I'll throw a monkey wrench into the discussion:

If you are in an area with no current or a current going with you, slowing down will improve fuel effiecency. If you are fighting a strong current (which is not always possible to avoid), faster may be more efficent. Example:

Two Throtle settings with no current:
2000rpm: 5 mph 1 gal per hour = 5 mpg
3000rpm: 8 mph 2 gal per hour = 4 mpg

3 mph current against you:
2000rpm: 2 mph 1 gal/hr = 2 mpg
3000rpm: 5 mph 2 gal/hr = 2.5mpg

Ideally you would develop a graph showing the relationship between speed, RPM and Fuel Consumption. By checking the difference between the GPS speed and speed thru the water, you could determine the most efficent cruise speed. Realistically, I use guesstimates. Trying to develop the graph mentioned would take a lot of time and effort and then as soon as a couple of barnicles sneak on board, the fuel level changes or wave action changes, the graph losses accuracy.

The other question is at what speed does it just start getting silly? Recently, I've had situations where I am at around 80% throttle and make 3.5 mph gps. If I ease up, I'm only going to make 1-2 mph.

I generally set my throttle and then if the speed drops below 4 mph gps I start throttling up to maintain at least 4 mph gps. If the current is going with me, once I get up to around 7.5 mph, I'll start backing off on the throttle rather than going faster (Recently coming down the Savanah River I was doing 7 mph with just enough throttle to maintain steerage. I'm guessing a little bit, but estimate I was getting around 30 mpg.). This maximizes fuel effiecency, while ensuring we are actually making some headway.

Mike & Tammy
Valhalla II

   Item 1: gph: Since this has been a discussion of being more fuel effient, gph doesn't take into account how far you are traveling per hour, so it doesn't tell you how efficent you are being with a current. With no current there is a direct correlation with mpg but that can get you into trouble if you don't realize the current is there. 

Example: At a particular throttle setting you are buring 1 gph and making 5 mph thru the watter. You have 20 miles to a safe port and 5 gallons left. Do you have enough fuel? If there is no current, you have a range of 25 miles, you should make it with a gallon to spare. Same senario fighting a 3 mph current. Since you are now only making 2 mph over ground, you will come up 10 miles short (and possibly run out after dark). Gph doesn't provide useful info until you account for speed, which is what  mpg gives you.

Another monkey wrench please, what would be the outcome if one were to figure gph rather than mpg? I never relate to mpg on the water even when the charts indicate such. For fuel consumption I find gph much safer.

     Item 2: Huck Finn had a boat with infinite efficency (not exactly but close enough for our purposes). Zero gph. If you run the math, it is always more effient to slow down going with the current. The practicle limit is enough headway to maintian control.  There might be a slight downtic in fuel efficency right around idle as internal engine friction becomes greater than the effort to turn the prop, but it should be pretty minimal.

quander this.,... If  you are going down current, are you better to go at a higher speed?

See you on the Waterways!

Capt. Sterling

Mike & Tammy
Valhalla II