Troubleshooting really bad gas mileage

Well, thanks to google this boat-newbie found your amazingly helpful forum. What google could not help me find was helpful advice for my issue.

I recently acquired an older boat (a 1970-ish 20ft fiberglass bayliner that was converted from an inboard to an outboard at some point) at a price too good to resist and have been fixing it up for about three weeks now. Yesterday was my first day-trip out of Whitter and considering I have no boat experience, don't know anyone with boat experience and the vast majority of my knowledge comes from one charter captain I had a couple conversations and google I'm not too sure what to do about the trouble I've discovered.

The engine (a 1993 115 Mercury) seems to run very well after a bit of warming up, it died once on the water and that was my fault entirely. The trouble is, the round trip I took was about 16 miles... the engine burned through at least 20 gallons of gasoline on the trip, running at full throttle or near it, during the majority of the traveling. That is abysmal.

As far as I could tell there was no leakage anywhere of fuel, no "glaze" on the water, none in the hull. I'm fairly certain the compression was good but I will be double checking it again. I'll admit, the previous owner said he had the RPM worked out but now I doubt him. I've started looking into RPM things but any helpful advice would be appreciated.

Other than that, I'm not sure what to do. Apparently googling "Poor gas mileage" on anything boat related produces a resounding "No Duh," which I knew already, but as far as I can tell the full throttle MPG of these motors should be around 11 mpg? Or am I mistaken? I know I can throttle down a bit to save some fuel (this was a test run after all) but I'd rather try and solve the issue.

I don't even know where else to began on this. I guess I'm still in shock.

Re: Troubleshooting really bad gas mileage

My '86 115 inline 6 gets 5 mpg at 3050 rpm spinning a 19p prop on a 15' speedboat.
That drops to a little less than 3 mpg at wot.
On a 20 footer you can expect 3mpg at cruise and less than 2 at wot.

Re: Troubleshooting really bad gas mileage

I have a '75 merc 850 85hp I-4, at WOT, I consume, on average right around 8 gallons/hr at around 31mph, which comes out to approx. 3.87mpg, this is pushing a 16' fiberglass tri-hull... a general rule of thumb for these 2-stroke OB's is hp = gallons per hr at WOT.. ex. 115hp = approx. 11.5 gph, or 3.91mpg at WOT, keep in mind these numbers are strictly a rough estimate...I would say for a better idea, go with what redjmp has stated as he has the same motor as yours.. Anyhow, by your calculations, that would put yours at 1.25 mpg.. does seem a bit much, but not too incredibly far off depending on prop pitch, weight of boat, how well it planes out, etc.

With that being said, when I'm cruising along at around 3k rpm, my gph increases to approx. 5 1/2 - 6, giving me somewhere around a 38-40% increase in fuel economy, while only knocking me back about 10mph. I'm FAR from an OB expert, in fact most of what I know I have learned from others on this forum, however I hope this helps you out some..

Re: Troubleshooting really bad gas mileage

Apparently my math is the first to go when im this tired.. just reread that and realized you were actually getting in the neighborhood of 0.8mpg, I would start by checking your carb adjustments to see if its running too rich, check timing, prop pitch.. a few ideas there anyhow.. sorry about the first response and bad math, been up far too many hours lol..