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Old 03 July 2015, 05:09   #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, 05:51   #2
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Quote:
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, 06:21   #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, 06:22   #4
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Ah forgot picture...

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Old 03 July 2015, 07:37   #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, 08:56   #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, 09:01   #7
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[QUOTE=69cmw;684545]
Quote:
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, 09:09   #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, 09:38   #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, 09:40   #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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Old 03 July 2015, 10:07   #11
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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
Al, does that mean the 10% of HP in gallons per hour thing (ie 40 litres for a 90 but you quote 30) does not hold true for larger 2 strokes?
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Old 03 July 2015, 10:17   #12
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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.
No - I reckon we were ashore about 1815 yesterday.

Thanks for everyone's comments. Don't think I often go that slowly or lightly loaded, was taking it easy hoping she'd sip the fuel a bit more gently.

I have a higher pitch (stainless) prop she'll spin quite happilly dropping the revs down by a further 300 rpm for same cruising speed. I don't really care about handling or acceleration or top end, just economy I guess. What's anyone's guesses whether the stainless prop would be more economical for this type of trip? (I don't use it often as its really for a larger hub, but I botch if to fit by using two thrust washers)
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Old 03 July 2015, 11:49   #13
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Fuel consumption 2 stroke / 4 stroke / opti / etec etc

Quote:
Originally Posted by diver 1 View Post
No - I reckon we were ashore about 1815 yesterday.



Thanks for everyone's comments. Don't think I often go that slowly or lightly loaded, was taking it easy hoping she'd sip the fuel a bit more gently.



I have a higher pitch (stainless) prop she'll spin quite happilly dropping the revs down by a further 300 rpm for same cruising speed. I don't really care about handling or acceleration or top end, just economy I guess. What's anyone's guesses whether the stainless prop would be more economical for this type of trip? (I don't use it often as its really for a larger hub, but I botch if to fit by using two thrust washers)

It won't necessarily be more economical, as the engine will be working harder to spin the prop, there's only one way to find out.
Engines have a sweet spot for economy/power. Old tech 2 strokes even more so. Get outside that sweet spot & you're either lugging the engine & having to give it more throttle just to get the revs into the power band, or you're revving it's nuts off & getting nowhere.


.....sh1t happens.......
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Old 03 July 2015, 16:23   #14
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Our two previous 16ft boats with the same 90hp 2T engine got much the same, 4.5mpg or 1Lkm which compared to my current 5.7L V8 in a 21ft boat gets much the same.


I'm sure some simple maths of speed x width x length / weight = x hp required & newer engines rarely have better BSFC than good old ones.
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Old 03 July 2015, 17:47   #15
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Wish I got that out of my 5.7 lol
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Old 03 July 2015, 19:33   #16
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Ive got two boats one with a new yamaha F70 which gives me 3.5km per litre at 20 knots on a new zodiac 550 pro tour.

My other boat has the latest suzuki 90 on a much heavier fibre glass boat pushing 1400kg for a fuel burn of 3km per litre at 20 knots.

The right engine height and prop makes a huge amount of difference, also at 3500rpm prop slip will be much higher than at 4500rpm. So you may use more fuel by going faster but because of less prop slip you may find you go further per litre.

This may come in handy to calculate different engines fuel usage, remember this goes off rpm and is calculated assuming the prop allows full range of rpm. This can be worked in litres or gallons etc Liters | Mercury - Evinrude - Yamaha - Suzuki - Honda - Tohatsu - Outboard Fuel Consumption per Hour LPH

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Old 04 July 2015, 04:05   #17
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Might have been a little better going a little faster. 22-25 knots.
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Old 04 July 2015, 04:17   #18
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OP, I wouldn't bother, re-powering rarely stacks-up financially. Whack the fuel in and enjoy the boat. I bet that you jointly spend as much in the pub post dive.

My advice would be to wait until you can afford/justify to change the rig and then look out for your preferred outboard/hull choice.
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Old 04 July 2015, 09:10   #19
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We are always looking how to squeeze that extra mile out of our fuel but when it come to ale at about £10 a litre who cares?
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Old 04 July 2015, 12:45   #20
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Max its a rule of thumb thats all.
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