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Old 05 September 2009, 19:11   #1
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Fuel consumption.

As part of my education I'm asking a question that I expect has been asked (in one form or another) before. I've looked in searches and can find quite a few discussions concerning petrol consumption, but unfortunately I'm none the wiser.

Here goes: I expected (based on experience of motorbikes over the years) that 4 strokes would be more economical than 2 strokes of a similar power output. In a recent boat magazine, a number of boats were tested, none of them RIBs, it has to be said, but, of the two that caught my eye, one boat had an E-TEC 130 and the other a Yamaha F350. What confused me was that while the Yamaha increased the rate (lph) it consumed petrol and decreased the range it was capable of (as I would have expected) the faster it went, the Evinrude, on the other hand, while also increasing its consumption rate actually saw an increase in the range it could travel.

Yamaha F350 - 1,000 rpm - 5.0 kn - 5.8 lph - 201.7 nm.
Yamaha F350 - 5,900 rpm - 51.9 kn - 127.8 lph - 95.0 nm.
E-TEC 130 - 1,000 rpm - 1.1mph - 5.3 lph - 26.9 nm.
E-TEC 130 - 5,750 rpm - 42.1mph - 44.0 lph - 120.4 nm.

My confusion may lie in my previous experience which has been road vehicles, where, as I'm sure everybody knows, that engine speed generally equates to mpg which equals range. In other words, the faster the engine runs (in a particular gear) the more fuel it uses
and the less the distance the vehicle can travel.

Just to add to my dilemma, is it necessarily the case that if one was to run a boat at, for the sake of argument at 25 kn. with, for example a 115 E-TEC that it would use less fuel (or more) than the same boat with the 130 E-TEC, which has the same basic architecture as its lower powered sibling. The thinking being that the more powerful motor would produce more power and torque at a lower engine speed?

I know this is a convoluted question but unfortunately that's how my mind works. In the meantime, I'll go back and look at previous threads on this topic.
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Old 05 September 2009, 20:07   #2
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The E-tec data for 1000 rpm looks a bit odd to me. My boat (E-tec) 115 gives a slightly higher top speed (45mph) with 46L/H at 5700 RPM but I am pulling around 2-3MPH at tickover (650 RPM). I'm prob doing around 5mph at 1000 RPM.

1.1mph at 1000 RPM for a boat that manages over 40MPH WOT seems a little slow to me especially when 1000RPM is 40% higher than tickover speed(~650).

I'd certainly agree with you that at the same speeds I would expect a 130 to use less fuel than a 115. I certainly cut my fuel consumption considerably when I went from a 40hp to a 50hp on my old boat.
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Old 06 September 2009, 03:21   #3
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Firstly you cannot compare these two engines the V8 Yamaha is a thirsty heavy beast and will use a lot of fuel at speed.

The tests seem to compare the perfomance of each engine at virtually idling speed then at full speed (WOT) which is a really odd coparrison.

Also the test figures refer to the Yamaha performance in knots and the E-tec in mph.

Finally the reason the E-tec is showing as increased Mileage at higher speed is that pushing a small planing boat along at displacement speeds will not be as economical as pushing along at a planing speed.

The reason the same does not apply to the Yamaha is that the sheer size of the engine and its ability to "gulp" petrol at WOT is what you are seeing if they were to compare the Yamaha at say 25 knots cruising speed when the boat would be planing but the engine would be running at a much more economical speed then you would see a more different picture

Similary if you were to run the boat with the E-tec on at 25mph you would see a further increase in range.
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Old 06 September 2009, 03:58   #4
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Quote:
Originally Posted by Chris View Post
Firstly you cannot compare these two engines the V8 Yamaha is a thirsty heavy beast and will use a lot of fuel at speed.
of course you can - you can compare any two engines, just as you can compare any two boats with different hull shapes - one will use more fuel than the other! What you may not be able to do is extrapolate findings for one engine or "style" of boat to a completely different one.
Quote:
The tests seem to compare the perfomance of each engine at virtually idling speed then at full speed (WOT) which is a really odd coparrison.
Whilst it seems an odd comparison here - I suspect it would be a reasonably sensible one in fast fisher circles - where the question is "how much fuel to get the fishing grouind" and "how much fuel when trolling".

Quote:
Finally the reason the E-tec is showing as increased Mileage at higher speed is that pushing a small planing boat along at displacement speeds will not be as economical as pushing along at a planing speed.

The reason the same does not apply to the Yamaha is that the sheer size of the engine and its ability to "gulp" petrol at WOT is what you are seeing
I'm not sure that really makes sense to me.

Quote:
if they were to compare the Yamaha at say 25 knots cruising speed when the boat would be planing but the engine would be running at a much more economical speed then you would see a more different picture

Similary if you were to run the boat with the E-tec on at 25mph you would see a further increase in range.
Agreed - for most rib users what matters is the fuel consumption at "cruising speed" and WOT. I guess safety boat users might also be interested in fuel consumption at slow speeds.

