Re: Model 850 Mercury Outboard. Fuel Comsumption???
Yes a lot of folks use the rule of thumb and it works for them. Personally I have had other things effect the number including boat type, hull shape, what kind of water you boat in, load, how do you operate your boat, and last but surely not least is type of fuel scavenging.
Cross flow is the oldest design for modern engines and it is the worst in fuel economy. Large displacements, like the old OMC 25's 40's and "Fat" V6 series were all guzzlers. Course you got the best low end performance out of them too. A 25 Johnson back in the 50-60 time frame would easily plane out (and then some) a 16' fishing boat with a couple of guys and their gear out in the bay where water was usually rough.
Newer engines with better engineering (not brute force like cubes will do for you) got better if you kept them lightly loaded and at higher rpm's. OMC hit the jackpot back in 1968-9 with the Rude Triumph 3 cylinder loop charged induction system. Sucker was tough and could run circles around a cross flow....great engine. Offspring still being made today. Merc at the time came out with what they called Direct Charge which was a ported piston and some other goodies, but piston still had crown (not flat top like loopers) and they did better but OMC still had the patent and Merc had to wait. I had an '88 115 tower that was DC and it was excellent on fuel, but I had the hull to allow it to shine. Then Merc came out with their loopers when the OMC patent expired and they were excellent on fuel also...i currently have one at 90 hp.
I had an 85 back in the late 70's-early 80's; may have been an 850. Cowl had 3 stripes front to rear, red, blue, and silver; went straight back till they got to the rear of the cowl then made a curve down and stopped. Anyhew, it was direct charged and excellent on fuel on a light bass boat with the right hull.
Hope this helps.
Mark
