Trawler Fuel Economy

Marin said:

The previous owner of our boat replaced the three original tanks with five smaller ones. Four are saddle tanks, the fifth one is a 60 gallon day tank. It is located in the bilge on the centerline of the boat under the engine room floor plates between the aft ends of the inboard engine stringers. In this position it is totally out of the way and uses space that was unused and empty before.

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I'm hoping never to have to replace my fuel tanks

bayview said:

Fly: A great picture of a boat using fuel to move water instead of the boat. That PIX should be shown to all the "hull speed" fans.

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Good eye! That shot was taken during a brief run at WOT (10 Kts) during the period when my port engine was running a bit warm. Note the steam from the port side. A subsequent Barnacle Buster flush resolved the problem.

I rarely operate like this except to test systems. My normal is 7.5 kts at an easy 1800 RPM/~3.2 GPH.

Britannia said:

I'm hoping never to have to replace my fuel tanks

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Don't blame you for not wanting to, but if you want to add a day tank to your existing system under the foor plates on the centerline of your engine room could be a potential location unless there is already equipment, plumbing, etc. occupying that space.

FlyWright said:

Good eye! That shot was taken during a brief run at WOT (10 Kts) during the period when my port engine was running a bit warm. Note the steam from the port side. A subsequent Barnacle Buster flush resolved the problem.

I rarely operate like this except to test systems. My normal is 7.5 kts at an easy 1800 RPM/~3.2 GPH.

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FlyWright at a sedate six knots:

Marin said:

Don't blame you for not wanting to, but if you want to add a day tank to your existing system under the foor plates on the centerline of your engine room could be a potential location unless there is already equipment, plumbing, etc. occupying that space.

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I have a single engine - so there's no space under it for a tank. There's not any room either side since it's a round bilged boat. I think I'd have room for one in the lazarette though - that's probably where I'd put it.

Richard

Haven't seen the need for a day tank. Fuel is filtered twice (twin Racors, using one at a time, as well as on-engine filters replaced annually) prior to fuel delivery to the engine's cylinders. Use FloScale meters to measure fuel-flow. Haven't adjusted their calibrated, so I apply simple math (times two-thirds in my instance) based on observed fuel consumption.

I suspect day tanks are mostly a passagemaker practice and have little value on a trawler.

manyboats said:

I suspect day tanks are mostly a passagemaker practice and have little value on a trawler.

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Probably depends on how big they are and how they're used. Ours is 60 gallons which is, I think, much larger than what is commonly thought of as a day tank. In our boat the day tank-- which will actually run the boat for a bunch of days--- serves as the main feed for the engines and the generator. Fuel is transferred to the day tank via gravity from the four saddle tanks using manually operated valves at the bottom of each tank.

However the fuel system can be valved to allow the engines to take fuel directly from either of the saddle tanks on the engine's side of the boat and the fuel return--- such as it is on an FL120-- can be valved to either of the saddle tanks on the engine's side of the boat. Normally, the fuel return from each engine is valved to go to the day tank.

So in our case the "day tank," is more a means of managing the fuel on the boat rather than serve as some sort of fuel-use monitoring device which is how it might be used on a passagemaker. Our "day tank" has an electric fuel gauge sender on it with the gauge at the helm. The four saddle tanks have their own sight gauges.

Marin said:

Probably depends on how big they are and how they're used. Ours is 60 gallons which is, I think, much larger than what is commonly thought of as a day tank. In our boat the day tank-- which will actually run the boat for a bunch of days--- serves as the main feed for the engines and the generator. ...

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Have four tanks at 79 gallons each. Would be silly to assign one as the "day tank." As the tanks are either starboard or port, I choose one which will keep the boat on an even keel.

I've thought about eventual need to maybe (hope not, hope not, hope not) change our two 100 gallon aluminum gasoline tanks. They look real good and are currently OK. Look so new they may have been replaced some time during the boat's 38 yr. history (hope so, hope so, hope so)... but, in reality, I think not!


Sooo, in addition to it being gas fumes I'd need to nullify, there is little to no room to remove nor replace them... in same size that is. Therefore; I would collapse the tanks via commercial grade negative-air suction machine. Then I would somehow "twist" them out having removed exhaust and top ends off each motor.

For replacements I would likely put in four 40 to 45 gallon tanks in same location. There is another area where a fifth 40 to 45 gallon tank could be placed. That would maintain 200 gallon capacity or maybe even 25 gallons more.

