Fuel tanks,  lines, fills, grounds,  pickup tubes, and gauges

Fuel Tanks:

Fuel tanks are as diverse as the vessels in which they are installed.  They can be built in,

molded into the structure, or secured in place. They are found manufactured from

various materials, copper, aluminum, steel, black iron, stainless steel, Monel, fiberglass,

rubber,  plastic, and probably other materials I have not come across. The contents of a

fuel tank is, more often than not, just plain fuel. In some cases there are other surprises

found in the fuel tank. Water is one of the most frequently uninvited guests, along with

various bacteria, and mixtures of rust and slime. Most fuel tanks are designed  so that

the fuel pickup tube does not extend all the way to the bottom, thereby missing the heavier

water or other debris which may be lurking along the bottom of the tank. There are types

of bacteria which form huge colonies and have the appearance of a jelly like material.

These globs of material remain suspended in the fuel tank at various levels. Once in a

while the blob will come up under the pickup tube and cut off the fuel supply to the engine.

Of course, this invariably happens in rough seas, miles from port.

Fuel tanks are very difficult to inspect. As most of them can not be inspected from the

inside, a peripheral external inspection is all that can be performed. Because of the

location of most tanks, three or four sides can not be inspected. So, in a large vessel

with two or three fuel tanks mounted in the bilges, a lot must be left to the imagination.

I have also had the occasion to note fuel not drawing from one of two full tanks.

Example:  A vessel with two fuel tanks usually is equipped with a manifold which

permits switching engine fuel feeds from either or both tanks.  This is a very handy system

to have aboard when fuel is running low in one tank. In this example, the vessel is about

to empty its primary tank and the operator decides to switch to the auxiliary tank,

which is full. A few minutes after the switchover the engines quit and it appears

that they have run out of fuel. Re starting fails and it is determined that fuel is not

reaching the engines. Imagine having a full tank and not being able to use it.

Here’s what happened next. The auxiliary tank was checked with a dip stick

 and was full. The filters, manifolds, and fuel lines were cleaned and checked for

 leaks or obstructions, and no problems were found.  The fuel pumps were tested and found

to be working properly.  After everything was tested and the engines were still not

getting fuel, it was suggested to switch back to the tank that was almost empty.

The engines again started and sprang to life without hesitation.  It happens that the

The design of all marine fuel tanks only allows for fuel line connection from the top

 of the tank. So, a metal tube, called a pickup tube is fitted to the top of the tank

and the fuel line is connected at this point.  Usually the pickup tube is fabricated

 from quality non corrosive metal tubing which is rigid and extend almost to the bottom

of the tank. Some space, at the bottom of the tank, is set aside for water and other

debris which collects over time. The tube on this tank was fabricated from an aluminum

 tube secured to a bronze cover plate. The tube corroded away at the bronze

 to aluminum connection due to electrolytic action. It is imperative to check out fuel

supply systems, including manifolds during the sea trial.On a recent sea trial I

conducted, the vessel owner, who was operating the vessel, mentioned that one

tank was about half full and the other was empty. He mentioned this after I asked to

try the manifold to check the feed from both tanks.  As I spend most of the time during

the sea trial in the engine compartment, I noticed that both tanks were accessible.

They were mounted outboard of the engines within easy reach. The tanks appeared

original and were about twenty years old. They had been painted some time ago and

 were well secured. I tapped on the tank that was supposed to be empty and found that

it was almost full. After we returned to the dock, the owner went off for coffee and

 I continued my inspection which included the control station wiring. I was interested in

why the fuel gauge indicated “empty”.  The back of the control station panel was easily

 reached from a main salon hatch. Imagine my amazement to find a disconnected wire

at the fuel gauge. So, I slipped the connector over the terminal, and, sure enough, the

gauge read full. It looked like the owner knew he had problems with the tank and

tried to defeat the inspection by disconnecting the fuel gauge. I showed the disconnected

wire to the prospective buyer and let him know that there was probably a problem in the

 fuel tank. When the owner returned we confronted him with the disconnected wire and

he said he did not know about any disconnected wires. We asked if we could re-connect

 the wire to see if the gauge would work, and he said sure.  After re-connecting the wire

the gauge sprang to life and indicated an almost full tank. So, we again asked to run

 the engines and check out the fuel manifold system. The owner, with a red face at

this point, reluctantly said yes and we tried the manifold system which, of course, did not

 permit fuel to be supplied from the almost full tank. The owner admitted that he had run

out of fuel on that tank some time ago and had the gauge disconnected because it was

giving him a false reading. One might wonder why he wouldn’t just add fuel to the empty

tank.This is another example of the owner trying to hide something from a possible buyer.

Another interesting situation that I observe on occasion is the presence of huge

 bacteria colonies in diesel fuel tanks. The colonies present themselves as

gelatinous blobs that float at various levels in the fuel, some close to the bottom

and some in the middle. In this unusual case I was operating a 1982 42 Atlantic Offshore

 with twin 3208 Cats. The sea trial  was to take us to Newport, a distance of about

twenty miles. The sea was almost calm for about half the trip and the vessel performed

better than expected at all engine RPM’s.  About half way the wind and sea picked up a

 bit, small sea, about two feet or so. I had already checked out the fuel delivery system

 before getting underway, and it was in great shape. As soon as we hit chop of about

two feet the starboard engine went back to an idle while the throttle lever was at

half throttle. Incidentally, the wind and waves began to increase as soon as the

engine decided not to operate properly. We decreased speed and went along for

a while on the port engine,  which would have been a pain without the auto

pilot. Both fuel gauges indicated full,  and I knew the delivery system was in good

condition, so what could cause the engine to idle. I left the throttle lever in the same

position it was in when the trouble began. Believing that the remainder of the trip was

going to be a drag I just sat back and watched the sea grow. Five or ten minutes

 went by and the engine was back to full RPM. What magic is this, I thought.  Another few

minutes went by and the same engine went back to idle again. I was navigating from the

Sound into the Bay then to Newport harbor. As I approached the harbor the engine

 revved up and shut down a couple of times, and I thought how nice it would

be to have both engines running as docking around the million dollar yachts is

 a bit tricky in all this wind. Well, luck was with me, and as soon I reached the harbor

and the waves went away the engine came back to its normal operating mode and

docking was not a problem. I let the new owner know about the details of the day

and he was upset that I did not pick up on this problem during the previous days

 original sea trial, which was conducted without a wave in sight. As he was with me

the first day, he admitted, the vessel operated as it should and I could not have noted

a problem if there were no symptoms. I had a real interest in this problem, so I decided

to find out why and what. After several calls to local Marine Diesel mechanics  who

 suggested everything from water in the filters to clogged injectors, I talked the new

owner into another sea trial, no charge, and I would remain in the area of the fuel

manifold so that I could change tanks when the problem arose. Sure enough, as soon

as we hit some mild chop the engine RPM dropped back to an idle. I switched

 tanks via the fuel manifold and the engine picked up speed and remained steady all the

way back to the dock. The new owner had the fuel tank in question cleaned and the cleaning

crew discovered several large blobs of what looked like black Jell-O. It appears that

the blobs were floating at a level where they could get under the pickup tube which had a

screen to prevent large particles from entering the system, and partially shut off the fuel flow.