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Old 27 December 2011, 04:57   #1
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Country: Australia
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Boat name: Lunasea
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Twin ME421STi's fuel economy

Today I decided to go up river for lunch and see what the economy is like.

Boat is a Ribtec 890SX. Absolutely awesome to drive across Botany Bay in the chop and really need to concentrate on the tighter bends in the Georges River.

Not sure of the distance but according to the hour meter exactly 3 hours running. Probably 2 hours at 3000 to 3500 and 1 hour at 8 knots.

I used 72 litres of diesel. That is a lot less than I expected. The 4.7m Zodiac with 50hp 2 stroke Merc used over 50 litres petrol plus oil for the same trip. The Yamaha jet ski used about 60 litres plus oil. The 20ft Bertram with twin 260 Mercruisers and shaft drives was about 140 litres.

I have no idea how big my tanks are, so that is the next thing I need to work on.
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Old 27 December 2011, 05:22   #2
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A lot of the yam twin set ups only burnt 1 litre per mile and the Ribtec hull is very efficient. I would always allow 1.5 per mile if I was out cruising and 2 per mile if is was flat and running hard. AP
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Old 27 December 2011, 06:57   #3
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Alan, I was cruising in the 30 to 40 knot range, so say 2 hours at 35 knots per hour is 70 miles in total. Another hour at 8 knots, that makes about 78 miles and at 72 litres, looks like we are right on the money of just over a litre per mile.

Any suggestions to work out the tank sizes as I can't really run them dry and I can't see them to measure them as they are under the fibreglass? The most I have put in with the gauges reading on the last red led has been about 80 litres per side and someone had mentioned they should be about 200 litres each. Maybe they meant 200 in total.
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Old 27 December 2011, 08:51   #4
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I think the 8.5 is the stretched version of the 7.4 and dont hold me to this but I think they around the 135 litres each. All the stringers and bulheads would be in the same places and we managed to get 3 x 135 under the deck which gave us a 400 NM range. The extra lenght would be for the engines. There is only one way which is to drain them down and then fill them. Alan P
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Old 27 December 2011, 15:25   #5
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Well next time we run the tanks low, that is exactly what we will do.

It is amazing, such a simple and logical idea that we just didn't think of. I guess that's what discussion is all about.

Any idea what material the tanks are likely to be made of?
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Old 29 December 2011, 07:28   #6
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Quote:
Originally Posted by Alan Priddy View Post
A lot of the yam twin set ups only burnt 1 litre per mile and the Ribtec hull is very efficient. I would always allow 1.5 per mile if I was out cruising and 2 per mile if is was flat and running hard. AP
I'm with Alan on this. Over the years I have always found them to be around the 1.2ltr per mile per engine actual but allow 1.5ltr crusing and 2ltr in a blow.
Never really changed whatever length of boat they were in.
Cheers
Andy
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Old 29 December 2011, 10:53   #7
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Originally Posted by kubcat View Post
Well next time we run the tanks low, that is exactly what we will do.

It is amazing, such a simple and logical idea that we just didn't think of. I guess that's what discussion is all about.

Any idea what material the tanks are likely to be made of?
They will be stainless steel made by Hilsea Engineering in Portsmouth UK Alan P
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Old 29 December 2011, 12:14   #8
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Quote:
Originally Posted by Andy Gee View Post
I'm with Alan on this. Over the years I have always found them to be around the 1.2ltr per mile per engine actual but allow 1.5ltr crusing and 2ltr in a blow.
Never really changed whatever length of boat they were in.
Cheers
Andy
I think he was talkIng about total consumption - not per engine?
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Old 29 December 2011, 14:45   #9
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Quote:
Originally Posted by Searider

I think he was talkIng about total consumption - not per engine?
My guesstimate based on the last consumption figures are just over a litre per mile total, not per engine. I could be way out on my distance calculation though as I am guessing distance based on speed multiplied by time at that speed.
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Old 15 January 2014, 06:08   #10
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Quote:
Originally Posted by Alan Priddy View Post
I think the 8.5 is the stretched version of the 7.4 and dont hold me to this but I think they around the 135 litres each. All the stringers and bulheads would be in the same places and we managed to get 3 x 135 under the deck which gave us a 400 NM range. The extra lenght would be for the engines. There is only one way which is to drain them down and then fill them. Alan P
Finally drained the diesel tanks and they are 190 litres per side, maybe I have a different bulkhead configuration.

I needed 8 days of use to almost empty them so I am certainly not displeased. The boat is running perfectly. 3200rpm seems to be the sweet spot for economy. I had to follow a friend for 3 hours in the ocean at 6 knots and it used a lot more per mile.

The digital fuel gauges are not particularly accurate as there only about 8 LEDs, also the first 'half' takes a lot longer to empty, could be because of the shape of the tanks. I may change the gauges to a regular needle type.

Thank you everyone for your input.
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