Fuel usage

[PhilipF]

I'm rather dismayed by the amount of diesel consumed by the D3 Volvo 160HP "common rail" engine installed in our Nimbus 280 'Familia'.

Yesterday we came down here to Brixham from Poole - seventy miles as I understand it. I'd brimmed the tank, and the only fuel used since was what was consumed for heating - just a very few litres for the week since filling I would have thought.

Leaving Poole by the 9.30 bridge, we arrived here spot on 1600 hours. The engine was kept running at a fraction over three thousand revs. So this gives an average of around 10.75 MPH SOG - we had the tide against us for around four hours.

A graph produced by Volvo for this engine indicates at three thousand under load, the consumption ought to be 15 litres per hour. 'Brimming' up today, took 146 litres. This gives a consumption of above 22 litres per hour. I'd better make clear the boat has only just gone back in the water, following anti-fouling. She went into the water last June for the first time. So all in all, she really ought to be in first class nick.

Quite apart from the fact that these figures rather put the kybosh on our running budgets, there are other considerations: Range of the boat for example. I had been planning on this being around 150 miles. The fuel tank capacity is 240 litres - so on my figures, it going to be far less than that. A considerable disapointment.

 

[Whitelighter]

Ok, first thing is the fuel usage is always theoretical. While an engine of certain HP will use a predictable amount of fuel in perfect conditions (it is often said that a physicist can predict the outcome of any horse race provided all the horses are spherical and the race is run in a vacume - i.e., in theory he can, but in the real world with variable conditions he can't), when you ad outside factors and different setips such as hull type, drive type, fouling etc etc the engine will use more (or less).

What uses fuel is the amount of power the engine is having to produce. Take your car. Drive along a flat section of road at 3000 rpm and it might be doing 35mpg say. Come to a ateep hill and to maintain the speed you need to open the throttle, but the revs don't increase as the gearbox maintians the same ratio - i.e 40mph is always 3000 rpm, or what ever. As you go up the hill, same revs same speed the fuel consumption will increase - you can see it if you have one of those clever computer thingies that BMW love.

Now, your D3 160hp revs to 4000 rpm if I am not mistaken? The conventional maths says that the most economical engine speed should be about 75-80% - probably around 3000-3200. Having done the conventional maths, I reckon at 3100rpm a 160hp motor would be using closer to 20 litres per hour. At full chat, assuming it is producing 160 Hp it would be using more like 32l/hr (I have cheated, I have a luttle spread sheet that does the maths for me - can't remember who sent it to me, but no doubt they'll pop up in a minute.

The fact that you were punching a tide means your SOG would have been well down, plus the boat would have been working harder. I personally don't think 22l/hr is that bad. I would however question SOG of 11knts. Sound very slow for 3000 rpm. Were you planing at this speed? If the boat was dug in sitting behind the hump, then this is the most ineficient speed for the boat anyway - either go for full displacement (about 7knts in a 32) or get on the plane (I guess 16knts ish) - any where in between and you will burn fuel uneccessarily. Weather and conditions permitting of course.

 

[[2068]]

The trouble is with those fuel consumption curves, is that they are "typical", which means that in practice, they match no boats whatsoever.

People have reported that the figures can be quite accurate on some boats, especially faster ones that can reach 30kts, and where 3000rpm is over the "hump" and gently planing.

In your case, I suspect that 3000rpm is pushing much higher up the load curve: at 11mph the hull is not planing, but faster than displacement speed, hence the poor consumption.

If it were me, I would try speeding up to say 16kts, and also try slowing down to see what effect that had on consumption. A fuel flow meter would help to figure out what is going on.

dv.

 

[[2068]]

How on earth did you type all that lot so quickly?

 

[Kawasaki]

Some quick questions first.
According to Volvo/ Nimbus manuals, what does Your D3 max out at, revs I mean.
What are Your reccomended cruising revs?
10.75 agin how much tide der rekon?
160 hp at 15 litres per Hour seems a bit ambitious to Me, whose ever eng we are talking about.
15 Litres = 3 Galls ish to Me (Can't do Metric yet!)
My old Nimbus 27 with a 105 hp old fash turbo Volvo consumes 4/5 galls an Hour at 3200 revs.
Max revs 3600
As Jez says, if You are displacing and not "over the hump" or at least Semi displacing (which I believe Your hull will be happier at) consumption will suffer.
K

 

[Whitelighter]

[ QUOTE ]

How on earth did you type all that lot so quickly?

[/ QUOTE ]

Skill, dedication and a willingness to help

 

[Kawasaki]

Well at 3000 odd posts (sorry I don't mean they were all odd!). He's had plenty of practice!
Plus these Deskbound Automotive Industry Types.
Do a lot of typing. /forums/images/graemlins/tongue.gif

 

[[2068]]

Hmmph. If there was wi-fi on Trains, I would have been able to post a lot more this week. A track of my movements would have pointed to someone that didn't know whether they wanted to be on Hayling Island, Canary Wharf, Leatherhead, or Maidenhead.

dv.

