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Old 09 March 2008, 13:43   #21
Country: UK - Scotland
Town: Ardfern
Boat name: Moon Raker
Make: Humber Destroyer
Length: 5m +
Engine: Honda BF 90 D
MMSI: 235035994
Join Date: May 2006
Posts: 694
I kept a very careful check on my consumption last year. Boat and engine as described on the left.

Cruising speed 3600 rpm, which gives 23 knots and seems to be the most economical speed.

Average Consumption over the season 1.75 miles per litre - yes mpl. Litres per hour works out at 8.9, but that's because I spend a fair amount of time pottering about finding places to anchor, so average speed over the year is around 15 knots. (Taken from GPS av spd at the end of each trip)

I think the consumption increases exponentially to enormous amounts between 4000 rpm and max 5600 rpm, but by then I'm doing 39 knots and struggling to hang on.

Take it steady and I think you'll be pleasantly surprised. Incidentally, my heavy 38ft. ketch used about the same amount of fuel per trip (under power) as my little RIB. (They both work out at around 8 nautical miles per gallon )

Even more useless info. 5 miles motoring with my Johnson 3.5 auxiliary used the same amount of fuel as the main engine at 3500rpm over the same distance. Took ten times as long though.

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Old 09 March 2008, 15:24   #22
Country: UK - England
Town: cornwall
Boat name: nothing
Make: rib eye 430
Length: 4m +
Engine: tatsu 50
MMSI: 666
Join Date: Mar 2006
Posts: 1,914
I don't know if its a difference with 2 strokes , but having the right pitch prop made a great difference to mine .

When i first had the boat it was overpropped a little and keeping the boat on a slow cruise to get down the river from saltash to the sea seemd to use a lot of fuel .
Same thing with the 4 stroke on my old boat didn't seem to change fuel consumption

I came to the conclusion the etec or maybe 2 strokes in general don't like to labour

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Old 10 March 2008, 06:42   #23
Country: UK - Scotland
Boat name: Wildheart
Make: Humber/Delta Seasafe
Length: 5m +
Engine: Merc 60 Clamshell
MMSI: 235068449
Join Date: Apr 2007
Posts: 4,614
I'll second Alystra on that one - My old boat (SR4 / Suz 25) would run forever on a tank at 3/4 throttle (about 16/17 Knots ish - no GPS) . Open up to get 21 (flat out) and I was onto my second tank before I knew what was happening.

So throttle back and the fuel consumtion will drop remarkably for a smal ldrop in speed. Also conversation becomes easier!!
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Old 10 March 2008, 07:57   #24
Simon B's Avatar
Country: UK - England
Town: Leicester
Boat name: Vixen
Make: Ribcraft
Length: 6m +
Engine: Suzuki OB 175
MMSI: 235071839
Join Date: Jun 2004
Posts: 1,621
Originally Posted by ian parkes View Post
I came to the conclusion the etec or maybe 2 strokes in general don't like to labour
4s dont either but any engine with a carb on is effectively using it's throttle like a tap independant of speed.

New boat is here, very happy!
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