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Old 03 July 2015, 06:09   #1
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Fuel consumption 2 stroke / 4 stroke / opti / etec etc

Yesterday we went offshore fishing from Poole and covered 90.7 miles for 85 litres of fuel giving 0.93 litres / mile

This was a 3 cyl 90 Hp 1998 Mercury 2 stroke pushing a wide beam Tornado 5.8 with two men and a heap of fishing gear but only cruising along at 20 knots mostly / 3500 rpm.

I must admit I was a bit disappointing not to get slightly better fuel economy especially as we were mostly on calm water and going very slowly.

What fuel consumption does anyone manage with more modern engines on this size of rib?

I'm tempted by a Yam F115 or Etec 90 currently but would either of these offer a suitably improved fuel ecomony to justify their cost (second hand) spread over say 4 years and 1000 miles per year?
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Old 03 July 2015, 06:51   #2
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Originally Posted by diver 1 View Post
Yesterday we went offshore fishing from Poole and covered 90.7 miles for 85 litres of fuel giving 0.93 litres / mile

This was a 3 cyl 90 Hp 1998 Mercury 2 stroke pushing a wide beam Tornado 5.8 with two men and a heap of fishing gear but only cruising along at 20 knots mostly / 3500 rpm.

I must admit I was a bit disappointing not to get slightly better fuel economy especially as we were mostly on calm water and going very slowly.

What fuel consumption does anyone manage with more modern engines on this size of rib?

I'm tempted by a Yam F115 or Etec 90 currently but would either of these offer a suitably improved fuel ecomony to justify their cost (second hand) spread over say 4 years and 1000 miles per year?
I don't think you can complain at that. 3500-4000 is about the optimum cruising rev range for most motors. I doubt that any slight improvement with a newer engine would justify the cost.
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Old 03 July 2015, 07:21   #3
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Fuel consumption 2 stroke / 4 stroke / opti / etec etc

If you see in the picture below, a similar sized boat with a modern 4 stroke (albeit a 140hp) is achieving 0.8 LPM whether its doing 20 or 30kn.

Compared to what your getting now that's a saving of..... Well....nothing really.
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Old 03 July 2015, 07:22   #4
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Ah forgot picture...

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Old 03 July 2015, 08:37   #5
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I get about 1.4 miles/litre, perhaps a bit more with 2 up and a 115 merc 4 stroke (brand new with 8 hrs on it) in a fairly wide 2.3m zodiac 530 at those sorts of speeds, with some slower and come faster bursts. With 7 in the boat it dropped to about 0.9 to 1m/l. Not worth changing the engine for, unless you have far more compelling reasons.

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Old 03 July 2015, 09:56   #6
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[QUOTE=diver 1;684514]Yesterday we went offshore fishing from Poole and covered 90.7 miles for 85 litres of fuel giving 0.93 litres / mile

should that not be 1.07 miles per litre
which i reckon is pretty good
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Old 03 July 2015, 10:01   #7
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[QUOTE=69cmw;684545]
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Originally Posted by diver 1 View Post
Yesterday we went offshore fishing from Poole and covered 90.7 miles for 85 litres of fuel giving 0.93 litres / mile

should that not be 1.07 litres per mile
which i reckon is pretty good
Its 0.937 litre/mile or 1.07 miles/litre
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Old 03 July 2015, 10:09   #8
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That consumption sounds very good to me. I always thought the general 2-smoke rule is 10% of HP in (imperial, not smaller US gallons) gallons per hour at WOT. Or approx half a litre per hp per hour at WOT for a 2 stroke.

I know you're not running at WOT but for you WOT would be theoretically 9 gallons or 40 litres an hour.

Considering I use a piddly 15 HP 2 stroke and probably average about 2.5 nm litre and you're getting 0.95 nm litre from a 90 I'm envious!
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Old 03 July 2015, 10:38   #9
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Hi diver, is your rib orange and black and were you returning about 7:30pm to Poole. If so I think I saw you.

Anyhow, I have a yam115 4 stroke (great engine btw) on a 6m ribeye and get about the same economy. Unless you wanted an excuse for a new engine, I would of said those figures were pretty good and you would not get a huge improvement vs cost of an upgrade.
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Old 03 July 2015, 10:40   #10
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If the OBM is in good/reasonable condition, as a rule of thumb, in litres per hour at wide open throttle, its about thirty litres per hour,Thats a 2/str
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