Twin ME421STi's fuel economy

[kubcat]

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.

[Alan Priddy]

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

[kubcat]

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.

[Alan Priddy]

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

[kubcat]

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?

[Andy Gee]

Originally Posted by Alan Priddy

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

[Alan Priddy]

Originally Posted by kubcat

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

[Searider]

Originally Posted by Andy Gee

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?

[kubcat]

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.

[kubcat]

Originally Posted by Alan Priddy

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.

[biffer]

The needle type gauges aren't any better, I'm assuming you have green LEDs on the gauge with red on the port side, they are wema gauges, the best thing to do is when you fill it from empty count the litres until it get to green, we use to put the senders in but not right to the bottom of the tank, that way when it read empty there was alway a bit left, we never used to tell the customer, the only true way to gauge your fuel is with something like a flowscan system, there are too many different variables to get it dead right

[Brian]

+1 for the Floscan system.
It's awesomely accurate (and finely adjustable) and you should be able to get a system for twins, fitted at around £1000 Sterling.

[Daniel]

+2 for flo scan. Excellent.

FYI Most of the "cheaper" nmea systems that work with plotters AERNT diesel comparable.

[kubcat]

There is no doubt that Floscan is the ultimate setup but for the cost of the Floscan I can purchase about 1000 litres of diesel which is probably about half my annual use.

If I was doing long distances where range was an issue, Floscan would probably be my choice from a safety point of view, but I doubt I will make significant fuel savings using it. This RIB's sweet spot is 3200 rpm, I have no usable adjustment of the drives and no trim tabs fitted, so there is not a great deal of fuel to save. For my other boat, a 38 ft Cigarette with twin 8.1 litre petrol engines, Floscan may be very worthwhile.

Cuzpro.com has some fuel gauges that are NMEA 0183 compatible but more importantly for me they 'learn' the tank shape. I do not expect them to be entirely accurate, but if they are within 10%, that suits me fine. Of course that will depend on my current senders and they are about 10 years old.

These gauges are on sale at the moment for under 100 pounds for a 3 tank gauge. I think it should be more accurate than what I have and at a cost that is worth taking a punt.

http://www.cruzpro.com/tl30.html

[kubcat]

Originally Posted by biffer

The needle type gauges aren't any better, I'm assuming you have green LEDs on the gauge with red on the port side, they are wema gauges, the best thing to do is when you fill it from empty count the litres until it get to green, we use to put the senders in but not right to the bottom of the tank, that way when it read empty there was alway a bit left, we never used to tell the customer, the only true way to gauge your fuel is with something like a flowscan system, there are too many different variables to get it dead right

Yes they are Wema's. Biffer, the weather is perfect, hurry up and get here.

[biffer]

Originally Posted by kubcat

Yes they are Wema's. Biffer, the weather is perfect, hurry up and get here.

Ha ha, sounds like you've got a million questions for me. My flights are booked and it's coming up fast. My son is telling me it's in the 40s over there at the moment

[Barrowboy]

Originally Posted by biffer

Ha ha, sounds like you've got a million questions for me. My flights are booked and it's coming up fast. My son is telling me it's in the 40s over there at the moment

It's in the 40's out here in the North Sea at the moment too Biffer...

41.72 degrees Farenheit to be precise.

[Chris Caton]

Originally Posted by Barrowboy

It's in the 40's out here in the North Sea at the moment too Biffer...

41.72 degrees Farenheit to be precise.

nice

[biffer]

Originally Posted by Barrowboy

It's in the 40's out here in the North Sea at the moment too Biffer... 41.72 degrees Farenheit to be precise.

Ummm let me think North Sea, Sydney, North Sea, Sydney, can I have a bit of time to think about it

[Chris Caton]

Originally Posted by biffer

Ummm let me think North Sea, Sydney, North Sea, Sydney, can I have a bit of time to think about it

times up Biff, it's the Nth Sea for you, I'll do the Sydney trip