Emergency diesel tank

[Alcyone]

We had diesel bug problems again at the weekend, this time at 4am off the Nash sands in the Bristol channel.

After ultimately changing filters, trying bypasses and so on, I eventually resorted to running a pipe directly from our 5litre 'emergency diesel' tank to the secondary filter. Even though our Beta 20HP normally uses around 1l per hour, I went through 5l diesel in less than an hour before I realised, and redirected the diesel return pipe from the engine into our emergency fuel tank.

I had no idea that so much diesel went back to the tank. In our case it seemed that only around 1/3 or so was being burned.

Couple of points, first, does this seem normal, and secondly, If I do (as expected) now make up a small tank with pipe just in case this ever happens again, should rediredcting the diesel return pipe be designed into such a system?

More detail on our club forum, here:
http://cardiffyachtclub.proboards.com/index.cgi?action=display&board=general&thread=1106&page=2

 

[multihullsailor6]

Don't have the answer you are seeking but regarding diesel bug: it was confirmed to me by a marine engineer and then by a chemical engineer that it is a good practise to add 1ltr of petrol to every 100ltrs of diesel to avoid / kill off the diesel bug. Much cheaper than all those additives!

 

[snowleopard]

I can confirm that 50-70% of the fuel drawn from the tank goes back. I have an arrangement that allows each engine to draw from its own tank or the opposite one but I didn't go to the trouble of arranging switching of the return pipes e.g. if I switch the starboard engine to draw from the port tank, the level in the starboard tank goes up.

 

[nimbusgb]

Yup the lift pump usually pumps LOTS more fuel than the engine uses. Usually the return comes from the filter mounted on the engine or closest to the injector pump. Yes a complete system with both feed and return would be the best bet. Trouble is how do you ensure that on discovering the bug that you don't divert dirty fuel from the filters to the emergency system and thus end up with two contaminated tanks.

BTW. You were fortunate that it lasted an hour!

I am building a fuel polishing system with an electric pump and separator and filters that will just run and circulate / clean the fuel independently of the engine system. 2 run-ins with the bug are enough for me.

 

[sarabande]

MH6


Sorry, that won't work. Petrol, being a distilled hydrocarbon, also suffers from similar bugs to diesel. And also, so does Avgas, and all the other aircraft fuels. They all use bug killer or very sophisticated filtration systems.

You need a purpose designed bug killer.


Perhaps your marine enginerror was thinking about the diesel waxing problem which also results in blocked filters and dead engines. Confusable, but quite different causes.

 

[Alcyone]

Thanks for your answers, guys. I haven't given the full story, that involved sucking diesel from the main tank, through blocked pipes using an oil sump pump, straining it through a tea towel and into the smaller tank, in a rolling sea, ruining the carpets in the process, but we got home.

The main point for me was that I always thought to keep a small amount of diesel in an emergency tank - 5l, or 5 hours, would get me to a safe port or anchorage, for sure. I just hadn't counted on so much not being used. I know now.

It only takes a few minutes to take off the fuel return pipe and redirect it, albeit it's a tight squeeze and a hot place. Better, for me, though, than calling the coastguard for a rescue.

 

[Alcyone]

Yup the lift pump usually pumps LOTS more fuel than the engine uses. Usually the return comes from the filter mounted on the engine or closest to the injector pump. Yes a complete system with both feed and return would be the best bet. Trouble is how do you ensure that on discovering the bug that you don't divert dirty fuel from the filters to the emergency system and thus end up with two contaminated tanks.

BTW. You were fortunate that it lasted an hour!

I am building a fuel polishing system with an electric pump and separator and filters that will just run and circulate / clean the fuel independently of the engine system. 2 run-ins with the bug are enough for me.

We've broken down outside Portishead, on a flood tide, in front of a car transporter ship under tugs, off Skomer, in the Wild goose race, and now off the Nash sands in the dark.

Fair to say, I've had a gutsful, too.

Engine, tank and pipes less than a year old.

 

[multihullsailor6]

MH6


Sorry, that won't work. Petrol, being a distilled hydrocarbon, also suffers from similar bugs to diesel. And also, so does Avgas, and all the other aircraft fuels. They all use bug killer or very sophisticated filtration systems.

You need a purpose designed bug killer.


Perhaps your marine enginerror was thinking about the diesel waxing problem which also results in blocked filters and dead engines. Confusable, but quite different causes.

Sarabande,

This is what this American chemical engineer had to say on another forum:

"Regarding the addition of petrol, I run these sorts of tests and there is basis for his comments; petrol is generally toxic to diesel bugs, even in relatively small amounts. Some desiel performance additive control bugs to some extent, simply because the solvent base is slightly toxic to them. However, diesel bibcides are far more dependable."

 

[Bajansailor]

A bit of thread drift perhaps - this is not related to diesel bugs, but to the rate of flow of diesel in the return line.

The set-up on our Challenger 35 has the S/S diesel tank (about 20 gallons capacity) tucked away at the aft end of the long keel, in a void space aft of the encapsulated ballast, underneath the engine.
It is possible to take the tank out, albeit with a fair bit of difficulty. It is out now (for the 2nd time in 12 years) - the reason?
There seemed to be air getting into the fuel line, as it always took a few 'goes' before the engine would start - checked all connections, all seemed ok.
A friend suggested hooking up a wee auxiliary tank to test it, so we connected a standard OMC (Johnson / Evinrude) 6 gallon plastic tank (with about 5 gallons within) to the supply line (the tank was resting on the quarter berth next to the engine) - and it started instantly, and ran beautifully.
This was at the mooring - about 5 minutes later we were about to cast off and go for a wee trial, to give the engine some load, when it conked out.

Oh dear, whats happened?
Three of us were scratching our heads, and then I accidentally kicked the fuel tank - and it moved easily. Ooops!
The fuel pump had pumped 5 gallons of diesel in less than 10 minutes out of the tank and transferred most of it to the main tank in the keel.
And the engine is only a 28 hp Volvo 2003.

So now I have decided (for the time being at least) to just use the 6 gallon tank (it now has the return hose going in to it via the filler cap) and top it up occasionally.
Much less hassle.
We found that the big tank was very gungy on the bottom, despite occasionally (trying to) pump out stuff on the bottom using my Pela pump. We use less than 1/2 gallon / hour, so there is a good 10 hours of motoring there in the little tank, and it is easy to keep it clean (and not have fuel sitting for a long period of time).
And we shall always have a few extra portable containers on board for replenishment.
(The 6 gallon tank now lives in a locker under the quarter berth).