I believe thinking out of the box is very necessary regarding fuel tank replacements.

One way or the other... need to do = need to be done = success!

Happy Fuel-Tank Daze! - Art

markpierce said:

Would be silly to assign one as the "day tank." As the tanks are either starboard or port, I choose one which will keep the boat on an even keel.

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The so-called day tank in our boat is on the centerline. We periodically fill it by transfering fuel via gravity from an opposing pair of saddle tanks. The four saddle tanks drain into the day tank at the same rate. The engines and generator are normally fed from the day tank. So the boat automatically remains on an even keel laterally no matter how much fuel we use or transfer.

The smart location for the day tank is ABOVE the engines and filter bank.

Then priming is fool proof and vacuum leaks can not happen.

A rotary hand pump to fill the day tank, and your boat can put- put forever without electric, after starting.

Gravity is your friend and will work for you!

Have two 350 gallon side tanks and no place (that I wanted to give up) for a day tank. Changed the plumbing so that both the generator and engine draw off the starboard tank. Will only fill to the port tank and transfer fuel though a Racor filter to the starboard. Both tanks have bottom drains that are valved to the fuel transfer pump / polishing system. In an emergency, both tank drains can be opened allowing the tanks to equalize through the manifold or fuel could just be added to the starboard tank through it's deck fill.

Ted

I have two 20 gallon poly tanks connected to a fuel manifold with two racors. The system valves let me pick which tank fuel is drawn from and which tank to return fuel to. I only have a fill on the port tank and transfer fuel to the starboard tank by running off the port tank, but returning fuel to starboard. That way the fuel in the stbd tank has all gone through a racor and the on-engine fuel filter. Except for fuel transfer, I normally run off the starboard tank. Given my fuel consumption, a full tank is good for about 30 hours cruising at 7 knots without any risk of running out.

Just a tidbit regarding "day" tanks: I worked on a power plant that had four Nordberg 4000kW gensets, each engine had a day tank. It was sized so it could run the engine 24hrs at full power. All fuel entering from the bulk tanks on site had to go through a good filter rig, and day tank was sumped on each shift. It was set up so only one shift had to transfer or otherwise manage fuel. Day tank was in the basement under the engines.

A smaller "saddle" tank of say 1000? gal was mounted right next to engine, it was supplied from day tank with a electric drive vane or gear pump that ran constantly. An overflow from the saddle tank drained back to the day via gravity, so it always kept the same about 3/4 full level. Level alarms sound if tank overfilled or ran down to half, in either of those cases something was wrong and operator had an hour or two to sort it out.

Engine driven gear pump sucked from saddle tank, through a duplex "last chance" filter, then to the fuel rails. The last chance filters were not much bigger physically than what we see on trawler Cats. But could flow over 300gph. Full load burn was about 220gph, about 22hp/gph, with about 60gph of return flow.

Neat old engines: V16, 13" bore, 16" stroke, 4900hp @ 514rpm. Had variable valve timing, too.

Too cold to work outside. So killing some time...

Nice to hear about systems like that, so far from our own.
Thanks Ski

Ski, that sounds close to the Learjet 60 cruise fuel burn...200-230GPH. We carried about 1000 gallons when full. We had 4600 Lb thrust at takeoff power, but ran with much less at cruise. I have no idea of any thrust/KW/HP conversion.

Do you remember how big the tanks were to support that monster?

FlyWright said:

Ski, that sounds close to the Learjet 60 cruise fuel burn...200-230GPH. We carried about 1000 gallons when full. We had 4600 Lb thrust at takeoff power, but ran with much less at cruise. I have no idea of any thrust/KW/HP conversion.

Do you remember how big the tanks were to support that monster?

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Hey AL - 200-300GPH = 2 +/- miles per gallon... or a bit less? If so, not too far off many boats on TF. Just a bit less travel time! LOL - Art

Art said:

Hey AL - 200-300GPH = 2 +/- miles per gallon... or a bit less? If so, not too far off many boats on TF. Just a bit less travel time! LOL - Art

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You're right, Art. I did the math in a prior thread, but it came out about the same NMPG. But I have to say, I can have a whole lot more fun with my boat when the engines are silent than I ever could have with a Learjet. That's when the adrenaline kicked in on the plane!