Fuel Tanks and Usage

[Guest]

My boat has two fuel tanks to feed the two (Mercruiser) diesel outdrives. Earlier this year when cruising to Denmark in 8 hour hops, one of the tanks appeared to empty much faster than the other. On filling the tanks up, one required constantly circa 25% more diesel.

Heating was not in use, both engines were running together at the same revs, and there was no sign of any leaks anywhere. Both engines had also been serviced beforehand and were apparently in perfect order. The switchable fuel lines were set as standard, port tank to port engine, s/b to s/b.

Not being very experienced with diesel engines, what might be going on? I understand that there is some sort of fuel return hoses - could the movement of the boat mean that both engines return to one tank? If a return hose was blocked, would this then mean fuel returns to the other tank?

Whilst I can switch the engines to pick up from the fuller tank, can this be done mid-channel whilst the engines are running or should they be stopped first to avoid air entering the system?

Sometimes I remember my single engined (and tanked) petrol powered Bayliner with great affection!!

 

[byron]

Sounds to me that the scavenge is going into one tank only. Try opening the balance pipe.

©2001
www.education-jobs.co.uk
www.alexander-advertising.co.uk

 

[Col]

A friend of mine had a Sealine 305/twin Volvo diesels that had the same probs.The fuel return on one engine was partially blocked,and as he had the crossover valve open,it returned more to one tank than the other.Worth checking?

 

[hlb]

Mine dose the same to some extent.
But then the rev counters tend to lie a bit too, so maybe one engine is working just a bit harder that tother.
My boat also go's much faster when driving from upstairs than down.

Haydn

 

[Piers]

Adrian,

I agree with what's already been said - the cause is more than likely the returning fuel from the engines' injection system.

One caution on opening the balancing pipe between tanks. If the boat has any lean on her whatsoever, the fuel will flow towards the lower tank which could cause the lean to worsen, more fuel to flow, more lean and so on. I know - it once happened to me!

You also ask if you can change over fuel tank supplies whilst on passage. Yes, provided the fuel flow is not interrupted. For this, the clue is always to open a valve first before closing another. i.e., there is never going to be a time when the fuel supply is shut off.

It would be good to hear the outcome.

Piers du Pre
MBM Cruising Club enthusiast

 

[miket]

On my last 3 boats (Brooms and a Turbo 36) all had a maze of fuel taps, but I did eventually get it sorted.

The correct position for each engine to draw fuel from and return excess fuel to, its own tank is:
Balance fuel tap closed.
Port and starboard taps open.

I have certainly changed permutations in mid passage with no problems.