Gas Guage Accuracy

[boatino]

I recently purchased a 2000 Grady Marlin. I have put about 20 hours total on the boat between trolling 1 engine at 2mph and cruising. The boat has twin 250 Yamaha Saltwater series engines and digital readouts. I have the dash board switch set on the auxillary tank. The gas gage on the readout has only dropped 1 notch. How much can I trust the gage and if the switch is set on auxillary is that the only tank being used? One last question, which fuel nozzle is which, the plackard is missing near the covers on the gunnel.

Thanks,

John

 

[Curmudgeon]

Not sure about a Marlin, but every other Grady I've seen has a switched gage that reads the tank selected, only. Has no control over which tank is being actually used ... :?

 

[Pez Vela]

How much you can trust your gauge depends upon what you expect it to do. If you expect it to give you precise readings, then you can place very little trust in it. If you expect it to give you a general idea of when it's time for a fill-up, then you can place a reasonable degree of trust in it, but then only with more time on the water getting to know your boat. Your auxiliary tank reading may not have moved much because you may be drawing fuel from your main tank. You need to locate and learn how to use the multiple tank draw valve which you probably have onboard.

 

[GTOMAN]

If you are 62 years old like me, I suggest trusting your gas guage about as much as you trust you memory. Two weeks ago I used my boat for the first time since January. After going a quarter mile on the main tank that was reading 100% full, both motors cut off. I was able to get one motor started by switching to the auxillary tank. I was convinced that some 5-month old ethanol had gone bad and I had to dispose of 150 gallons of it. After several hours of trying different things, changing the fuel filters and trying to siphon gas out the tanks via a 3/8 hose routed thru the drain plug, I removed the sending unit on the main tank and stuck a tube down into the tank. It was bone dry. Then I remembered that when I used the boat in January, I purposely tried to use up all the ethanol so it would not be left in the tank over the winter. My neighbor wisely advised me to start keeping a written log of my fillups, repairs, etc. so I don't rely on my memory anymore. I also learned that my primer switch was wired to the opposite motor it said on the switch. Also got me a new sending unit! The sad part is this cost me a perfect weekend to go to the Big Rock for dolphin. It was so calm, a guy at the dock said he saw a Carolina Skiff(and some fools) out there.
It sounds like you are unaware how you switch from the main tank to the auxillary tank on the boat. On my Sailfish, the levers are in the bilge, accessed via the rear seat. One lesson I learned in reading them(the hard way) is to read them via the indicator arrow at the bottom of the lever, not by the handle at the top of the lever. If you read them by the handle at the top you will be on the opposite tank you think you are on.

 

[Gary M]

You hold 306 total gallons. The Main tank is 156 and the Aux is 150. On your filler caps, the fwd one is the Main. Your Grady dealer may have new decals for them. Think of the Fwd/Main being the leader/bigger and that should help you remember.

Back in your bilge area under your aft seat, stick your head in there and look to your left and you'll see the two Fuel Selector Valves for each tank. They should be labled Main & Aux. One engine runs off of each of those or both if you select the valves that way. If you run one engine off each tank, the fuel gauges at the helm will drop at half the rate that they would if you run both engines off of one tank.

At your helm you will have a gauge labeled Fuel Management.

When you fill your tanks, with the keys "on" press both bottom buttons momentarily. This will "zero out" your fuel burn totalizer and reset it to "0" gallons. You can read that by pushing the Mode button until the arrow points to TTL. As you burn fuel, this will show you how many gallons you have burned so far.

When running to the Bahamas I always top off and reset the Fuel Totalizer to zero. Using 300 gallons to be a bit conservative, I then can accurately keep track of how much total fuel I have left in the tank/s. I've had to recently replace that gauge on my 2001 Marlin but that gauge was always within about 1% of actual fuel burn. FAR more accurate than depending on those little 8 bars!

If you want to switch tanks back in the bilge, you can switch them with the engines at idle and they will not die.

Sediment, sludge, crap, whatever usually lurks in the bottom of the tanks so I never run them near empty.....

Before I add any gas, I add my Stabil, Ring Free, etc first, then add fuel. I feel that this helps to stir the elements up rather than adding the additives on top of a full tank........ but that's just me!

Hope this helps!

 

[Curmudgeon]

.. so I never run them near empty.

Isn't the tank pick-up near the bottom, whether the tank is near full or near empty? Just askin ... l :?

 

[grady23]

If you want some level of trust in your fuel sender(s) then replace it with a WEMA unit. I've done this on both tanks and they read fairly close. The best thing is an ACCURATE fuel flow or fuel management system. My Yamaha setup is accurate to within 2% and actually thinks I've burned slightly more fuel then I have. Better that way then me thinking I've got more left then I do. It's very easy to burn much more fuel then you think when you get in to rough water. Invest in a Flo-Scan ot the yamaha system --- You won't regret it!

 

[boatino]

Gary M.,

I stopped down at the boat last night and noticed the fuel switches in the bilge. My are currently set one engine for each tank. Is that how you run yours? Or do you use up the auxillary tank first then switch over to the main? My reasoning is the main is more in the center of the boat vs. the rear hopefully giving a better ride. Any thoughts?

Thanks,

John

 

[LUNDINROOF]

My Mercury Optimax gage always showed that I burned about 8 to 10% less fuel than I took on when filling the tanks. That drove me crazy because it meant either I never knew how much fuel I could depend on having or the pumps at my local marina were registering more fuel than they were pumping.

I have filled my new 283 up twice, once it took about 150 gallons and the other time it took 50 gallons. The Yamaha fuel system was within 1/10 of a gallon both times. Either Yamaha has a better system or they fixed the pumps.

 

[Ron Semenza]

I have two tanks on my Seafarer. Also have Yamaha fuel monitering system. Both tanks are filled and treated. I keep a note pad, Actually two. One on boat and one at home. At the end of the day I subtract the gallons used from the tank I ran on. If I put gas in I add that to the note pad. So far it works well.

 

[Gary M]

.. so I never run them near empty.

Isn't the tank pick-up near the bottom, whether the tank is near full or near empty? Just askin ... l :?

Yep, the pickup is near the bottom but I meant that I try to not run out of either tank when it gets down to just one bar, or 1/8 of a tank.

As far as what tanks to burn from, I usually will burn out of the fuller tank. There has been some discussion here about the fwd/aft tank and weight distribution with some various opinions but my only long runs are to the Bahamas and the boat's so loaded down with so much crap all over the boat and the cabin that I don't worry about that!

However, for Island trips, I try to run over on the Main and then only fill/run from the Main until it's time to go home. The theory being here that I KNOW that I've got at least 150 gallons of fresh US gas in the Aux and by not adding any Bahamian gas to that tank, I know that I can rely on the good stuff to either get me home or to a fuel dock/marina over there, if I were to take on some bad gas in the Main tank over there.

Because of Ethanol at home I only fill up when I'm leaving for the Bahamas. Otherwise, over the winter I usually only add 100-150 gallons at a time. This means that I'm putting in fresh gas more often rather than waiting to burn down a full load of the 306 gallons at home. So far, this has worked well for me.

 

[boatino]

Thanks for your input Gary.

John