Dramatic increase in fuel consumption?

[titsey]

Help please!

I have owned a 10m sailing cruiser from new for 5 years. The vessel has a Volvo D1-20 inboard which has up to 4 weeks ago used just 1.25 LT per hour virtually irrespective of how it is driven. Current engine hours are 600 and it has always been regularly serviced.

Last month (on hols in France) the consumption jumped to just under 2LT per hour and has remained at that level since.

There is no blue smoke coming out of the exhaust. The boat goes as quick as it has ever done at cruising speed of 2,000 rpm (so no excessive fouling on hull or prop?) The boat is loaded the same as always - just SWMBO and self.

Does anyone on the forum have any ideas as to the cause? Ideas gratefully appreciated!

 

[sighmoon]

I once had the experience of having to increase the throttle (and hence fuel throughput) to maintain the same revs. The culprit was water in the fuel.

 

[alahol2]

Is it just that you are spending relatively more time motoring at cruising revs (being on holiday) than at tickover? My engine averages 1 Lt/hour overall but I would probably need to plan on 1.5Lt/hour for a long trip under motor. ie normally my time at tickover is relatively long compared to my time at cruising revs because I don't use the engine very much.

 

[nigelmercier]

...used just 1.25 LT per hour virtually irrespective of how it is driven...

Not possible. Fuel consumption is approximately related to the RPM to the power of around 2.5.

 

[[2574]]

Ideas gratefully appreciated!

You haven't just added a diesel powered heater to the boat by chance have you?

 

[david_bagshaw]

re filled with **** euro diesel? (with lots of bio content)

 

[Twister_Ken]

Fuel return pipe come adrift? But you'd probably smell it if it's pumping 3/4 litre of diesel into the bilge every hour.

 

[johnalison]

Not possible. Fuel consumption is approximately related to the RPM to the power of around 2.5.

Clearly true, but I have also noticed that my fuel consumption tends to be fairly constant within a wide range of uses. I get the impression that my 2-bladed folding prop is more efficient when driven a bit harder and that this partly compensates for the greater revs, ending with an average consumption of about 2L/hr for a 34ft boat.

 

[Pye_End]

Sounds as though it is not squirting into the bilges or remaining unburnt in the exhaust, so probably load related. A diesel will use more fuel if loaded more, even at the same revs - eg alternator loading up; fouling on hull; towing a dinghy; motoring into a headsea etc. At this time of the year I would look at the hull and prop for fouling (although this is not supported by your comment over boat speed). The only other thought is how you are calculating fuel usage - is it from a guage that is miss-behaving, or from topping up your tank to the top, and using an hour meter.

 

[Tranona]

Agree with Pye End. The amount of fuel used is directly related to the amount of power demanded by the prop. The control lever sets the revs and the governor varies the amount of fuel required to maintain those revs and therefore the power required from the engine. So at your 2000 revs you could be using anywhere between about 7 and 13hp (the latter the maximum it can produce at those revs). Could also be mis-measurement of the amount of fuel used for the hours.