plane with the bow up, right?

The calibration procedure must start with an empty tank. I can siphon the gas out, but won't really know if its empty because I wont really know if the siphon tube is in the bottom of the V.
So, can I run the tank dry or will that get me in trouble? Boat is new with only 50 hours on it, so there shouldn't be any crud in the tank bottom, right? Or is there some other pitfall running it dry?

 

[seasick]

Don't take a chance on running out of gas, Fill the tank and then run for a while until you see a noticeable drop in the gunge reading. Make a not of that reading after the boat sits at rest with a typical load (mainly passengers).
Next trip, fill up again and compare how much gas it takes. Over time you will get a good feel for how much gas you really have and what to expect the gauge to read

The display has no calibration. The level it reads depends entirely on the resistance of the sender and the range of resistance is a standard for all senders (except European)..
Regarding the crud in the tank; The pickup may or may not reach the absolute bottom of the tank. Therefore, there can still be some gas in the tank but the level is below the pickup. In addition, the bottom of the sender may be even higher than the pickup.
Bottom line is that you can't rely only on the gauge reading. There is too much variability.

 

[seasick]

YES! SkunkBoat and seasick, I get it. I've been staring at seasick's awesome diagrams for an hour thinking my tank angle was opposite because at rest water flows down the
deck toward the aft drain holes. BUT, bilge water collects midship due to the SeaV2 hull design, so I bet the front of the tank is lower than the rear when the boat is at rest, consistent with the drawings.
I think the sender is in the front, and the pick-up must be in the rear closer to the engine. So like SkunkBoat, its perfectly normal to have more room for fuel after the float has topped out.
So for the fuel to displace the air, the vent must be in the rear of the tank (pardon my nerdy physics problem, but this is so interesting)

Great, 1 mystery solved and I learned a lot. Do you think the calibration is still off? I don't remember seeing any gauge readings between 100-110 even on plane.

You should have two deck plates over the tank. Open the rear one and you should see the hose to the pickup, some wires and a 5 bole round sender flange. The pickup and sender will be aft. Pull the front deck plate and you should see two hoses and connections. The fatter hose is the fill and the smaller diameter is the vent.

The gas tank is usually installed parallel to the deck but the deck at rest may angle back and probably does. That helps rain and such flow back towards the scuppers. Don't assume that at rest the tank is level to the seas, it probably isn't.

 

[Jack236]

Seasick, I opened the 2 deck plates and it is Exactly as you said!

I disconnected the sender and attached a variable resistor to the leads going to the CLPlus. ABYC senders have a resistance value of 33 to 240 ohms that represent full to empty. As I varied the resistance thru that range, sure enough, the gauge changed accordingly,
and I could see values in the 95-110 gal range that have never showed up before, so I'm sure the dealer did a slop job of calibrating it. Its a little tricky, readings don't change instantly, probably the CLP uses some algorithm to smooth the readings
so it doesn't jump all over the place when the attitude of the boat changes abruptly.
According to the manual, to do a fuel tank calibration, it requires the tank to be empty to start. Incredibly inconvenient to say the least. To do a "Single point calibration" that's all you need. To do a "Multipoint point calibration"
you have to start empty and hit a button at 0, 25%, 50%,75%, and full as you are filling it. Sounds like Single point calibration assumes a square edged tank, and Multipoint gives the CLP more fuel volume vs sender resistance
data to calculate more accurate readings for different shaped tanks. Since the boat is on land for the Winter, I tried a Single point calibration by using the resistor to fake an empty tank. It looked somewhat better than originally,
but I'm limited to what I can do on land by changing the angle of the boat on the trailer to test it. Unfortunately, I have all Winter to think about it. . . I'll do the Multipoint in the Spring and report on how that goes.