2 stroke fuel efficiency

Fisherman Rob

Fisherman Rob

Well-Known Member
The other day we did a long run around the Southern Gulf Islands in our 17' runabout. We set out northward from Sidney, and stopped at Ganges market on Saltspring, on to Montague Harbour on Galiano for a picnic lunch, then south to Cabbage Island via boat passage, rounded boiling Reef off Saturna Island, then back to Sidney. Beautiful sunny day, mostly flat seas. In total we covered 70nm and consumed 75l of fuel, averaging 1.07 liters per nautical mile at 25 knots.

Boat is a 1996 17' DE powered by a 1996 2 stroke 115 hp Yamaha. I have heard that these old two strokes are not particularly fuel efficient, but seems consuming 1.07l/nm isn't too bad? Anyone have a boat of similar spec and power for comparison? What improvement could be expected from a newer 4 stroke?
 
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Interesting, I have almost exactly the same fuel economy on my boat.

1993 Hourston 17' with a 1993 Johnson 70hp two stroke. My last trip I went 86 miles, cruising around 25mph and burned 76 litres. That miles though, not nautical miles.

Supposedly you can expect up to 30% better fuel economy with the newer motors. Would be interested to hear if anyone has a similar boat to ours with a newer 4 stroke or ETEC.
 
I think the conventional wisdom is that a four stroke motor will improve your fuel efficiency by something in the order of 30%, perhaps up to 40%. Your fuel efficiency is actually quite good with that 2 stroke. I calculated it as approximately 3.7 nmpg. I have a DE 16 powered by a 1996 Johnson 90 hp 2 stoke and I get about the same, maybe a little better, if the seas are calm and I keep the speed reasonable. Full throttle gets me 32-34 knots, but that's more speed than I need. I can cruise comfortably at around 18-21 knots at 3500 RPM and use a lot less fuel. If I had a 4 stroke, my efficiency and range would be a lot better. But my 2 stroke is really reliable so far, and I can buy lots of fuel for what a new 4 stroke would cost, so it will be run until it dies a death that is too costly to repair. But I hear that the 2 strokes are easily rebuilt, so who knows?
 
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Interesting comments so far. I suspect better than expected fuel economy might be attributed to flat sea conditions and long runs at conservative rpm (ie 70nm at 4200 rpm without much stopping and starting).
 
I run a 1998 Double Eagle 17ft with a 150hp etec great package. Last week I ran 38m and burnt 30L cruising at 35mph one way but slower on the way back due to the weather. I don't pay close attention to my numbers just ball park my distance and my GPH to know if I need to fuel up on my route. I usually get between 4-6 GPH depending how I run the boat.
 
I have a 1994 17.5 double eagle and running a 2012 115 hp 4 stroke merc with the smart meter. When cruising at 28 mph burns 4 gallons a hour and at 32 mph burns 5.2 gallons an hour and wide open 42 mph a whopping 11 gallons an hour. Trolling at 1000 rpm 2knots burns 1.2 gallons an hour. This is with two people and their fishing gear.
 
So for a comparison, Chero's smart meter indicates the same boat at same cruising speed with a new 4 stroke should average 0.6 l/nm. That seems far better fuel economy. With gas at $1.32/l a new four stroke would pay for itself in only 20,000 nm! :). Is the smart meter accurate compared to actual consumption?

Meanwhile Leftcoast got 1.15 l/nm with a 150hp etec on the same boat, probably averaging slightly faster speeds.
 
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Speed makes the big difference. I think the rule of thumb for all outboards is to take the first number (for outboards less that 100 hp) or the first 2 numbers (for those over 100 hp) and that's what it burns in gallons per hour at full throttle. Much less at lower RPM.
 
We did d a big run from Nanaimo to the San Juans a few years ago and I kept close track of fuel and distance.
I had been told that the most efficient speed for most boats was that which just kept you on plane and so that is what I did.
My 16 foot BR with a 115Merc 2S averaged 1.7km per litre. The speed was kept down to approx 27kmh whenever possible.
I have compared this number to other boats and the motor doesn`t make any significant difference. 2S v 4S etc.
 
Smiley that calculates to 1.08l/nm, so two stroke numbers so far are all similar. What kind of boat is a BR? While Chero's smart meter suggests double the fuel economy with a four stroke, I'd be interested to compare some actual measurements of consumption per distance.
 
Chero said:
I have a 1994 17.5 double eagle and running a 2012 115 hp 4 stroke merc with the smart meter. When cruising at 28 mph burns 4 gallons a hour and at 32 mph burns 5.2 gallons an hour and wide open 42 mph a whopping 11 gallons an hour. Trolling at 1000 rpm 2knots burns 1.2 gallons an hour. This is with two people and their fishing gear.
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Fisherman Rob said:
So for a comparison, Chero's smart meter indicates the same boat at same cruising speed with a new 4 stroke should average 0.6 l/nm. That seems far better fuel economy. With gas at $1.32/l a new four stroke would pay for itself in only 20,000 nm! :). Is the smart meter accurate compared to actual consumption?

Meanwhile Leftcoast got 1.15 l/nm with a 150hp etec on the same boat, probably averaging slightly faster speeds.
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In cases where there is a fair and somewhat equal comparison done, I have not found there to be much more than 25% in fuel saving with 4 stroke and in many cases less. For example Chero's 4 stroke 115 figures above are not that far better than a 2 stroke 90 I had on the same hull as Chero. My older boat, a 1990 DE 17 hull with a 1997 90 hp Johnson carbed 2 stroke would run at about 25 mph and consume about 3.5 gallons. At 32 mph almost 6 gals/hour. Although not the same hp, in this comparison the 2012 merc 115 4 stroke compared to the 1990 carbed 90 hp 2 stroke the actual fuel consumption difference is barely 10% ( not counting oil consumption)

Comparing a Yami F225 4 stroke results with the Yami 200hp HPDI ( 2 stroke, fuel injected) I have found that given the same hull ( Grady 228 hulls ) that my HPDI is just about as fuel efficient as a F225 - both about 9 gallons/hr at similar cruising speed. The oil injection cost for the 2 stroke is of course extra. The only variables may have been load weight differences and if motors were equally tuned/maintained.
 
My 200 Yami carbed 2 stroke/22 Seasport used exactly 28 gallons from Sooke Harbour to the Kingfisher fuel dock in Bamfield. 23-30 mph...did 30 when the seas allowed. Got the same result on a round trip from Sooke to Port Renfew.
 
On my 17.5' Campion I switched from a 70hp Johnson (2s) to a 100hp Yammi (4s) a couple of years ago. Under same conditions the new motor uses slightly less than the old. The main advantages for me are: no 2 stroke stink and noise, bit more power and no oil mixing anymore. Glad I switched!
 
135 Optimax on 18' FGlass runs about 50-60l fuel from ambelside to thrasher and back. So about 1l / Nmile. 3400-4000rpm bout 30-38mph
 
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