Fuel consumption

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Caladh

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Can’t decide if I’ve had fuel siphoned out during winter ashore but - filled up with fuel the other day and since our last fill up in September last year I seemed to have gone from 2.5 litres per hour to over 5 !! Is there any other reason my trusty 4Jh would increase mpg by that much ? Pretty sure in September I’d fully filled the tank.
 
Caladh said:
Can’t decide if I’ve had fuel siphoned out during winter ashore but - filled up with fuel the other day and since our last fill up in September last year I seemed to have gone from 2.5 litres per hour to over 5 !! Is there any other reason my trusty 4Jh would increase mpg by that much ? Pretty sure in September I’d fully filled the tank.
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2.5 ltrs per hour seems a bit low for a 4jh. What revs are you running it at? At normal cruising revs, using engine alone, I would have expected it to be 5 ltrs per hour.
 
My Beta 60 gives 2.25-2.5l per hour over a season - 2.5 litre 4 cylinder. A jump to 5l/hr sounds extreme. Either heating or a leak (which you would know about) or did you have the tank properly full at the previous fill? If not you could have added more this time to get back to fully full, especially if it was only a top up.
 
Caladh said:
No dirty bottom 😂 and 2.5litres is our average. Heater had been run up twice over the winter for 30 mins.each time. Supertramp may have the answer - the pumps at St Peter Port seems always to be somewhat foamy.
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Garage forecourts actually deliver fuel by weight, not volume, it's the law and why many pumps have a sticker saying the (equivalent) litres delivered are calibrated at 15 degrees C.

I would be surprised if marina pumps didn't operate the same way. The calorific value of fuel is by weight, not volume.

I've got a Yanmar 3YM30AE and for around the last 500 hours (bought boat when it had 200 hours total), it's averaged 2.5 litres an hour.
 
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fredrussell said:
Hmm - gonna have to check my fuel consumption figures!
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My 4.4L 4 cylinder Perkins is direct injection and averages 3.3l/hr at 6kts. Drop to 5kts and we only use 2.5L per hour. I have confirmed these figures many times by filling up and noting hours and refilling full.
I am surprised that so many with smaller engines use so much fuel. Direct injection does help though
 
The particular engine doesn't matter much for consumption, it's how much power you need (usually less than the engine is capable of) to push your boat along at that speed (and whether with or against wind and waves).

(The revs aren't directly related to the fuel consumption, either. It is how much power is required (and hence quantity of fuel provided by the governor) to maintain the revs you have set, so again boat and conditions dependent.)

If you do not have a fouled bottom or prop, and do not have a substantial fuel leak, I think it highly unlikely you have doubled your fuel consumption. (Where did all that potential energy go?) Perhaps you misremember filling your tank, or perhaps the nozzle triggered off due to a bit of back flow of fuel in the filler making you think it was full when it was not?
 
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Mt Perkins 4-107 (43HP) pushing 4 tons at 4.5kts (nice cruise setting) burns about 1.8 - 2.0lts / hr .... 7kts ... she's likely hitting about 5lt/hr ..

My 2GM Yanmar in my 38ft 5ton is about same at 4kts ~1.5lt/hr ... no point pushing her harder than that - its a small engine IMHO for the boat ...
 
LittleSister said:
The particular engine doesn't matter much for consumption, it's how much power you need (usually less than the engine incapable of) to push your boat along at that speed (and whether with or against wind and waves. nor are the revs directly related to the consumption.

(The revs aren't directly related to the fuel consumption, either. It is how much power is required (and hence quantity of fuel provided by the governor) to maintain the revs you have set, so again boat and conditions dependent.)

If you do not have a fouled bottom or prop, and do not have a substantial fuel leak, I think it highly unlikely you have doubled your fuel consumption. (Where did all that potential energy go?) Perhaps you misremember filling your tank, or perhaps the nozzle triggered off due to a bit of back flow of fuel in the filler making you think it was full when it was not?
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Exactly. (y)

Small diesel engines have specific fuel consumption in a fairly narrow range between about 200g and 250g/kWH, which is 2 to 2.5l/h for every 10hp consumed at the flywheel. There are very few conditions or even defects which can change that much, and certainly not without its being very noticeable.

OP has either lost track of some of his fuel, or has a really fouled bottom, which indeed can double fuel consumption.

Or is operating faster than he remembers. Wave-making resistance of the hull goes up very steeply within a couple of knots of hull speed, so small variations in operating speed can produce large variations in fuel consumption.
 
At 6 knots my 4JH3E returns 2.5 litres/hour at 2,000~2,200 rpm depending on sea state and bottom cleanliness. I'm not sure I could even get it to consume 5 ltres/hour!

PS. I've added a .pdf file showing the performance curves for some variants of the 3 & 4JH series engines.
 

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Refueler said:
My 4-107 will push my Sunrider25 to 9kts at full throttle ... but the black smoke and fuel consumption goes WOW !
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Try backing off slightly the engine control lever (which doesn't directly control the fuel, it sets the rpm target). You will likely maintain the same max speed with less smoke and lower consumption. It sounds like your boat is over-propped or whatever, such that your engine can't achieve the revs corresponding with the lever full forward, while the governor is chucking in more fuel than it can burn trying to reach the revs it can't actually achieve for whatever reason.
 
wonkywinch said:
Garage forecourts actually deliver fuel by weight, not volume, it's the law and why many pumps have a sticker saying the (equivalent) litres delivered are calibrated at 15 degrees C.

I would be surprised if marina pumps didn't operate the same way. The calorific value of fuel is by weight, not volume.

I've got a Yanmar 3YM30AE and for around the last 500 hours (bought boat when it had 200 hours total), it's averaged 2.5 litres an hour.
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I’m not querying the amount delivered by pumps at all. Just foamy diesel makes the pump filler click with the foam rather than actual fuel.
 
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