Fuel Gauge Accuracy

I never trust fuel gauges. Never. Ever. I don’t care how fancy they are even Headhunter pricey gauge system can be inaccurate.

the most reliable way to know is to have a calibrated dipstick. Of course that only works of the top of the tank is accessible... Lazzara always delivered their boats with a nicely carved wooden dipstick. Ok my personal 53 Hatteras the tanks are under the companionway. I just Pull a small hatch, undo the mech gauges and put in a stick I calibrated. Accuracy guaranteed. Boats with tanks above the floors usually have sight gauges which are just as accurate.

I also keep a fuel log in a spreadsheet on my phone. I record every trip including fuel totalizers from the Engine ECUs, distance covered (from the (GPS odometer), gen hours etc. it calculates fuel used, fuel left as well as NMPG. I m usually within 5 to 6% of actual fuel used.

here is a screen shot

Look at a gauge from CruzPro(com).
I've had their fuel gauge for many years. Use a reed style sending unit with their gauge. After installing get the tank down as low as you dare, then refill tank as per instructions. Did mine in 5 gal increments, it's easy, 5 gal push button, rinse and repeat till full.
The wonderful thing , I'm accurate to a half gallon over 400 gallons.
What I don't know is if one gauge will support 3 sending units via switch.
They're based out of Australia.

I would hope your new Cats show a fuel use log.

One issue your having is bow angle affecting the reading. The best senders will always have this issue.
With your lack of comments, I can guess you have never worked on these senders during your ownership.
Without records, you may never have any service history but it sounds nill or poor at best.

I have tried the Centroid products, What a PIA. Never again.
All of these have been replaced with KUS / WEMA products and all are working very well years later on ours and many customers boats.
https://kus-usa.com/products/liquid-level-sender/

Centroid, Wema and some others do not have a swing arm and float twisting a rheostat. Probably what you have and installed poorly.
A single vertical rod just off the tank bottom to the tank top is all there is with this newer stuff. You do need vertical clearance above the tank to install these senders.

The best level check is with a dip and gallons per inch log starting from the bottom of the tank.
Sadly, Not able to do so on my boat.

Our forward tank is T shaped. When full, the fuel gauge takes for ever to make 1/2 tank, then the lower half of the tank disappears 3 times the top half rate.
Our aft tank is long (for/aft) and skinny. Forward end is lower in the hull several inches. Fuel pickups are in the after end. Sender in the middle.
You get nervous quickly when you slow down (lowering the bow) and suddenly one of the engines starts to stumble.
We never run both engines from the same tank for several reasons, this is one example.
The best fuel gauge in the world will not help this mess.
Hours on the gen-set(s) and mains is my only unit of measure. 18 years (thousands of hours) later, just starting to get any good at this.

Yep, I hope your fuel consumption logs in those new Cats work.

Another idea on many multi tank boats, Sight tubes on the tank sides.
You can also remote mount these sight tubes but have to be at the tanks top & bottom levels.

One more idea, You have 3 tanks. Use one as a day tank and meter what you transfer from the other tanks to your day tank.

Sadly, Without spending some quality time and rum on your boat, These are just out loud ideas.

That s why you have to calibrate the stick. I have a similar issue with my Hatteras keel tanks as they taper at the bottom. If a gauge or linear stick shows 1/4 you re actually at 1/8.

first refueling, I paused every 50 gallons and marked the stick

as I explained earlier, with flow meters and totalizers which most modern engines have, you can just keep a log. Generator burn vary with load which you can easily can an average that’s close enough. Getting within 5 to 10% accuracy is easy.

I ve never seen a boat where the limber holes where so small that it took a long time to refuel. What you may to do is fuel the problematic tank first, then the other one(s) and top off the tank after.

nowadays it s difficult to know when a tank is full as spilling a couple of drops will land you in jail... when I redid my old hatt I moved the fillers and vents high above deck in the house side which allows me to catch any spill. Much better than having the vent down on the hull side. Not always easy to set up though.

Generator are pretty consistent. I used 1.75 GPH on the 27 KWs. On my 15KW I average 1.25. So far on the “new” boat I m going to try 2.5 GPH average on the 45s

don’t over think it. Keep a log and you ll be fine

If you don't look at these, your making a mistake.

http://www.cruzpro.com/f60.html

Have you considered ultrasonic? Basically you mount an ultrasonic transducer on the top of the tank and it measures the distance to the oil. I have used them before for measuring tank levels in a stationary environment but you should be OK on a boat if it isn't too rough.

https://www.flowline.com/product/echopod-ug06-ug12-reflective-ultrasonic-liquid-level-transmitter/

There are many types and manufacturers available. The above is just an example. All it requires is a 2 inch hole in the top of the tank and some wiring. I am sure there are other form factors available. They should be pretty reliable and accurate particularly if the sea is calm.

You stated the your gauges read full when full, then always seem to be reading low.
Reading into this http://www.cruzpro.com/fu60.html , the FU60 has a learning mode.
It's more than just a display meter.

Reliability and repeatable accuracy.

Kus/Wema also has a capacitance sender if you like this technology.