Were both trips at about the same time, or similar outside temperature? I don't know too much about it, but while I was reading up on another outboard I considered buying, I remember reading a thread where somebody complained that leaving in the morning he was getting a better speed than coming home in the afternoon. Or vice versa. I didn't really pay attention, as it didn't pertain directly to the motor that I was researching. But, it was mostly due to temperature of the air, and what the air temp does to the density of the fuel, or something along those lines. So, that is one possibility.
The other would be the different places that you bought gas. You may have bought all your shore gas from a place that for whatever reason, hasn't started putting large quantities of ethanol in the gas. The marina may have had gas with a heavy ethanol concentration. If you do some readings on some of the numbers as far as fuel consumption goes on the newer trucks that have a consumption meters on the dash, they can see that they get significantly MPG readings on fuel without ethanol. On a truck, the difference is seen in consumption, as it just has to work a little faster, thereby eating more fuel, to achieve the same purpose, as it isn't constantly run at the redline. But, on an outboard, you are always working it as hard as it goes, so when faced with lesser quality fuels, it can't work any faster, so the results are shown in a lack of speed.
Thirdly, it could just be because you have a merc. :shock:
