Fuel managment with twin tanks

[forever young]

New to having two fuel tanks. Should I run both engines off of the main or aux tank or should I run one on each? If I run both off of one tank which one should I run down first. I have a 1998 30 Marlin and this is a new boat for me so I'll probably have lots of newbie questions on it.

 

[gradyfish22]

In reality, you need to experiment, not sure how your boat will run the best, but with most Grady's, running the main tank full and only filling the aux when needed is how the boat was designed to be operated. In most cases this is the optimal way for performance numbers except on the 22 seafarer where tanks were actually changed over time to switch locations. If you make long trips, I'd run off your aux first since that will give you your worst results, then save the main for after, burning the other way will be worse on most models. I would advise against using both tanks at the same time, I've seen guys run out of gas doing so, it is much harder to manage your fuel and since the tanks are not the same size and may not be fueled the same, that can lead to problems, and will also involve more monitoring for you. I will leave about 1/3 tank in my aux at all times, and maybe once every 5 trips I will fill it more and run the tank just so the gas does not sit, but my main tank is always over 1/3 or full. I found my 265 express operates with way better this way, and from most other Grady's I've been on, running the main full and the aux with less fuel has provided better efficiency. For the sheer fact of having to manage two tanks at the same time, I would avoid running one engine off each tank. I use the Yamaha fuel management a lot, I always monitor my mpg so I can optimize my performance. I also will fill my main tank, and when I am about 15-20gal from being empty I switch to my other tank on long runs, that way I never run the tank down all the way unless absolutely necessary in an emergency. Like I said, main should give the best results, but if the boat has been rigged with after market parts or re powered engines that can alter the performance, I'd experiment on tank levels and see how the boat runs. Maybe fill both tanks then run the aux out and see how the mpg and fuel burn changes as the tank gets burned off, chances are it should improve as the tank empties. Do the same for the other tank. Also, use the same method for trimming the boat out, as I come on plane for the first time I trim my engines all the way down, once you reach your speed you want to cruise at or close to it, trim the engines up while monitoring the fuel management gauge, it will begin to increase your rpm's and give a more efficient mpg and burn rate. Once you see those numbers change for the worst, trim the engine down a hair and leave her. If it is rough at, you may need to trim them a little lower and sacrifice efficiency to keep the engines from ventilating. Most Grady's like to run with the engines trimmed out high, but that all depends on how the boat is loaded and the sea conditions. Enjoy the new boat and let us know what your results are after you run the tanks in various configurations.

 

[BobP]

If you have ethanol fuel where you are, just like perisables at the market, rotate stock!

 

[gerrys]

On my Marlin, I always ran starboard on one tank and port on the other. An empty tank (either main or auxiliary) had no noticeable effect on ride or performance. The fact is, you're always trimming the boat for sea conditions anyway. By running separate tanks, you wouldn't share contaminated fuel should one tank get a slug of water or garbage. I also think it helps keeping fresh fuel in both tanks without keeping track of what tank ran last, etc.
On the Marlin @ 306 gal capacity, you have to do a lot of running to wear out two tanksfull. With the Yami fuel management system, I could always guess within about 15 gallons, how much fuel I had in each tank.

 

[gradyfish22]

I agree with Bob, you never want the fuel to sit in one tank too long, so you do need to sue both. As far as no results running off both tanks at the same time, I find that very hard to believe, depending which tank you run off, it will change the draft of the boat both at amidships and the transom, this effects how the boat rides and its efficiency. Yes, you can trim the boat with tabs and engines, but that is not a reason to use both tanks. I would not run both tanks at the same time, the boats were not built with the intent to do so, if so the tanks would be the same size and likely two big tanks split in half down the middle of the boat. If Grady thought it was ok practice to run of two tanks at once they would have stated that in their manuals. I know for a fact how you run your tanks will effect how the boat performs and I know that running off both tanks is standard practice on most boats unless the boats are cats or can only accommodate saddle tanks. I would experiment with the tanks but I am certain you will notice a difference in how the boat performs between the two. If not you can let my bosses know since I do this for a living placing tanks in hulls and designing boats.
I do agree that running off separate tanks can prevent one from running contaminated fuel through it, but chances are you burn the same on both engines and fuel with the same amount of fuel and at the same time, so you are either placing contaminated fuel in both or none. Regardless of how you fuel the engines or run them, you can still end up with bad fuel. Your best precaution is to add star-tron enzyme to the tank to pass any water through the engines safely and add ringfree to the engines to keep them properly lubed. Other then that and not letting fuel sit in a tank for too long are the only ways you as an owner can prevent bad fuel, everything else is up to where you get your fuel from which you do not really control.

 

[Grog]

With the smaller (less than a Marlin) the main and aux are quite different but in the Marlin you have two 150's. If you're local it may be better using two tanks simultainously to keep the gas fresh. For some who go to areas like the Bahamas, they use one tank and have one in case they get a bad load of gas there.

 

[SlimJim]

I have the 265 and one tank (the main) is 130 gallons and the aux is 120 gallons, I run only one tank at a time, I am using the aux first as I have both tanks full but my aux tank has older fuel and I want o use it first and then I will keep my main full and play with the amount in my aux tank. I always will use Ring Free and PRI-G. I wonder if I can also add Star-Tron as well?

 

[jehines3]

My marlin runs terrible with an empty back tank and full forward tank. I'm usually running off the main (forward tank) first. The extra weight forward with an empty rear creates noticable bow steer. However my marlin is a 280 with euro transom (stepped rear), your 300 has a different rear waterline so YMMV on the fuel load issue.

My Marlin is equal 153 gallon tanks. I always run off one tank at a time. I was told but have not confirmed that the tank pick-ups are at different heights for each tank, meaning if you run to the bottom of a tank one motor should cut out before the other. I have not been able to verify this. I plan to change my fuel pick-ups this season so I'll be checking then.

 

[Gary M]

For some who go to areas like the Bahamas, they use one tank and have one in case they get a bad load of gas there.

Yup.......I run both off the 156 gallon Main when I run to the Bahamas. Depending on if I may need fuel over there, I try to save the 150 gallon Aux for the last day or so of fishing and the run home to ensure that I have a known quality of gas to get me home. If I need to add fuel over there, I add it to the Main only. Luckily, on short trips to Bimini (60 nm each way), if I don't fish much, I don't need to take any on. In that event, I'll run the Main down to about 15 gallons and then switch tanks......

Also, I have shown total fuel burns of say 200 gallons off of the Yamaha FMS and when I top off back home, it might take 198.5 to fill it back up! Within 1% of actual fuel used gives me good piece of mind of exactly how many gallons are actually in the tank!