Propellers, RPMs, and Fuel Consumption

[Jason2tpa]

I recently re-propped, ridding of the 21" pitch that only got me 4700 @ WOT. Now with the 18" pitch I've got on the motor I'm able to reach top end of rpm spec @ WOT (5500). When I did have the 21", my normal cruising rpms were 3700-3900. With the 18", in order to cruise at the same speed as when the 21" was used, I cruise at 3900-4100.

I've definitely noticed a big difference in fuel consumption since running with this 18" prop (consuming more fuel). Yet I'm only cruising 200 rpms more than with the 21" prop.

Both 18" and 21" props are SS and 3 blade, and no changes to boat load or any other factors effecting rpms. I had thought that once the boat/motor is properly propped, no longer being over propped, I'd see better fuel economy or at least the same fuel economy.

Unfortunately I'm not able to accurately measure fuel consumption per mile because my fuel gauge is analog. And for those of you who are not familiar with analog fuel gauges, the needle sways as the boat pitches. So taking an accurate reading of fuel consumption is difficult, if not impossible.

Is the added 200 rpms of engine work sufficient enough to cause a big difference of fuel consumption? Any thoughts?

[99yam40]

fill tank the way up, run, and then fill back up to see how much fuel was used.
same way you do it in a car.

why do you need to run 200rpm higher than you did before?

You do not really need to be doing the exact same speed ,Do you?

I normally just find the RPM the boat and motor are happy at for the conditions

[panasonic]

How about using a portable tank for a few runs? A lot easier to get an accurate reading then....run a few tanks full at different rpm.'s note your mileage and speed with GPS...find your most efficient speed and rpm.

[Jason2tpa]

99yam...The boat/motor is happiest cruising the 200 additional rpms. Not a speed thing. Good idea about topping off, running, then topping off again to determine gallons used.

panasonic...the portable tank is a good idea. However, to convert the fuel lines would definitely be a pain. There is a junction in the fuel line, about 3 foot forward of engine. This would be the ideal spot to tie in the portable tank line. But again, very arduous task. Think I'll do the lazy method 99yam suggested.

Appreciate the input.