 

[Kawasaki]

I think You would rather be near a post code that ended in S 28! Methinks

 

[hlb]

Tend to agree with others, 10/11 knots is an awfull speed to travel. Perhaps the most expencive speed possible.

Experiment, first get planing, then keep pulling the throttes back slowly. The speed should drop, but hold, once around 13/14 knots, any further reduction in revs, will produce a marked reduction in speed. Even increasing the revs slightly will show little or no difference in speed.

So, find minimum planing speed. But you can not cruise at that. Waves knock you down all the time. So add two knots. 15/16 knots Else course you can stick to 6/7

 

[Kawasaki]

Willingness to help I'll give Yer.
First two----- Gerght aght of it! /forums/images/graemlins/tongue.gif

 

[Kawasaki]

Yep agree but if the tide is banging against one, one must stiil operate the vessel at the most economical rev range, ie that which would carry one at 18 knts with or 11 knts agin one. Doesn't one think?
That's if one is operating in places that have such strong tides, like one does.
Should the One's be begun with big O's like the last one when One posts like this?

 

[Whitelighter]

[ QUOTE ]
Willingness to help I'll give Yer.
First two----- Gerght aght of it! /forums/images/graemlins/tongue.gif

[/ QUOTE ]

 

[hlb]

Dont see how tide makes any difference. Less working on SOG, which is a bit daft really for this perpose, or even really, really daft. Less wanting to do hundreds of sums every few minutes. /forums/images/graemlins/confused.gif

 

[JKay]

Hull speed or planing for economy no where in between /forums/images/graemlins/mad.gif

cheers Joe

remember The Major and the lack of bass /forums/images/graemlins/blush.gif

 

[PhilipF]

Many thanks to all who have replied - yes you are fast typers!

To Kawasaki:

>According to Volvo/ Nimbus manuals, what does Your D3 max out at, revs I mean.
Not checked in the engine manual - not sure it gives these figures anyway. In practice though: I cannot push the throttle lever past the stop - but when it up against it, the counter says 3500. It would go beyond this of course if it was out of gear.

>10.75 agin how much tide der rekon? A couple of knots against me I would have thought.

I understand what is being said about planing and displacement. But we are restricted by hull design and conditions. I would have termed the boat as being "displacement". No mention in manual or brochures which dispute this. At these speeds I do of course set the trim tabs as low as they will go.

The Volvo main agent suggests I run it at a couple of hundred revs below maximum. My setting might have been marginally less than that. I never like running any engine flat out anyway.

Once again, many thanks for comments, but no more typing for me tonight!

 

[hlb]

"I would have termed the boat as being "displacement"."

Well, yer just pissing against the wind past about seven knots then.

Yer engines are working harder and harder, trying to achieve the imposible.

 

[contessa26]

Thanks for your thread.
I think I agree with the other posters.
Not an expert on motor vessels - and fuel consumption is always very variable - but 22 ltr/hr equates to a little under 5 gallons per hour (doesn't it?!), which I thought would be about right for a single 160 hp diesel engine running under great load at approx 3000 rpm...
I think I'd be more worried about the speed of 10.75 mph SOG, that you state.
I know of an older, well loaded up Nimbus 280 Coupe (same hull as yours, similar weight) with a 150 hp that does 13 kts through the water at 3000 rpm. It's doing approx 5 gal/hr. It's a semi-displacement, or semi-planing hull (it can run up to nearly 20 knots...).
Consumption? When you read MBM boat reports and see fuel consumptions of a lots worse than 5 gall/hr, you should be glad you've only got one engine! Range? That might have something to do with your fuel tank size. Speed? Now that's a different subject.
All the best.

 

[Hurricane]

Sounds that you could save money if you fitted one of these

You've got a D3 so presumable with EDC or even EVC.

We've got them fitted and use them a lot - particularly to refine the sort of problem you outline here.

An alternative would be to fit the ones Jez has just fitted - perhaps he can advise.

You would then quickly see what speed was the most economical - BTW SOG means nothing - it is speed through the water and sea conditions that make the real difference - compare your speeds using the log.

I would think that you will settle down at about 7 or 8 knots to give the most economical speed - this means planning the tides carefully to make the most of things - very rewarding when you get it right.

 

[hlb]

Umm. Thort that was what I said, though you put it much more eliquently. /forums/images/graemlins/tongue.gif

We do not seem to have established whether this boat plains or not.

If it planes, the engine seems to small. 3000 equals 10.5 umm knots. Flat out maybe 12/13/14. Still only on the edge of planing.

Alternately, it's a displacement boat, in which case, the engine is far to big. 50 hp would easily give it 8/9 knots.