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Old 09 March 2007, 15:46   #1
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Fuel consumption

I've just measured the fuel consumption of my engine (Johnson 2 stroke carb, 115 V4) for the first time.

58.6 litres to cover a GPS measured trip of 45.2nm seems a bit on the high side to me, anybody else with similar old-tech carb motor of a similar size able to comment?

I thought the rule of thumb was about 1ltr per nm, I know carb 2 strokes are thirsty but 30% more thirsty? This was not flogging it in fact a fair bit of time was spent at about 10 knots looking at a nice bit of coastline I'd not seen before, average speed on the whole journey was 15.something knots.

Didn't see any whales either (which was the object of the trip)
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Old 09 March 2007, 16:07   #2
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Probably about right, but 10 knots will be your problem, boat isn't planning but running bow up using a lot of fuel but mot much speed.

You could try this:

http://cgi.ebay.co.uk/Navman-F41-mar...QQcmdZViewItem

Pete
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Old 10 March 2007, 08:56   #3
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Cool...

Ebay won't let me register from here as they now require a credit card to authorise new users and they don't like my address (!) how much does one of those cost new?

Could be right I suppose, 8-10 knots is probably not a "best fuel economy speed" - though we probably only did about 10 miles like that (between looking for whale spouts, and looking at coastline).

I will keep a log of fuel and distance for a few tankfuls and see what the long term average is I think.

Just out of interest I suppose the best fuel economy speed is "just on the plane" is it? My feeling after a year of using but not measuring is that about 18 knots is probably the most economical cruise, fairly low revs but still well up on the plane.
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Old 10 March 2007, 09:23   #4
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With the vodka and tonnic about 160 over hear. I think if you start plotting the fuel figures then the graph will be pretty much flat for the mpg. Have a look at some of the US boat test sites, mpg figures only seem to vary at slow and flak speeds. In between its almost flat.

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