I think the ETEC data is probably misleading - either the speed is wrong, perhaps they were fighting against 3 or 4 knots of tide? or the RPM is wrong (did they check the tacho?) or it has a prop that was set up for top speed and has absolutely no grip or something. Would probably be more meaningful to compare its fuel consumption when doing 5 kts than doing the same revs (perhaps this is what Chris means by "you can't compare").
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Old 06 September 2009, 21:08   #5
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Thanks for all your replies so far. The tests were in the latest edition of Sports Boat and RIB magazine. In the interests of brevity, I only wrote down the two extremes of 1,000 revs at one extreme and full throttle at the other. The report in actual fact gave figures at 500 rpm intervals up to full throttle. But, I felt that the principle I was interested in was reasonably well represented by the two sets of figures. In fact the editorial actually made the following comment of the E-TEC powered boat:

"I discover that, on a day of mixed and aggressive helming in lively sea conditions, by far the most efficient running speed (by which we mean the speed that brings us the greatest effective range) is at wide open throttle. Here with the revs peaking at 5,750 rpm, you see a speed of 42 knots bringing you a useable range in excess of 120 nautical miles. Drop down to 5,000 rpm and about 39 knots and your effective range, leaving ten per cent in the 140 litre tank, is reduced to just 84 nautical miles. It seems then that, not only is this boat configured for the keen driver, it actually rewards aggressive helming with radical increases in economy."

In writing this reply, I've noticed that the figures in the rpm/speed/fuel flow/range chart that the speed is shown as mph, yet in the text it's shown as knots. I think there is a typographical error there as in the text it mentions "42 knots" and "about 39 knots" whereas the chart shows 26.1 mph @ 5,000 and 42.1 mph @ 5,750 rpm.

As a matter of interest, the E-TEC is on a 5.9 mtr Nordkapp Enduro weighing 810 kg. all up weight and the F350 is on an 8.1 mtr Yamarin 80DC weighing over 1,000 kgs more.

My question about more powerful engines possibly being more economical was brought on by a trial a company I had dealings with almost 30 years ago who trialled a 38 ton Bedford TM artic with the Cummins L10 290 engine and an identical truck with the Cummins N14 350
diesel. The result was that as long as the driver didn't ask too much from the bigger engined vehicle, it was more economical than its smaller engined sibling. The advantage was that with more power and torque, the 14 litre wagon lost less speed on hills and required fewer gear changes in built up areas and was therefore not only more economical but also made better time.

The raison d' etre behind the question was that when I eventually get a boat, I'd like an E-TEC on the transom. The reasoning is entirely by the heart and not the head. I think I'm maybe looking for reasons to get one as opposed to the more sensible 4 stroke option.
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Old 07 September 2009, 05:36   #6
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I'm not convinced by the ETEC data that you describe (not only because of the inconsistencies you decirbe 39/26 and mph/knots) but also because if they got all that information then they were presumably reading it off a fuel flow gauge which are notoriously unreliable unless carefully calibrated. The only accurate way is to use a full tank drive a significant distance at a given speed/rpm and top up the tank again.
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Old 07 September 2009, 06:11   #7
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I am not convinced by those figures either.
I can only go off my real life figures for my eTec 115 tickover in gear 0.7 ltr/hr speed 2-3 Knts, 4000 revs 28 Knts 24 ltr/hr wot 5700 revs 44 ltrs/hr 40 Knts
160 ltr tank. You work out the ranges.

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Old 07 September 2009, 06:38   #8
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Using less fuel per mile at WOT than at slower speeds seems incredible to me - assuming the boat is fully planing at the slower speeds.

In the truck example, I'm not convinced that the bigger engines are more efficient, they're just being used more efficiently, i.e. less time at full throttle (not necessarily at maximum revs.).

I would have thought the same applies to outboards, so it follows that one should fit the maximum power engine the boat is designed for and then you will achieve your comfortable cruising speed at economical throttle settings.

I cruise at 3600-3800 rpm, which gives just over 23 knots and over a three year period has averaged 1.65 miles per litre. At WOT, 5600 rpm, 41 knots, it get's a bit scary and I need a tanker to follow me.
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Old 07 September 2009, 16:46   #9
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The principle of the more powerful engine being more efficient stems from the fact that 2 engines, especially if based on the same architecture, ie block, head and displacement but producing different levels of torque and power, means that the more powerful engine should be producing comparable figures to the less powerful one, but at lower revs.

Therefore, it may well use less fuel and have a longer lifespan.

What really confused me was the assertion that the e-tec used less fuel the faster it went. It looks, from your replies as if there is some doubt about that. But, that is mitigated to a degree by the writer's statement that the most efficient speed, with regard to range, was at Wide Open Throttle. From my experience of other forms of transport lower engine speeds usually equate to greater fuel mileage, hence my confusion in this case.

Slightly off-topic I know, but as I type, the TV programme "Warship" is on in the background. The narrator has just discussed the trans-shipping of diesel from a supply ship and has said that the 1,000 tonnes of diesel will allow Bulwark to travel 2,000 miles.

Half a tonne (500 litres) per mile.
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