Gents,
We're talking about a couple different ways of computing fuel burn rate. Dave, you mentioned 'miles-per-gallon', like commonly used for trucks and cars. While I was talking about 'gallons-per-hour', or gph.
I think most mariners use 'gph' rather than 'mpg'. Either method could be used, but since most nautical charts are set up in nautical miles, using 'mpg' may get confusing.
Let's take your halibut fishing hole as an example. You wrote...
Running to my favorite halibut hole which is top secret, it is 120 miles round trip. It takes me about 2 1/2 hours at 25 mph. I think I burn around 25 gallons.
Even here, there is some confusion. You state your running 25mph, but if I divide 120 miles, by 2 1/2 hours (120/2.5) I get a speed of 48mph. So you must mean that that it takes you 2 1/2 hours each way. That closely corresponds to another example you gave in your post above.
If it takes you 2 1/2 hours each way, then that means it takes you 5 hours to run the whole 120 mile round trip. This sounds better, because 5 hours, multiplied by your given speed of 25mph, equals 125 miles. Very close, and sea conditions can cause bigger differences that we see here.
Since you state that your boat burns 25 gallons for the whole trip, and we know the trip takes 5 hours, then simply divide the gallons burned by the hours run (25 divided by 5) and I get a figure of 5 gallons burned per hour (5gph).
What makes this important, when traveling long distances by boat, is that tides, currents, and winds can really change just how far (miles) we may actually travel in a given hour. But, the fuel consumed in that hour is usually pretty constant. So at least in my opinion, it's better to compute fuel burn in 'gallons-per-hour' than by 'mile-per-gallon'.
Another way to put it is...
Say your heading due north at 30mph for 2 hours, on land you would have covered 60 miles. But lets say that the whole time on water, you were bucking a 5 mph current, which was heading due south, your actual distance traveled would only be 50 miles (as plotted on a chart.) Since we already figured that your boat burns 5 gallons per hour, you would have burned 10 gallons so far. Pretending we were on land, your gas mileage woud be would be 6mpg. (That woud suck for a car!) But since we're on water, and due to the current, your boat has actually only traveled 50 miles, now your only getting 5 mpg (worse yet!) So the 'mile-per-gallon numbers can change due to current, etc., but the gallons-per-hour numbers stay constant.
In this example, in a mere 2 hour run, there's already a 10 mile difference. Multiply that by a 5 hour run, and we're talking possibly running out of gas 25 miles short of the intended target (the fuel dock.
).
This is why I don't care for using 'car-like' miles-per-gallon figures for calculating fuel burn rate while on the water. In the example above, the constant factors were that you were running for 2 hours, and you burned 5 gallons-per-hour, for a total of 10 gallons. Actual miles covered varies greatly.
BTW Dave, I know you run long distances regularly, and am not 'preaching' to you!
Just thought I'd spell this out a bit better (maybe) for those who might need the advice. Also, if your only burning 5gph with your 90hp engine, then I over estimated the fuel burns for the first posters twin 70hp set up. That boat is probably only buring 10gph for both engines combined.
Clear as mud!!!
